Maize streak, maize stripe
and maize mosaic virus diseases
QJ
in the tropics (Africa and islands
■ h in the Indian Ocean)
■ Im pact o f m aize virus
diseases and curre nt research
■ Vectors
and e p id e m io lo g y
■ M a iz e virus diagnosis and
m aize streak virus v a ria b ility
■ M a iz e resistance
and breeding
Dossier prepared b y f C. JOURDAN-RUF, J.-L. MARCHAND, M. PETERSCHMITT @ ® C A , BP 50 35, 1 • 3 4 0 3 2 Montpellier Cedex 1,iranee B. REYNAUD, J. DINTINGER I C tR i0€A , station, de Ligne Pqljdis,■ 9 7 4 1 0 Saint-Pierre, Réunion, F w ce
The main research scientists that c o lla b o ra te d in these studies are:
G. KONATE, O. TRAORE and S. TRAORE (Burkina Faso); #> f r; M. ESSEH-YOVO ¡Togo); C. THE (Cameroon): H. PHAM ¡Zimbabwe); P. MARKHAM (UK); C. BUDUCA, B. CLERGET and A. RODIER [Réurtion, France) = } M i ' Photos D. Debert:
B a g g in g m a iz e in flo r e s c e n le s . p /V a iz e fie ld s in B u rkin a F a sfx Photo B. Reynaud: P e re g rín u â m a id ts .
Impact of maize virus
diseases and current research
Regions
The th re e m a in t r o p i c a l m a iz e viruses are: maize streak virus (MSV), maize stripe virus (MStpV) and maize mosaic virus (MMV). Their impacts vary m arkedly between countries (Figures 1 & 2).
MSV was fir s t de te cte d in South A f r ic a at the b e g in n in g o f the century, and similar virus diseases have been described in sugarcane and in many wild Poaceae species. It is transmitted by homopteran insects of the genus Cicadulina (Homoptera, Cicadellidae). MSV infections have been n oted in m a n y d if f e r e n t countries, with varied incidence. It is endemic to all East African countries,
Evolution of host
plant/vector/virus
complexes
P. m aidis a n d the viruses it transm its evolved together in a few plant species. M a iz e is th e f a v o u r e d h o s t o f th is vec to r/v iru s com plex; the o rig in a l host was teosinte o r sorghum. The close host p lan t/vector relationship perpetuates the c o m p le x w ith the v ir u s , d e s p ite the n a r r o w r a n g e o f h o s t p la n ts a n d susceptible species.
The e v o lu tio n o f M S V , tra n sm itte d to m aize b y Cicadulina mbila N a u d e , is d iffe r e n t: the v iru s w a s re v e a le d in m a iz e c r o p s , b u t a ls o in fe c ts w i ld grasses through the leafhopper vector, which is com m on in A fric a and has a w id e host range.
Madagascar, Réunion, and central Africa. The disease is also sometimes found in West Africa, p a rticu la rly in M a li, T o g o , Côte d 'I v o i r e and Senegal. In W e s t A fr ic a , there was a w idespread outbreak of MSV in 1983-1984, sometimes c o m p le te ly d e s tro y in g all m aize crops. This disease also occurs in India in wheat and millet, but it has n o t been d e te c te d in th e South American tropics.
MStpV is not as common. Peregrinus m a i d i s A shm ead ( H o m o p t e r a , D e l p h a c i d a e ) , the insect vector, is present in many African countries (Côte d 'I v o i r e , Z a ire , N ig e ria , Burkina Faso and Cameroon). Since 1936, this insect has been reported
in East A fr ic a , T a n z a n ia , Kenya, Mauritius and Réunion, but MStpV has caused very little damage. The v iru s was r e c e n tly id e n t if ie d in W e s t A f r ic a (Côte d 'I v o i r e , Togo, N ig e ria , B urkina Faso and Cameroon). However, it seems to have had a greater impact in Latin America, particularly Venezuela and the West Indies. It also occurs in the Philippines and Australia.
M M V , f ir s t d is c o v e r e d in 1921 in H a w a ii, is also tra n s m itte d by
P. m a i d i s . It was la te r d e te c te d in Cuba, G u a d e lo u p e , S u rin a m , Venezuela, Guyana, Puerto Rico, Burkina Faso, Côte d'Ivoire, Nigeria, M o z a m b iq u e , T a n z a n ia , Kenya, R é u n io n , M a u r it iu s and In d ia .
Organizations
involved
CIMMYT, Centro Internacional de M ejoram iento de M a íz y Trigo, M exico CIRAD, Centre de coopération
internationale en recherche
agronom ique pour le développement, France
CORAF, Conférence des responsables de la recherche agronom ique africains IRA, Institut de la recherche
agronom ique, Cameroon
INCV, Institut national des cultures vivrières, Togo
INERA, Institut national d'études rurales et agricoles, Burkina Faso
IITA, International Institute o f Tropical A griculture, N igeria
John Innés Institute, N o rw ich, UK
The incidence of M M V is generally lo w in a ll o f these c o u n tr ie s . N e v e rth e le s s , serious M M V e p id e m ics have been reported in some re g io n s o f m a iz e g r o w in g c o u n tr ie s such as USA ( H a w a ii, Florida), Brazil and Mexico.
Control
Virus-resistant varieties of maize are of considerable econom ic interest due to the extent of damage caused by m a iz e viru se s in the tro p ic s ( A fric a , South A m e ric a ) and the lack of e ffic ie n t a g ronom ical and chemical control techniques. Higher
and more regular maize yields could be obtained with such varieties. Maize breeding studies began in the 1930s in East and South Africa with the aim of obtaining MSV resistance. Moreover, the 1972 MSV epidemic that occurred in West Africa promp ted certain research organizations (IIT A and C IM M Y T ) to c o n d u c t similar studies.
D u r in g the 1 970s, C IR A D (in c o ll a b o r a t i o n w i t h A fr ic a n and European partners) focused research on this topic in order to deal with the serious m a iz e v iru s e p id e m ic problems — the three mairi maize viruses and their vectors are present including local resistant varieties.
The research
and who is involved
Since the 1980s, CIRAD and partners have been focusing on the following topics:- in se ct v e c to rs and t h e ir epidemiology;
- maize virus diagnosis;
- b re e d in g and in v e s tig a tin g th e g e n e tic fa c to rs d e t e r m in in g resistances, and transferring them into susceptible maize genotypes. These investigations revealed the v e c to rs and fa c to rs in v o lv e d in t r ig g e r in g and s p re a d in g v ir a l epidemics in maize crops and led to the development of specific maize virus diagnosis techniques. Resistant ecotypes in Réunion have been used
in resistance transfers to create maize varieties that w ill be more efficient under cropping conditions found in A fric a and th ro u g h o u t the Indian O c e a n r e g io n . The d e v e lo p e d diagnostic techniques can be applied at a variety of sites, without special equipment, to identify viruses and assess varietal susceptibility.
CORAF, during the general assembly m e e tin g o f its m a iz e n e tw o r k in Yaoundé (Cameroon) in 1987, deci ded to launch an MSV research pro ject because of the urgency of the situation. For resistance transfers, n a tio n a l CORAF c o rre sp o n d e n ts chose a num ber o f varieties from a m o n g a lr e a d y (or soon to be) widely grown maize varieties. This multidisciplinary project brings together virologists, entomologists and geneticist/breeders:
- virus studies by John Innes Institute, N o r w ic h (UK) and C IR A D , Montpellier (France);
- interactions between viral isolates and resistant genotypes in Réunion (CIRAD) and Burkina Faso (INERA); - insect vectors in Réunion (CIRAD), Burkina Faso (INERA) and Cameroon (IRA);
- m a iz e v iru s d y n a m ic s in host plants in the tropical phytovirology laboratory (CIRAD);
- resistance genetics in R éunion (MSV, MStpV, MMV) (CIRAD) and in Togo (MSV) (INCV);
- creation of resistant varieties in Réunion, Togo and Cameroon. ■
Symptoms
Maize streak disease
M a iz e s tre a k is c h a r a c t e r iz e d b y ch lo ro tic spots a lo n g the le a f veins, f o r m in g d is c o n tin u o u s s tre a k s o f v a r y in g th ic k n e s s . D w a r fis m is very marked in young infected plants.Dwarfed young plant. M SV leaf symptoms.
Chlorotic stripes on maize leaves due to MStpV.
Maize mosaic disease
M a iz e m o s a ic is c h a r a c te r iz e d b y r e g u la r c o n tin u o u s c h lo r o t ic lin e s running along the w hole length of the le a f , p a r a ll e l to th e v e in s . The s y m p to m s v a r y a c c o r d in g to the thickness o f these lines, their spacing and discontinuous appearance.Limited growth o f infected plants. Continuous chlorotic stripes
caused by M M V .
Maize stripe disease
M a iz e stripe occurs in two forms. The f ir s t , s im p ly c a lle d s t r ip e , is c h a ra c te riz e d by c h lorotic stripes of various widths along the leaves, and th e a p e x is t y p ic a lly c u r v e d . The second form, called chlorotic stripe, is c h a r a c te r iz e d b y the f o r m a tio n o f c h l o r o t ic s tr ip e s , a n d th e le a v e s e v e n tu a lly becom e to ta lly c h lo ro tic , with the reappearance o f thick green discontinuous stripes. Young infected plants present severe d w a rfis m , and then d ry up and die.Photos P. Baudin and B. Reynaud Infected plants are dw arfed and dry up.
S t # * *
Vectors and
epidemiology
As early as 1 9 7 3 ,
severe epidem ics of several
maize viruses w ere noted
in Réunion. In 1 9 8 5 , three
viruses w ere identified
by serological techniques:
MSV, M StpV and M M V.
Insect vectors are required
for their transmission.
M SV is transmitted
by at least eight different
C i c a d u l i n aspecies.
M StpV and M M V
are transmitted by
Peregrinus m a id is .Research carried
out to date has
mainly focused
on the Sudano-Sahelian
region (particularly
Burkina Faso) and Réunion.
Insect vectors
In Réunion, Cicadulina mbila is the main MSV vector even though this insect o c cu rs in r e la t iv e ly small quantities on maize, the final host of the disease. It is a very active and e ffic ie n t Poaceae-specific vector.
C. storeyi is the next important MSV vector in Réunion.
In B u rk in a Faso, fiv e C i c a d u l i n a
species have been id e n t if ie d in a ll e c o lo g ic a l z ones: C. m b i l a , C. similis, C. arachidis, C. triangula
and, less commonly, C. hartmansi.
The viru s tra n s m is s io n p o te n tia l varies in the d iffe re n t leafhoppe r species, with C. mbila being the most efficient and C. arachidis the least.
P. maidis is a sedentary vector that is s p e c if ic to m a iz e and s o rg h u m w h i c h it c o lo n iz e s in r e la t iv e ly hig h d e n s itie s . Its m a iz e v iru s transmission e fficie n cy is in trin s i cally low.
MSV host plants
and vector
epidemiology in the
Sudano-Sahelian
region
A detailed study, representative of Sudano-Sahelian regions, was set up in Burkina Faso in 1984.
H i g h I y M S V -s e n s iti ve m a iz e cultivars (Safita II and Jaune flint de Saria) were used for all tests of virus transm ission w ith the le afhoppe r
C. triangula. The serological ELISA ( e n z y m e - lin k e d im m u n o s o r b e n t assay) technique was used to detect MSV in collected samples.
Virus reservoir plants
A 3-year (1988-1990) wide ranging survey o f herbaceous plants was conducted in Burkina Faso, resulting in th e d e te c tio n o f 41 Poaceae species with MSV symptoms. ELISA tests revealed the presence of the virus in 25 species.
A n a ly s is o f th e s p a tio te m p o r a l distribution of virus reservoirs (host plants) highlighted three main points: reservoirs were greater in maize crop z ones; th e in c id e n c e o f M SV in maize fields was proportional to the number of reservoirs; and there were very few infectious reservoirs in May, with an increase 3-4 weeks after the onset of the rains to reach a peak in A u g u s t. In B u rk in a Faso, the incidence of MSV is generally 6% or less in maize plants presenting virus symptoms; it increases to 30-40% during severe epidemics.
In Sudano-Sahelian regions, the risk of early infection (often severe) could be m in im iz e d by e a rly s o w in g ( m id - M a y to mid-June), d u r in g a period when the inoculum source is minimal. However, this recommen d a tio n c a n n o t a lw a ys be met on a c c o u n t o f the c o tto n and ric e cropping calendar.
The importance
of clim atic factors
In B u rk in a Faso, 10 years of agroclim atic studies demonstrated the effe cts o f c lim a t e on MSV incidence, in order of importance: r e la tiv e h u m i d i t y in F e b ru a ry ; relative h u m id ity in January; and temperatures in May.
In January, at the beginning of the dry season, insect densities are high and they are obliged to migrate to refuge areas (sites w ith permanent w a te r s u p p lie s , ir r ig a te d crops) because of the lack of host plants. The relative air humidity decreases through January and February, which can a ffe c t in s e c t b re e d in g and survival and lead to high mortality. The n u m b e r o f MSV v e c to rs a v a ila b le in the f o l l o w i n g ra in y season is determined by the size of
o p t im a l fo r C. m b i l a and p r e c i p i t a t io n fa v o u rs d e v e lo p m e n t of Poaceae hosts. W ith hot temper atures, th e re is a h ig h e r risk of epidemics of MSV than MStpV, with a reversal o f the s itu a tio n at the end of the dry season. The cooler w e a th e r is m ore s u ita b le fo r P. maidis than C. mbila.
P. m a i d i s can s u rv iv e at m ore m o d e ra te te m p e ra tu re s , w h ic h means that epidemics can occur at high e le v a tio n s . Since P. m a i d i s
generally infests maize, areas where this crop is grow n intensively are seriously affected by MStpV.
The M M V infection rate of maize is not very high in irrigated low land zones. However, it should be noted that the symptom-based technique for evaluating viral infection levels in maize plants often underestimates M M V incidence; this is particularly true when there are secondary MSV and MStpV infections, since these t w o v iru s diseases mask M M V symptoms.
Efficiency of virus
transmission
to the insect vector
Peregrinus maidis adult. Photos B. Reynaud
Cicadulina mbila nymph and adult.
the residual insect population, which in turn determines the extent of MSV development. May precipitation was found to be very im p o rta n t in the MSV e p id e m ic s o f 1 9 8 3 - 8 4 . In Zimbabwe, March-June precipitation is also a f a c t o r th a t d e te rm in e s
Cicadulina spp. population densities from July to September, and thus MSV infection rates. Moreover, in Zim babw e and Réunion, the rising temperatures during the southern hot season promotes d e v e lo p m e n t of vector populations.
Impact of three
maize virus
diseases in Réunion
In Réunion, the population dynamics of maize virus insect vectors were s tu d ie d by s o w in g m a iz e on a bimonthly basis from 1983 to 1985, and on a w e e k ly basis from 1985 to 1988.The virus symptom observations and vector trapping results for lowland areas in R é u n io n in d ic a te d th a t epidemics of the three virus diseases d iffe r in terms o f th e ir locations, extents and seasonal patterns. At low elevations, MSV causes very high to total maize losses. It is the most com mon maize virus disease during the hot season, when temperatures are
The in trin s ic transm ission a b ility expresses the s p e c i f i c i t y o f the infectious agent/vector relationship. With persistent viruses, the cycle of v iru s tr a n s m is s io n to the insect occurs in the following steps:
- acquisition access period, when the insect acquires the virus after feeding on an infected plant;
- latent period in the insect, or time required for the insect to become infective;
- inoculation access period, when the plant is contaminated through an
infective insect puncture.
Moreover, there is a virus retention period in the insect, during which the vector remains infective.
The cycle of MSV transmission to the vector is short, i.e. 24 h or less; those o f M S tp V and M M V are a b o u t 15 days.
Virus acquisition can be followed by a latency period and the inoculation. Depending on the virus and insect vector, various transmission barriers have been identified. In different insect organs, some cell membranes can act as barriers or as pathways linked to a specific virus recognition mechanism.
Figure 3. Diagram of the infection process in the insect vector (from REYNAUD, 1988).
Transmission studies revealed that MSV, M S tp V and M M V have characteristic circulative-type virus patterns, with a latency period in the insect, maintenance of the infective potential during moulting, presence of the virus in the insect and inocula tion of the plant w ith insect saliva (Figure 3). Very little is known about the latency period in the insect — it includes the transit time of the virus th ro u g h the insect's body and its uptake in the salivary glands to a sp e cific c o n c e n tra tio n threshold. Percentages of infective insects in a population are determined to assess v e c to r tra n sm issio n e ffic ie n c ie s . C a p tu re s and tests o f d if f e r e n t leafhopper populations showed that 50% (on average) of C. mbila insects in a p o p u la tio n are MSV vectors, w h ile 20% of P. maidis insects are MStpV and M M V vectors.
MSV transmission
by
C. mbila
In Réunion, C. mbila is an excellent MSV v e c to r w ith a high in tr in s ic transmission capacity (above 50%), which corresponds to levels noted in populations studied in South Africa and Burkina Faso.
No MSV multiplication in the insect was noted since, after a short acquisi tio n access p e rio d , there, was an o v e ra ll re d u ctio n in transm ission rates and viral concentrations. This insect vector is efficient because of its rapid uptake of the virus during a c q u is it io n and the lo w v ir a l c o n c e n t r a t io n s re q u ire d d u r in g inoculation. The virus takes a long time to disappear in the insect, thus explaining its excellent long-standing infective potential.
M SV c o u ld be c o n s id e re d as a circulative non-propagative virus.
MStpV transmission
by
P. maidis
P. m a i d i s p o p u la t io n s have a relatively low MStpV transmission capacity, i.e. about 20% in Réunion, which is quite close to levels noted in other regions.
MStpV transmission by P. maidis is relatively inefficient. Transmission barriers have been detected: the first involves hindering penetration of the v iru s th ro u g h the in te s tin a l w a ll d u r in g a c q u is it io n fe e d in g , and secondly through the salivary gland walls. Some females can transmit the virus to their offspring through their ovaries. The virus can m u ltip ly in most of these insects from the egg stage onwards.
For artificial infestations (in varietal s c re e n in g tests fo r resistance), it is now possible to select for the alimentary and ovarian acquisition ability. Populations of vectors with a high intrinsic transmission ability can thus be o b ta in e d w i t h i n a fe w generations.
M M V transmission
by
P. maidis
In R é u n io n , P. m a i d i s has a lo w M M V intrinsic transmission capacity, i.e. about 2 0% infectious insects. Tw o transmission barriers, sim ilar to those d e s c rib e d fo r M S tp V transmission, were found. ■
Maize virus diagnosis
and maize streak virus
variability
Symptomatic diagnosis of virus diseases is often imprecise
and should be confirmed with various serological tests
or electronic microscopy. This is especially true for early
stages of infection when diagnoses could be faulty if not
confirmed by reliable serological techniques. ELISA
(enzyme-linked immunosorbent assay) techniques w ere used
to obtain clear diagnoses of maize virus diseases present
in maize selection plots in Réunion. MSV, M StpV and M M V
symptoms can be characteristic of each of the three viruses
at relatively advanced stages of the disease,
but the appearances sometimes vary, especially
for M M V . Secondary infections of several viruses
can also occur.
d 'I v o ir e , M a li and B u rk in a Faso.
Virus purification
and diagnostic
techniques
The th re e m a iz e v iru s e s (MSV, M S tp V and M M V ) have been purified. The extracts were injected in to ra b b its to p r o d u c e s p e c ific immunoglobulins.
ELISA tests were developed for maize virus diagnosis. These quick simple tests are suitable for routine virus disease diagnosis.
These tests helped to pinpoint MSV in e ig h t A fr ic a n c o u n tr ie s . The presence of MStpV and M M V has rarely been proven in Africa, but they w e re r e c e n tly d e te c te d in Côte
Conventional ELISA tests can only be c a rrie d o u t in the la b o r a to r y . Nevertheless, CIRAD (Montpellier, France) has developed a modified ELISA test fo r d e te c tio n s o f all th re e viru se s on n i t r o c e llu lo s e membranes, w ith o u t requiring any la b o r a to r y e q u ip m e n t. This new technique w ill facilitate studies on geographical distributions of these viruses and h e lp in d e t e r m in in g specific requirements for breeding resistant varieties (Figure 4).
MSV variability
Virus isolates that induce streak leaf symptoms have been described in several grasses, in c lu d in g maize.
Features of
monoclonal
antibodies
A n tis e ru m p re p a re d a g a in s t a g iv e n v iru s w i ll n a t u r a lly c o n ta in a set o f antibodies that recognize several capsid p r o te in sites in the v ir a l p a r tic le . In contrast, a m o n o c lo n a l a n tib o d y o n ly recognizes one capsid protein site, thus enabling detection o f slight variations in such pro te in s. M o n o c lo n a l a n tib o d ie s a r e p r o d u c e d t h r o u g h fu s io n o f ly m p h o c y te cells a n d m y e lo m a cells. These hyb rid cells (hybridom a) have the perpetual traits o f myeloma cells along w ith the a b ility o f lym p hocyte cells to p r o d u c e s in g le a n t ib o d ie s . C IR A D , in c o lla b o r a t io n w ith the In s titu t de b i o l o g i e m o lé c u la ir e e t c e l lu l a ir e (S trasbourg, France), have developed f iv e m o n o c lo n a l a n t ib o d ie s th a t recognize MSV.
In itially, on the basis of biological tests, s e r o lo g ic a l te c h n iq u e s (with m on o clo n a l antibodies) and electronic microscopy, the isolates w e re a ll c o n s id e re d to be MSV strains. Later, nucleotide sequence analyses o f the v ir a l genomes revealed that some of these strains are actually distinct viruses.
In 1986 at CIRAD, serological tests on streak viruses of grasses led to the characterization of three different serotypes.
>%eS
Virus diagnosis kit
A kit was developed for serological field diagnosis o f viruses using nitrocellulose membranes. It can no w be purchased (from CIRAD ) b y a n y o n e w a n tin g to serologically confirm a visual diagnosis. This diagnosis kit is very easy to use: - th e te st k it c o n t a in s e v e r y th in g required for the analysis;
- it is very compact;
- the test can be stopped a fte r p la n t e x tr a c ts a r e d e p o s it e d o n the membrane, the membrane just has to be d r ie d . In th is fo r m , th e test c a n be completed w ithin the next 2 0 days; - these dried untested membranes can be sent b y m ail in a sim ple envelope and tested later.
For instance, someone not w a n tin g to spend the time required to perform the serological tests can simply deposit plant extracts on the m em branes a n d send them to a correspondent for analysis.
Sugarcane, Digitaria sp. and Setaria
sp. isolates could thus be distingui shed fro m v a rio u s m a iz e and Poaceae isolates belonging to the same serotype.
The gene sequences also confirmed these d is tin c tio n s w ith respect to other isolates — even indicating that sugarcane and Digitaria sp. isolates are distinct streak viruses that have been called sugarcane streak virus (SSV) and D i g i t a r i a streak v iru s (DSV), re s p e c tiv e ly . S im ila r ly , a
Panicum sp. isolate was reclassified, i.e. after it was initially identified as an MSV isolate, it is now considered as a d istin ct virus called P a n i c u m
streak virus. Further viruses could thus be distinguished in the future w i t h o th e r iso la te s. S e t a r i a sp. isolates have not yet been analysed in detail, only serological tests have been carried out. In summary, the s e r o lo g ic a l te c h n iq u e s used in Réunion did not enable serotype differentiation in MSV isolates, or in other maize isolates even though they o rig in a te d from 11 d iffe re n t
If, on the o th e r h a n d , the use r a ls o w a n ts to c o n d u c t th e a n a ly s is , everything required fo r the testing and in te rp re ta tio n is s u p p lie d in the kits, i.e . u n r e v e a le d m e m b r a n e s a n d positive/negative controls.
Samples from Z im b a b w e and Réunion were tested in various different ways to c h e c k th e v a l i d i t y o f k it f o r f ie ld analyses:
- tests p e r fo r m e d c o m p le te ly a t the sampling site;
- p la n t e x tr a c ts d e p o s it e d a t th e sampling sites and membranes analysed elsewhere;
- tests perform ed com pletely at CIRAD (M o n tp e llie r, France) w ith leaves sent from Réunion and Zim babwe.
T he re s u lts s h o w e d t h a t th is v ir u s diagnosis kit is highly reliable and valid.
countries, or in wild Poaceae isolates — some of which showed different virulence when tested on maize. The serological technique was found to be unsuitable in several locations, particularly Réunion, for distingui s h in g isolates w i t h d if f e r e n t pathogenicities.
Studies c a rrie d o u t in the Sudano-Sahelian zone confirmed the presence of one dominant serotype in m a iz e crops. In m ore than 300 maize isolates analysed, 99% showed a characteristic serological maize profile (which was previously determined by CIRAD). However, half of about 100 isolates tested were fo u n d to c o n ta in both the maize serotype and another serotype that had been identified in wild Poaceae species. This m o d ifie s p re v io u s conclusions, indicatin g that major serotype in maize can occur in the a s s o c ia tio n w i t h o th e r strains w h o s e s e r o lo g ic a l p r o file s and pathogenicities might be masked by the m a iz e se ro ty p e . As in o th e r lo c a tio n s , it is l i k e l y th a t in the 1 2 3 4 A 1 2 3 4 B
Figure 4. Immunoenzyme detection o f M M V (A) and M SptV (B) using ELISA from four crude extracts of maize from Réunion, infected by MSV (1 ), M M V (2), MStpV (3) and uninfected (4). Each plant extract was tested a t dilutions o f 1 /1 0 , 1 / 1 0 0 and 1 / 4 0 0 (leaf w e ig h t/b u ffe r volume).
Photo M . Peterschmitt
Sudano-Sahelian region, where the maize serotype and two others have been identified, molecular analyses w i l l reveal the presence of streak viruses o f grasses th a t d if f e r from MSV.
MSV resistance
mechanisms
in maize
V a rio u s m e c h a n is m s can be involved in a plant's virus resistance. Such mechanisms were compared in the re sista n t m a iz e v a r ie ty IRAT 29 7 and the s u s c e p tib le variety INRA 508.Maize resistance
and breeding
The main aim of the research carried out in Réunion
is to transfer the resistance of some local maize varieties
to three m aize viruses (MSV, M StpV and M M V ) into other
susceptible varieties of high agronom ic interest.
MSV-resistance transfers are also under w a y in Togo, and
very recently in C am eroon. This breeding w o rk with varieties
that are a lready being grow n by farmers, or promising ones,
interests many African CORAF-member countries, and also
countries in Latin Am erica and the W est Indies.
Location of MSV
in the maize plant
Virus colonization processes in the plant (MSV capsid antigens) have been d e fin e d using the ELISA t e c h n iq u e . A fte r a p la n t le a f is inoculated by C. m b i l a , the virus migrates through phloem canals to the stem. MSV colonizes the leaves th r o u g h y o u n g d i v i d i n g ce lls in w h ic h viral D N A re p lic a tio n can o c c u r. V iru s spread can o n ly be d e te c te d in tissues th a t fo rm after infection.
W ith this m ode o f in fe c tio n , the younger the plant is at inoculation, the h ig h e r is the p e rc e n ta g e o f infected tissues and the greater the yield losses. Since the virus is able to migrate from the inoculated leaf to the stem in less than 2 h, no prepro pagation of the virus seems necessa ry. Leafhopper virus transmission tests showed that the virus can only be acquired from leaves show ing symptoms.
Resistance mechanisms
Analysis o f virus d is trib u tio n s in maize plants in d ic a te d id e n tic a l d is t r i b u t i o n s in the s u s c e p tib le variety (INRA 508) and in a resistant v a r ie ty (IRAT 297). O n the other hand, a viral c oncentration difference between these varieties was detected by measuring virus n u c le o p r o t e in s using the ELISA technique, i.e. these nucleoproteins were 10- to 90-fold more concentra ted in INRA 508 than in IRAT 297. These results indicated that virus resistance in cv IRAT 297 involves m e c h a n is m s th a t h in d e r v ir a l propagation rather than b lo c k in g migration of the virus.The lower viral propagation in IRAT 297 is thus responsible for reduced severity of the initial infection; this is shown by less severe symptoms and a longer period before symptoms appear as compared to the suscep tible variety (in comparative tests, 5 days versus 3 days, respectively). Secondary infections are less severe in resistant plants since they are a poor inoculum source. ■
Resistance sources
found in Réunion
In Réunion, resistance was discovered in lo c a l m a iz e v a r ie t ie s , e s p e c ia lly in coastal varieties. This a d a p ta tio n was a c q u ire d throu gh na tural selection. In 1 9 7 4 , the Réunion variety "Revolution" was found to be M SV resistant in Benin. This was confirmed in several countries, i.e. in Kenya in 1 9 7 6 , USA in 1 9 8 3 and N ig e ria in 1982.
A bo ut 100 ecotypes were collected in a 1 9 7 9 -8 0 survey conducted in Réunion and Rodrigues. They were tested under natural infestation conditions o f M SV, MStpV and M M V , and 41 o f them were found to be resistant to m aize viruses. This re s is ta n c e w a s la te r c o n fir m e d under artificial infestation conditions. These e c o ty p e s w e re p o ly c ro s s e d to p r o d u c e th e " C o m p o s it e v ir o s e s ré s is ta n t" (C V R ), w h ic h w a s fu r th e r im p r o v e d t h r o u g h t h r e e r e c u r r e n t b re e d in g cycles, a n d then used as a resistance do nor in transfers.
O v e r a ll, m ore th a n 80 0 m aize varieties from tropical and temperate re g io n s have been screened in Réunion under natural virus infection c o n d i t i o n s . N o n e o f th e m are sufficiently resistant for cultivation in Réunion, except for ecotypes from Rodrigues w h ic h are g e n e tic a lly similar to those found in Réunion. One long-term objective is to improve yields and ag ro n o m ic qualities in lo c a l m a iz e v a rie tie s . The m ain problem is that these varieties have low yield potentials and they are all early producers. They have a poor im provem ent potential because of t h e ir v e ry n a rro w g e n e tic bases. Hence, resistance transfers would be q u ic k e r and m ore e f f ic ie n t , and enable creation of a w ide range of re s is ta n t v a r ie tie s fro m a lre a d y im p ro v e d p la n t m a te ria l o f high agronomic value.
Preliminary studies
The f o l l o w i n g f o u r c o n d itio n s had to be met to be able to transfer resistance to these three viruses‘ 1^
; ^ e
Rating
through backcrossing:
- pinpoint resistance in germplasm of the Indian Ocean islands, and select the most resistant populations to polycross them in a "Composite virose résistant" (CVR);
- carry out mass rearing of insect vectors to be used for inoculations. C. m b i l a , present in R é u n io n , is c o n s id e re d to be the m a in MSV vector and was thus chosen as vector fo r the v a r ie ta l im p r o v e m e n t programme. P. m a i d i s is the o n ly known vector of MStpV and M M V, w h ich is also present in Réunion; - develop a system to screen plants for virus resistance, through artificial infestation techniques using C. mbila
( c u r r e n t ly b e in g p e rfe c te d fo r
P. maidis). A resistance scoring scale is established;
- serological tests were developed to check, when necessary, whether the transmitted virus is the right one. Prelim inary studies demonstrated that resistance to MSV from Réunion e c o ty p e s was h ig h in m a n y countries, and that the Réunion MSV strain was especially virulent.
Virus symptoms are graded visually on a s e m i-q u a n tita tiv e 0 - 5 scale, w h ile taking the seriousness o f the symptoms into account.
Plant symptom scoring:
0 - the plant shows no symptoms; 1 - a few chlorotic spots are detected on the plant during a detailed inspection; 2 - slight streaks are clearly visible on the plant;
3 - moderate streaks are visible;
4 - the plant presents serious streaking with dwarfism ;
5 - the plant presents very serious virus infection with highly marked dwarfism . The s y m p to m s d o n o t c h a n g e a f t e r 3 5 days po stinfectio n. S ym ptom s are thus scored a n d plants fo r resistance transfers are chosen between this stage a n d flo w e rin g . O v e ra ll scores can be attributed to families o r varieties on the
basis o f these in d iv id u a l p la n t scores. For in s ta n c e , in tests c a r r ie d o u t in Réunion, it is considered that plants with scores o f 2 o r less, c o n tra ry to those s c o re d 3 - 5 , w i ll n o t p r o d u c e lo w e r yields because o f the presence o f the v ir u s . The o v e r a ll s c o re s ta k e th is threshold into acco un t, e.g. a v a rie ty w ith no p la n ts scored 3 o r h ig h e r is a ttrib u te d an o v e ra ll score o f 0 a n d c o n s id e re d as a re s is ta n t v a r ie ty . In contrast, a variety with all plants scored 5 is very susceptible and is allotted an overall score o f 5 0 0 . The overall score is calculated as follows: 100 X (3 X N 3 + 4 N 4 + 5 X N 5) / I N| (0 to 5) w h e re N¡ = n u m b e r o f pla n ts scored ¡ ( 1 , 2 , 3, 4, 5).
Artificial infestation
MSV resistance
research
C. m b i l a is easy to breed and artificial infestation can be carried out w ith o u t difficulty. Research on MSV is therefore more advanced than on M M V and M S tp V in Réunion.
This virus is also m a in ly found in Africa, thus explaining the develop m ent o f MSV resistance tra n sfe r p ro g ra m m e s in T o g o , and m ore recently in Cameroon.
CVR improvement
Since preliminary tests revealed high internal v a r ia b ilit y in resistance, recurrent selection work was under taken to im prove CVR, first under natural viral MSV pressure (cycle 1, CVR-CI), then u n d e r a r t if ic ia l infestation (cycles 2 and 3, CVR-C3). This markedly increased the level of MSV resistance in CVR.
Agriculture et développem ent ■ Special issue - December 1995 A r t if ic ia l in fe s ta tio n s a re essential fo r p la n t s c re e n in g since the p ro p o rtio n s o f the three m a iz e viruses can v a r y d u r in g the y e a r: n a tu ra l v ir a l pre ssure is not homogeneous o r constant enough for precise screening. These a rtific ia l infestations require highly productive mass rearing and active vectors to lim it the number o f insects t h a t h a v e to be p r o d u c e d . The p e r c e n ta g e o f in fe c tiv e in s e c ts is in c re a s e d by lengthening the virus acquisition period on diseased plants and by breeding insects to enhance their transmission ability. Insects used for transfers o f MSV resistance are obtained from a selected C. mbila population in which all insects are able to transmit the viru s . These a re p la c e d on
p la n ts p re s e n tin g v e r y severe symptoms and left for 3 days for virus acquisition. Carbon dioxide anesthesized insects are set in the w h o rl o f each p la n t. A p la n t's s u s c e p t ib ilit y d e c re a s e s w ith a g e , w h ic h m e a n s t h a t v e r y e a r ly in fe s ta tio n is necessa ry. In f e s t a t io n s a r e c a r r i e d o u t 10 d a y s a f t e r s o w in g f o r practical reasons (at this point the plants have grow n to a sufficient size fo r d e po siting the vectors). The infestations must be efficiently conducted to certify that all plants a r e in fe c te d . E x p e r ie n c e has
shown that com plete infestation ____ __________ ____________________________ can be o b ta in e d b y d e p o s itin g
three viruliferous leafhoppers on A rtificia l field infestation with C. mbila.
Breeding techniques
The three main m aize breeding techniques are based on certain features in this plant's floral biology, i.e. its cross-pollination and selfing potential.
Pedigree selection. Pedigree selection involves selfing a certain number o f plants, thus producing SI families. Families o f interest are chosen according to various criteria and then selected plants selfed w ith in these fam ilies. This produces S2 fa m ilie s , w ith subsequent selfing and choosing to produce S3 and S4 families, etc. The breeding process is generally carried out up to S6-S8, thus producing homozygous lines with low vigour, but w hich can be used to create hybrids through heterosis.
Recurrent selection. Recurrent selection involves choosing some plants o f a variety or composite. Seeds o f cobs from these plants can be sown in "e a r to rows" for instance w ith each cob pro du cing a seed line. Choices are then m ade according to one o r several criteria and chosen families are crossed to produce a new variety. The variety is progressively improved through several choosing and crossing cycles.
Backcrossing. Backcrossing is used to transfer a trait to an interesting variety that does not already have it. The FI progeny of the receiving variety and the trait d o no r are screened for the trait. Plants with the trait are backcrossed with the receiving variety. A fte r several backcrossing cycles, varieties a re produced that som ewhat resemble the initial variety but contain the sought-after trait.
In 1986, variety CVR-C1 was registe red under the name IRAT 297 with the Crop Science Society of America. Its MSV-resistance value was verified through comparisons of susceptible varieties with IRAT 297 top-crosses.
Genetic factors
determining MSV
resistance
Based on the im p r o v e d CVR-C3 form, 500 S1 families were produced by selfing, and tested through artifi cial MSV infestation. The very low percentage o f MSV sym ptom -free plants, the variability in the symptom scores in each fa m ily and a gauss d is t r ib u t io n cu rv e p lo tte d fo r all fa m ilie s in d ic a te d th a t fa c to rs d e t e r m in in g MSV resistance are complex and likely controlled by a polygenic system.
Nevertheless, a few S5 lines w ith very high levels of resistance (even complete resistance in one line) were o b ta in e d after fo u r m ore s e lfin g cycles. Genetic analysis of this mate rial revealed a major system of very heritable complete resistance, with p o s itiv e in c o m p le te d o m in a n c e , in v o lv in g at least three d iffe r e n t genes. M oreover, the presence of p a rtia l resistance was in d ic a te d during the breeding of some lines —
MSV resistance
research conducted
by IITA in Africa
In 19 72 , MSV resistance was detected b y IITA in the c u ltiv a r T r o p ic a l Zea Yellow (TZY). This variety was then bred a n d re s is ta n c e t r a n s f e r r e d to o th e r higher-yielding varieties. The im proved line IB 32 (obtained in 1979) was found to have high p a rtia l resistance. It was used as a resistance donor. A selection p r o g r a m m e w a s c a r r i e d o u t b u t c o m p le te ly sym p to m -fre e plan ts w e re r e je c te d ; m a n y v a r ie tie s h a v e been p ro d u c e d a n d d is trib u te d th ro u g h o u t A frica.
but this cannot yet be attributed to a separate (probably polygenic) system or to partial expression of a major system.
Resistance transfers
MSV resistance transfers are perfor med through a series of conventional backcrossings.
In R é u n io n , fo r each s c re e n in g , 5 0 0 0 F1 p la n ts are sow n and infected 10 days later by depositing three preinfected leafhoppers in the whorl of each plant. Resistant plants are chosen before flowering through the elimination of susceptible plants. The chosen plants (only 250-300) are in te rc ro s s e d . The F1 and F2 generations of each breeding cycle are thus screened.
The same procedures are used in the research programme carried out in Togo, but the a rtificia l infestation technique differs. Maize plants are sown in pots. Once the maize plant emerges, the pots are placed in cages containing infected leafhoppers. The plants are then planted out in the field. The chosen plants are selfed and then crossed again. The resistan ce s c re e n in g is v e ry e f f ic ie n t . Resistance is at least p a r t i a l l y
maintained after the first backcros sing (Figures 5 & 6). It is useful to begin screening at the F1 generation in order to discard as many recessive genes as possible from the outset. The resistance transfer procedure is limited to a single initial cross with the donor and then two backcrossing cycles. The fin al variety therefore contains the resistance trait and 90% of the genes of the receiving variety. Selfing is necessary to recover the c o l o u r and g r a in - t y p e o f the receiving variety — when they differ from those of CVR. This was done for each screening in the Togo trials. In Réunion, selfing was carried out after the second backcrossing cycle.
Current results
P re se n tly, f iv e v a r ie tie s fro m th e b a c k c ro s s in g p ro g ra m m e in R é u n io n , i.e. C N 7 , T ié m a n tié , Pool 16 SR, IRAT 171 and Suwan 8331, are considered to be modified after tw o backcrossing cycles and three selfing cycles. In Réunion, they have been found to possess excellent resistance. Most plants showed no sym ptom s under severe a r tific ia l infestation conditions with a highly virulent virus isolate.
500 400 -O 300 200 -100 0
m
Tiémantié • IRAT 171 Suwan 8331o
C N7 □ Pool 16 SR ir
O riginal FI variety ~ r F2 ~ r F3 T T T 1---1 ----BC1F1 BC1F2 BC1F3 BC2F1 BC2F2 BC2S1 Transfer stepsFigure 5. Efficiency o f M SV resistance transfers: variations in the overall susceptibility scores during different transfer steps.
Origin of converted varieties.
Origin Mali Burkina
Faso
Benin CIMMYT CIMMYT Côte
d'Ivoire
IITA-Nigeria
Variety Tiémantié IRAT 171 CN7 Suwan 8331 Tuxpeño CPJ 16 SR Pool
o 4 500 —i
Tiémantié IRAT 171 CN 7 Suwan 8331 Tuxpeño 16 SR Pool
Varieties
O riginal form H Resistant form (F3 of the starting cross)
Figure 6. Transfer efficiency. G ram yields o f converted varieties com pared to those of the orig in a l varieties (k g /h a ).
C o n c e r n in g the m a iz e b re e d in g carried out in Togo, five varieties (V io le t de K a tio la , ZL 2BD , Pool 16 DR, early Blanc 2 and AB 21) are at the second backcrossing stage and are therefore converted. Different b re e d in g p r o c e d u re s are b e in g assessed in terms of MSV resistance and preservation of grain texture and husk coverage of maize cobs, traits that w ere sought in the selection process.
MStpV and M M V
resistance
Comparison of the performance of the IRAT 297 composite with that of the s usceptible IN RA 508 h yb rid under natural conditions highlighted a high le vel o f M S tpV and M M V resistance in the local composite, as shown by c o m p le te resistance in more than 80% of the plants. The g e n e tic fa c to rs th a t d e te r m in e resistance to these viruses seem to differ, despite the fact that they occur in the same geographical zones and are transmitted by the same insect vector. IRAT 297 is currently the only known source of MStpV resistance. M M V resistance is n o w d e fin e d and p la n t resistance to v e c to r transmission of the virus has also been detected in IRAT 297 hybrid lines. Studies on this latter type of resistance are now under way. These two levels of resistance, i.e. to the virus and to- transmission, should be jointly taken
into account for developing efficient genetic control techniques.
Prospects
In the short term, it is essential to make between-country comparisons, under artificial MSV infestation, of resistance levels in varieties that were selected for MSV-resistance in Réunion (final forms obtained by crossing S3 lines, with two backcros sing cycles) and Togo. Such tests are b e in g c a r r ie d o u t in C a m e ro o n , Togo, Zimbabwe with CIMMYT, and Réunion. The same type of test w ill then be carried in more countries under natural infestation conditions.
The results s h o u ld h i g h l i g h t the extent o f s im ila rity — in terms of recovery of agronomic traits, grain types, resistance other than to MSV — between resistant and susceptible forms of different varieties.
In the longer term, transfers of MStpV and M M V resistance are planned, e s p e c ia lly in o p e n p o l l i n a t i o n varieties and hybrid varieties; the re s u ltin g p la n ts c o u ld th e n be distributed w id e ly th ro u g h o u t the tropics. Research is currently under way on P. maidis transmission, resis tance transfers into elite maize lines, and on the genetics of resistance to these tw o viruses. The preliminary results are promising. ■
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Abstract... Resumen...
Résumé...
J.-L. MARCHAND, M. PETERSCHMITT, B. REYNAUD, J. DINTINGER— Maize streak, maize stripe and maize mosaic virus diseases in the tropics (Africa and islands in the Indian Ocean).
Im pact o f m a ize virus disease a n d current research; vectors and their epidemiology; maize virus diagnosis and variability; m aize resistance and breeding. The three virus
diseases have been known since the beginning of the
century and are found in the tropics. They were studied in Africa and Reunion. Research in virology, epidemiology and genetics is carried out by CIRAD in collaboration with several African institutions (CORAF, INCV, INERA and IRA), international institutions (CIMMYT and IITA) and the John Innes Institute in Great Britain. Maize streak virus (MSV), maize stripe virus (MStpV) and maize mosaic virus (MMV) are always spread by insects. MSV is spread by members of the genus Cicadulina (numerous species have been identified). MStpV and MMV are spread by Peregrinus
maidis. These insects have been identified in Africa.
Streak epidemics cause serious crop damage (in Reunion and in Africa in certain years) but the impact of strike and mosaic is still not well known. The effectiveness of virus transmission to the insect vector was high for MSV and
C. mbila (50% of insects were infectious) and lower for
MStpV and MMV and P. m aidis (2 0 % of insects were infectious). The viruses were identified by enzyme-linked immunosorbent assay (ELISA). The serological method was the most reliable. A diagnosis kit was developed to test the presence o f the t h r e e viruses w ith o u t special equipment. With the new techniques (monoclonal isolates, range of hosts and g e no m e sequencing) used, MSV serotypes were identified. Detection of the virus in plants using ELISA made it possible to understand the mechanism of resistance to MSV. Varietal resistance appeared to be caused by resistance to propagation of the virus. The varietal selection work carried out in Reunion and in Togo was aim ed at tra nsferring resistance (found in local varieties) to varieties of clear agronom ic interest. A "Composite viroses resistant" was form ed from local varieties and then improved. The work on MSV is more advanced than that on MStpV and MMV. Genetic analysis of resistance to MSV revealed partial polygenic resistance and total resistance by virtue of two or three genes. Research has been developped on the three viruses resistance in Reunion, five varieties have been completed. Transfers have been carried out for the MSV resistance in Togo, fiv e varieties have been form e d . Trials are in progress in several countries (Cameroon, Togo, Zimbabwe and Reunion) to compare the levels of resistance achieved and the agronomic conformity of the resistant varieties. Key words: maize, streak, stripe, mosaic, virus, insect,
Cicadulina mbila, Peregrinus maidis, varieties, resistance,
ELISA, tropics, Africa, Reunion.
J.-L. MARCHAND, M. PETERSCHMITT, B. REYNAUD, J. DINTINGER — Las virosis del estriada, del stripe y del mosaico en el maíz en región tropical (Africa y las islas del Océano Indico).
El impacto de las virosis y las investigaciones actuales; los vectores y su epidem iología; el diagnóstico de las virosis y la variabilidad del virus MSI/; la resistencia del m aíz y la selección varietal. Estos tres virosis, conocidas
desde comienzos del siglo y difundidas en todas las regiones tropicales, han sido estudiadas más precisamente en Africa y la isla de la Reunión. El CIRAD ha llevado a cabo investigaciones de v iro lo g ia , e p id e m io lo g ía y genética, en colaboración con varios organismos africanos (CORAF, INCV, INERA, IRA), internacionales (CIMMYT, UTA) y el John Innes Institute de Gran Bretaña. El estriado
es transmitido por el m aize streak virus (MSV), el stripe es transmitido por el m aize stripe virus (MStpV) y el mosaico por el m a ize mosaic virus (M M V ). Los tres virus son transmitidos obligatoriamente por insectos, del género
Cicadulina para el MSV (se han identificado numerosas
especies) y por Peregrinus m aidis para el MStpV y el MMV. Dichos insectos se han identificado en el continente africano. Las epidemias de estriado provocan grandes daños (e n la Reunión y, algunos años, en A fric a ), mientras que el impacto de las virosis provocado por el MstpV y el MMV todavía se conoce mal. La eficacia de las tronsmiciones virales al insecto vector es elevada para el MSV y C. mbila (50% de los insectos infecciosos) y más baja para el MStpV y el MMV y P. maidis (20% de los insectos infecciosos). Los síntomas de los 1res virus están descritos, y la epidemiologia está estudiada en Burkina y en la Reunión. Los virus son identificados m ediante prue ba s in m u n o e n z im á tic a s ELISA (E n z y m e -lin k e d im m unosorbent assay). El m étodo serológico es más seguro que sólo la descripción de los sintomas. Se ha elaborado un kit de diagnóstico para probar la presencia de estos tres virus sin necesitar un equipo de laboratorio. Otras técnicas utilizadas (gírmenes aislados monodonales, numerosos huéspedes, sequenciación del genoma), se han distinguido serotipos de MSV. La investigación de virus en la p la n ta m e d ia n te la técnica ELISA ha p e rm itid o com prender el mecanismo de resistencia al MSV. La resistencia varietal al virus se manifiesta aparentemente por una re sisten cia a la m u ltip lic a c ió n v ir a l. Las investigaciones en selección v a rie ta l, tuvieron como ob jetivo r e a liza r tra nsferencias de las resistencias, presentes en variedades locales, a variedades de buen interés agronómico, en la Reunión y en Togo. A partir de variedades locales resistentes, se constituyó, y mejoró, un "Composite viroses résistant". Los trabajos relativos al MSV están más avanzados que los relativos al MStpV y al M M V . El análisis genético de la resistencia al MSV demuestra que existiria un sistema mayor de resistencia total oligogenica (3 factores geneticos) y tal vez una resistencia parcial poligenica, la qual implica genes minores. Se están realizando transferencias en variedades africanas y efectuando investigaciones en la Reunión y en Togo. Se han efectuado las investigaciones en los tres virus en la Reunión, cinco variedades se han terminado. Se han realizado transferencias para la resistencia al MSV en Togo, y se hon constituido cinco variedades. También se están haciendo pruebas en diferentes países (Camerún, Togo, Zimbabwe, la Reunión).
Palabras clave: maíz, estriado, stripe, mosaico, virus, insecto, Cicadulina mbila, Peregrinus maidis, variedad, resistencia, técnica ELISA, región tropical, Africa, Reunión. J.-L. MARCHAND, M. PETERSCHMITT, B. REYNAUD, J. DINTINGER — Les viroses de la striure, du stripe et de la mosaïque sur le maïs en région tropicale (Afrique et îles de l'océan Indien).
L 'im p a c t des viroses e t les recherches actuelles ; les vecteurs et leur épidémiologie ; le diagnostic des viroses et la variabilité du virus de la striure; la résistance du maïs et la sélection variétale. Ces trois viroses, connues depuis le
début du siècle et répandues dans toutes les régions
tropicales ont été plus précisément étudiées en Afrique et à la Réunion. Des recherches en virologie, en épidémiologie et en g é n é tiq u e sont co nd uite s pa r le CIRAD en collaboration avec plusieurs organismes africains (CORAF, INCV, INERA, IRA), internationaux (CIMMYT, IITA) et le John Innes Institute de Grande-Bretagne La striure est causée par le m a ize s treak virus (M S V ), le stripe est provoqué par le m aize stripe virus (MStpV) et la mosaïque pa r le m a iz e m o saic viru s ( M M V ) . Ces v iru s sont o b lig a to ire m e n t transm is par des insectes, du ge nre
Cicadulina pour le MSV (de nombreuses espèces sont
identifiées) et par Peregrinus maidis pour le MStpV et le M M V. Ces insectes ont été identifiés sur le continent africain. Les épidémies de striure provoquent des dégâts importants (à la Réunion et certaines années en Afrique) ; l'impact des viroses causées par le MStpV et le MMV est encore mal connu. L'efficacité des transmissions virales par l'insecte vecteur est élevée pour le MSV et C. mbila (5 0 % des insectes infectieux), et plus faible pour le MStpV et le M M V par P. m aidis ( 2 0 % d'insectes in fe c tie u x ). Les symptômes des trois virus sont décrits, et l'épidémiologie est étudiée au Burkina et à la Réunion. Les virus sont identifiés à l'aide des tests immunoenzymatiques ELISA
lE n z y m e -lin k e d Im m un oso rb ent assay). La méthode
sé ro lo g iq u e est la seule m é th o d e fia b le . Un k it de diagnostic a été mis au point pour tester la présence de ces trois virus, sans nécessiter un équipement de laboratoire. D 'a u tre s techniques (is ola is m o n o d o n a u x , g a m m e d'hôtes, séquençage du génome) ont permis de distinguer des souches différentes de MSV. La recherche du virus dans la plante par la technique ELISA a permis de comprendre le mécanisme de résistance de la plante au MSV, qui se traduirait par une résistance à la multiplication virale. Les sélections variétales, ont eu pour objectif de transférer des résistances dans des v a rié té s d 'u n bon in té r ê t agronomique, à la Réunion et au Togo. A partir de variétés locales, un « Composite viroses résistant » a été constitué, puis amélioré. Les travaux sur le MSV sont plus avancés que ceux sur le MStpV et le MMV. L'analyse génétique de la résistance au MSV montre qu’il existerait un système m ajeur de résistance totale oligogénique ( 3 facteurs génétiques) et éventuellem ent une résistance partielle polygénique impliquant des gènes mineurs. Des transferts sont réalisés sur des variétés africaines ou introduites à la Réunion et au Togo. A la Réunion, les travaux de sélection sont conduits sur les tro is viru s, cinq v a rié té s sont converties. Au Togo, des transferts sont réalisés pour la résistance au MSV et cinq variétés sont converties. Un essai multilocal est en place dans différents pays (Cameroun, Togo, Zimbabwe, la Réunion).
Mots-clés : maïs, striure, stripe, mosaïque, virus, insecte,
Cicadulina mbila, Peregrinus maidis, variété, résistance,
technique ELISA, région tropicale, Afrique, la Réunion.
