Leaf scald of sugarcane
Iri
Leaf scald, a bacterial
disease of sugarcane,
is currently spreading
rapidly worldwide.
It can only be controlled
preventively with
techniques to propagate
and distribute certified
healthy plants, along
with strict varietal
selection.
P. ROTT
CIRAD-CA, BP 5 0 3 5 , 3 4 0 3 2 Montpellier Cedex 1, France
L
eaf ( in t e r s p e c if ic scald o f h y b r id s sugarcane of S ac c ha ru m sp.) is a bacterial v a s c u la r disease caused by X a n t h o m o n a s a l b i l i n e a n s (Ashby) D o w s o n . It can have a serious economic impact when susceptible varieties are affected in sugarcane c ro p zones (R IC A U D & RYAN, 1 989). This d ir e c t e ffe c t is most o b v io u s when the first outbreaks occur in a country, or when a new strain of the pathogen appears in an already contaminated area.Symptoms
The symptomatology includes two m a in fo rm s, c h r o n ic and a cute, and two distinctive phases, latency and e c lip s e (ROTT e t a I. , 1 9 8 8 ; RICAUD & RYAN, 1989).
Chronic form
The chronic form is characterized by lines on the leaf blades, ru n n in g parallel to the ribs. These lines, or streaks, resemble s tra ig h t p e n c il lines, but can be as w ide as 1 cm. They gradually spread along the leaf blade and vary in colour from white to yellow on the blade. They become p u rp lis h w h e n re a c h in g the leaf sheath (AUTREY et al., 1992b), and can turn reddish with age. This is the o n ly v is ib le external sym ptom in resistant varieties.
W ith progress o f the disease, the streaks broaden and become more diffuse on leaves that are reaching m aturity. The leaf tissues w h ite n . Chlorosis of all o f the leaves can
occur and they w ill turn a w hitish c o lo u r. In a d d itio n to the loss of c o lo u r , the le a f tip s can w i t h e r and curl inw ard, g iv in g a spindly appearance to the shoots.
O n m a tu re c h r o n i c a l l y in fe c te d stalks there is almost simultaneous development of side shoots with no apical dominance. Shoots at the base of the stalk may thus be more deve loped. This differs from situations w h e n ap ica l d o m in a n c e is in te r rupted on a healthy stalk by clipping off the apex, when only the youngest sh o o t lo c a te d b e lo w the a p ic a l m eristem w i l l d e v e lo p . The side shoots that form e x h ib it the same symptoms as the main stalks.
In longitudinal sections, infected stalks are characterized by reddened vessels in the nodal and internodal areas.
Acute form
The acute form is characterized by sudden w i l t i n g o f m a tu re stalks. When there is no chronic symptom expression, apparently disease-free stalks can begin withering, looking like they lack water.
The onset o f th is fo rm is o ften subsequent to a rainy period fo llo w e d by p r o lo n g e d d ry w e a th e r, but seems to be lim ite d to h ig h ly susceptible varieties.
Latency and eclipse
phases
During the long latency period of the disease, many infected plants exhibit no symptoms or sometimes just a few white lines on their leaves. It is not
sugarcane disease
Variability in the causal organism
Recent studies have focused on variations in X. albilineans (ROTT & DAVIS, 1994). This is a critical factor for the development of diagnostic and detection techniques, and determining the most virulent strains, when screening for leaf scald disease resistant varieties of sugarcane.
Development or introduction of new pathogen strains has often been blamed for loss of varietal resistance to leaf scald. However, this has never been categorically proven. There are pathogenicity variations in X. albilineans, and AUTREY et al. (1992a) recently published evidence on the presence of different races of this bacterium in Mauritius.
Types of variation
Several types of variation concerning different traits have been identified, e.g. in the morphology of bacterial cells and colonies, total cellular proteins, fatty acid methyl esters, reactions to bacteriophages, reactions to antisera (serological variations), and in the genome (DNA profiles).
Bacterial strains from 33 different geographical areas have been classified into three serovars (serological groups) and at least four genomic groups (ROTT et al., 1994b; DAVIS et al., 1994; DAVIS & ROTT, unpublished results). There is just one type of variation in some zones. However, two serovars and four different genomic groups were found in Mauritius. No correlations have yet been established between the different types of variation.
These results confirm that it is essential to halt transmission of the causal agent between geographical areas.
Serological variations should be taken into account when serological techniques (e.g. ELISA) are used to diagnose leaf scald disease. Efficient diagnosis requires sera that will react to different serological types of the bacterium.
The most certain way of investigating pathogenicity in X. albilineans would be to focus on its genomic variations, as indicated by the following observation in a study conducted in Florida: the increased incidence of leaf scald is directly associated with a genetic variant of the bacterium (DAVIS, 1992; DAVIS et al., 1993). Studies are under way at the Centre de coopération internationale en recherche agronomique pour le développement (CIRAD, France) and the University of Florida on the relations between genomic variability and pathogenicity in the bacterial organism. A positive correlation between these two X. albilineans traits would facilitate identification of highly virulent pathogenic strains, and enhance the efficiency of screening for improved varieties of sugarcane.
f u l l y u n d e rs to o d w h a t trig g e rs e x p re s s io n o f s y m p to m s after a latency period. The usual explana tion is that different types of stress, (e.g. nutritional or clim a tic stress), prompt progress of the disease. This la te n cy phase is one of the c h ie f obstacles to o v e r com e fo r c o n t r o llin g leaf scald. Sugarcane plants can to le ra te the presence o f the causal agent for several weeks or even months w it h o u t any sign of sym ptom s — som etim es there is expression, but it is to o s lig h t to be v is ib le
(ROTT e t a l . , 1 9 8 8 ; R IC A U D & R YAN, 1 9 8 9 ). The disease has thus spread w o rld w id e via cuttings from healthy-looking plants.
The eclipse phase can occur at the same time as latency. It is characteri zed by w h ite lines appearing and disappearing on the leaves during plant growth. These symptoms are no longer visible after senescence and death of the oldest leaves, and then there are no signs on new ly- f o rm e d leaves. H e n c e , a p la n t can appear diseased or h e a lth y , depending on when it is inspected.
Geographical
distribution
Leaf scald o f sugarcane was first reported in 1911 in A u s tra lia by N o r th (M A R T IN & R O B IN S O N , 1 961 ). However, it seems that the disease appeared in Fiji in 1908, or even much earlier.
This leaf disease induced substantial c r o p losses at the o u ts e t o f the c e n tu r y in n o b le cane v a rie tie s (S. o f f i c i n a r u m ) . It was p a r t ia lly controlled through the introduction o f re s is ta n t h y b r id s c o n t a in in g S. s p o n t a n e u m genes (R IC A U D & RYAN, 1989). H o w e v e r, there w e re f r e q u e n t o u tb re a k s , fo r u n k n o w n reasons, in several countries. For instance, the 1964 epidemic in Mauritius seemed to be related to a loss of resistance in two w id e ly -g ro w n sugarcane varieties, and the likely appearance of a novel X. a l b i l i n e a n s strain (R IC A U D & PEROMBELON, 1964).
In the last 10 years, leaf scald outbreaks have again been reported in G ua d e lo u p e , Florida, D o m in ic a n Republic, Mauritius and Taiwan. In Florida, it was associated with a new pathogenic strain (DAVIS, 1992). In Mauritius, it seems that the serious 1989 epidemic was caused by aerial tra n sm issio n o f the b a c te riu m (AUTREY et a i , 1992b). There is a c u rre n t increase in the num b e r of regions affected by this disease. The latest reported new locations are M e x ic o (IRVINE e t a i , 1 993), Louisiana (G R ISH AM e t a i , 1994), Texas (ISAKEIT, personal c o m m u n ic a t io n , 1994) and Guatemala (OVALLE et a i , 1995). Leaf scald has now been detected in at least 61 d iffe r e n t g eograph ica l locations (Figure 1).
Economic impact
Leaf scald can totally destroy planta tions of susceptible varieties within a few months or years. Spectacular drops in cane yields have been noted with infections of the acute form of the disease.Direct losses: cane yields and juice quality
This disease affects cane yields at harvest and the quality of extracted ju ic e (HOY & GRISHAM, 1994). In G u a d e lo u p e , COCHEREAU & JEAN-BART (1989) estimated crop losses of 13 t/ha of cane as determi ned by yield differences between miliable healthy cane and diseased cane. On the same is land, fie ld sugarcane yield losses of 15-20% were noted fo r the su sce p tible variety B69379 (ROTT et a i , 1995). In Mexico, the disease was detected for the first time in 1992, when it was responsible for destroying 800 ha of cv Mex64-1487. At the present time, cv M ex6 9 -2 90 , w h ic h is grown on 150 000 ha (25% of the sugarcane area), is re la tiv e ly unaffected, despite the fact that p re lim in a ry
studies highlighted its susceptibility (scalded cane w ith side-shoot develo pm ent). There could be a considerable fin a n c ia l im pact if cv Mex69-290 is actually susceptible and the disease progresses in Mexico. A 1% yield loss would represent an economic loss of about 1 5 m illion new Mexican pesos, or about 24 mil lion FF (October 1994 estimate, based on a mean field yield of 100 t/ha of cane and 100 pesos/t paid to farmers).
Indirect losses: replanting and breeding costs
Leaf scald also has an in d ire c t impact, e.g. cost of replanting des troyed plantations, propagation of healthy plants (micropropagation and heat treatment) and selection of resistant varieties. Moreover, some h ig h -y ie ld in g va rie tie s th a t are
9 0° I 0°l 90° N o r t h a n d C e n tra l A m e r ic a a n d th e W e s t In d ie s "'•Barbados Belize Cuba Dominica Florida G renada G uadelupe Guatemala Jamaica Louisiana M artinique ____ Mexico 4 5 ° Panama Puerto Rico Dominican Republic Saint-Kitts Saint Lucia Saint Vincent Texas Trinidad S o u th A m e r ic a A rgentina Brazil Guyana Surinam * «> Uruguay Venezuela
Figure 1. Current locations affected by sugarcane leaf scald.
Aspect of a diseased stalk: whitening of leaves that curl inward, giving a spindly appearance.
Photo P. Rott
General aspect of a sugarcane stool affected by leaf scald.
Photo P. Rott
Leaf scald symptoms in sugarcane
Brownish-red discoloration of vascular tissue colonized by Xanthomonas albilineans.
Photo P. Rott
Bevelled stalk section.
Longitudinal section of the stalk.
Diseased plant with side shoots also exhibiting foliar symptoms of leaf scald. Photo P. Rott
White pencil line on the leaf blade. Photo P. Rott
White pencil line
Chlorosis Necrosis
Foliar symptoms of sugarcane leaf scald. Photo P. Rott
sugarcane disease
susceptible to the disease should not be g ro w n , e.g. Q 6 3 in A u s tra lia , B 6 9 3 7 9 in G u a d e lo u p e and M 695 69 in Mauritius (EGAN, 1971 ; ROTT & F E L D M A N N , 1 9 9 1 ; AUTREY e t a l . , 1 9 9 2b). Recent outbreaks of leaf scald in Louisiana killed prom ising clones that were being bred, thus modifying varietal im p r o v e m e n t strategies (H O Y & GRISHAM, 1994).
Control
Pathogen in tr o d u c tio n s can be avoided in regions where leaf scald is not yet present by o n ly im p o rtin g disease-free plant material. This is also true for already infected areas since there is always the risk of transmission of a new and more virulent bacterial strain. Strict measures should be taken to control distribution and exchange of plant material, especially cuttings.
For the purposes of efficient preventive control, sugarcane plants should be shipped by a q u a ra n tin e station (BAUDIN, 1984; FRISON & PUTTER, 1993).
Heat treatment of cuttings
No chemical control is used to treat leaf scald at the present time. Heat treatment of cuttings is an effective p re v e n tiv e c o n tro l te c h n iq u e fo r d e s tr o y in g b a c te ria in in fe c te d tissues. It in v o lv e s s o a k in g the cuttings in water at room temperatu re fo r 48 h and then in hot w ater (50°C) for 3 h.Varietal screening
The most suitable and easy means of controlling leaf scald is by using healthy plant material and planting disease-resistant sugarcane cultivars.
It is also c r it ic a l to p in p o in t and eliminate susceptible varieties during the sugarcane selection process. Varietal screening tests have been set up in several countries where the disease is p re v a le n t. Resistance levels are g e n e r a lly assessed by checking for symptoms after artificial inoculation of stalks. Other methods which involve determining bacterial population densities in host plants are c u r r e n t ly b e in g in ve s tig a te d (ROTT et a i , 1994a, 1994c).
However, propagation of the disease c a n n o t be c o m p le te ly c o n tro lle d th ro u g h the se le c tio n and use of resistant plants. Further prophylactic measures are also required: destruc tion of diseased material, disinfection of cutting tools by brushing them with or soa kin g them in b a c te ric id e s , phytosanitary control prior to introdu cing plant material (quarantine).
Sugarcane quarantine
S uga rca n e p r o d u c t i v i t y can bem a in ta in e d th ro u g h steady efforts to e n h a n c e th e v a rie ta l status (i.e . all varieties cultivated in a zone at a given time) by introducing and assessing new v a r ie tie s . S u g a rca n e c ro p s are also especially vulnerable to diseases,
beca u se o f v a rio u s fa c to rs :
p ro p a g a tio n fro m cu ttin g s fa c ilita te s spread o f pathogens, m o n o c ro p p in g o v e r larg e areas is fa v o u r a b le to the d e v e lo p m e n t o f e p id e m ic s , the p lu r ia n n u a l a s p e ct o f th is c ro p (4-10 years on average) lengthens and complicates breeding.
Exchanges and transport of sugarcane cuttings is a serious risk that should be fu lly c o n tro lle d . CIR AD thus set up a sugarcane quarantine unit in Montpellier (France) in th e 1970s. T h is u n it supplies disease-free cuttings, mainly to sugar-producing areas of western and c e n tr a l A fr ic a , s u g a rc a n e b re e d in g stations in the West Indies (Guadeloupe) and Réunion, and there are prospects for s u p p ly in g o th e r re g io n s. M o re o v e r,
C IR A D , in a d d it io n to h a v in g
greenhouses set up to monitor sugarcane p la n ts and p ro p a g a te c u tt in g s , has d e v e lo p e d in d e x in g te c h n iq u e s (s e ro lo g ic a l tests such as ELISA and immunofluorescence) for reliable and p re c is e d ia g n o s is o f th e f o l l o w i n g diseases: mosaic, red leaf mottle, streak, leaf scald and ratoon stunting.
Clones distributed by this quarantine u n it com e from three sources: clones fro m e a rly stages o f s e le c tio n fro m CIRAD research stations in Guadeloupe and the West Indies Central Sugarcane Breeding Station (WICSCBS) in Barbados and commercial varieties from various origins worldwide.
V a rie tie s to be a nalysed are sent in directly by the source countries (about 10 buds/variety). The sugarcane cuttings are th e n le ft to g e rm in a te in a climate-controlled room at 30°C. Once they have grown a few centimetres, the young plantlets are replanted in pots on a r t if ic ia l m e d iu m , d rip w ate re d and placed in quarantine greenhouse n°1. The canes are sprayed w ith fungicides
and insecticides at planting and several times during their growth cycle. They are visually inspected regularly and tests are carried out to detect the presence of any causal agents. The sugarcane stalks are heat treated (cuttings immersed in water at 25°C for 2 days, and then at 50°C for 3 h) at the end of the growth cycle, after 9-10 months in the greenhouse.
T hereafter, canes are c u ltu re d again from cuttings in quarantine greenhouse n°2 to produce a new set of cuttings. The plant material is once more inspected, treated and tested. After 9-12 months growth, the cuttings undergo a short heat treatment (soaking in hot water for 1 h at 50°C) and are dipped into a fungicide. These cuttings can then be dispatched fro m th e q u a r a n t in e u n it w it h a phytosanitary certificate issued by the French crop protection service.
For fu rth e r in fo r m a tio n , c o n ta c t the Head of the Sugarcane Quarantine Unit, CIRAD-CA, BP 5035, 34032 Montpellier Cedex 1, France. Tel. (33) 67 61 65 55; fax (33) 67 61 56 03.
sugarcane disease
* CTICS: Centre technique interprofessionnel de la canne à sucre.
Figure 2. Propagation steps of sugarcane in Guadeloupe.
Disease-free
commercial fields
Nurseries are specifically set up to supply farmers with cuttings for their c o m m e rc ia l fields (sugar p ro d u c tion). They aim at p ro v id in g their customers w ith p e rfe c tly healthy p la n t m a te r ia l, and th e re fo re represent a crucial step in controlling plant diseases, particularly bacterial diseases, w h i c h are c o m m o n ly transmitted through cuttings.
Cuttings planted in these nurseries usually come from mother nurseries. The latter should thus be under strict control, certifying that all planting material is healthy or sanitized, i.e. controlled plants obtained through micropropagation or from cuttings that have undergone the previously described long hot water treatment.
An example: Guadeloupe
The goal o f the n e w sugarcane propagation system in Guadeloupe is to sanitize plantations contamina ted with various diseases, especially leaf scald (Figure 2).The system in c lu d e s a m o th e r nursery, w h ic h provides cuttings fo r a second propagation nursery (pre-nursery), which in turn supplies the c o m m e r c ia l n u rs e ry w it h cuttings.
This latter nursery produces c u t tin g s fo r the s u garcane farm ers. The first tw o nurseries should be remotely located with respect to the commercial nurseries. Disease-free micropropagated plants, produced in the C IR AD research station at Roujol (Guadeloupe), are planted in the m o th e r nursery (FE LD M A N N & ROTT, 1991). This propagation system provides sugarcane farmers w ith cuttings o f e xce lle n t genetic (no varietal blends, new improved v a rie tie s ), p h y s io lo g ic a l (yo u n g c u ttin g s ) and p h y to s a n ita ry (disease-free) quality.
Heat treatment of cuttings.
Photo T. Erwin
sugarcane disease
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ROTT P., KLETT P., DE SAINT-ALBIN A., S O U P A A ., F E L D M A N N P., 1 9 9 4 c. P o p u la tio n s o f X a n t h o m o n a s a l b i l i n e a n s
in stalks o f sugar cane clo n e s d if f e r in g in resistance to leaf scald disease. Abstracts of papers presented at th e 4th I n te rn a tio n a l S o c ie t y o f Sugar C a n e T e c h n o lo g is t s w o r k s h o p , A p r i l 4 - 9 , 1 9 9 4 , B ris b a n e , A u s tra lia , p. 29. I n d o o r o o p i ll y , A u s tra lia , BSES.
R O TT P., S O U P A D ., BR U N E T Y., F ELDMANN P., LETOURMY P., 1995. Leaf scald ( X a n t h o m o n a s a l b i l i n e a n s ) in cid ence and its e ffe ct on y ie ld in seven sugarcane c u ltiv a rs in G u a d e lo u p e . Pla nt P a th o lo g y
(in press).
Growth of side shoots with no apical dominance.
Photo P. Rott
sugarcane disease
Abstract... Resumen... Résumé
P. ROTT — Leaf scald of sugarcane.
Leaf scald of sugarcane is a bacterial disease caused by
Xanthomonas albilineans (Ashby) Dowson. The economic
consequences may be serious when susceptible varieties a re a ffe c te d in produc tion a re a s . S y m p to m a to lo g y includes a chronic form and an acute form as well as latency and eclipse phases that make diagnosis difficult. The disease is currently found in at least 61 sugarcane production zones. It spreads p a rtic u la rly r a p id ly in cuttings. P re v e n tiv e control m ethods a re used i. e. selection of resistant plants, control of the exchange of plant material by the use of a quarantine station, nursery p ro d u c tio n o f p e rfe c tly h e a lth y cu tting s an d h e a t treatment of cuttings. Control of leaf scald of sugarcane is not possible without a strict system of multiplication and diffusion of plant material from the production of tissue culture plants for planting out in a "mother nursery" to the supply of healthy plants for setting out in commercial fields.
K e y w o rd s : l e a f s cald, s u g a r c a n e , X a n th o m o n a s
albilineans, quarantine, varietal selection, heat treatment,
vegetative propagation, Guadeloupe, tropics.
P. ROTT — La escalcadura de la hoja de caña de
azúcar.
La escaldadura de la hoja (le a f scald] de caña de azúcar es una enferm edad bacteriana vascular provocada por
X a n th o m o n a s a lb ilin e a n s (A s h b y D o w s o n ). Las
consecuencias económicas pueden ser graves cuando resultan afectadas algunas variedades sensibles en las zonas de producción. La sinto m atolog ia incluye una forma crónica, una forma aguda y fases denominadas de latencia y eclipse que hacen el diagnóstico delicado. Actualmente, esta enferm edad está presente en por lo menos 61 zonas productoras de caña de azúcar y se propaga con gran rapidez puesto que es transmitida por los esquejes. Los métodos de lucha son preventivos: selección varietal de plantas resistentes, control de los intercambios de material vegetal mediante una estación de cuarentena, producción de esquejes perfectam ente sanos a partir de viveros y técnicas de termoterapia de los esquejes. El control de la escaldadura de la hoja de caña de azúcar sólo es posible en un esquema riguroso de multiplicación y difusión del material vegetal, desde la producción de vitroplántulas para crear el vivero madre hasta el suministro de esquejes sanos para la plantación de campos comerciales.
Palabras clave: escaldadura de la hoja, caña de azúcar,
Xanthomonas albilineans, cuarentena, selección varietal,
termoterapia, multiplicación vegetativa, Guadalupe, zona tropical.
P. ROTT — L'échaudure des feuilles de la canne
à sucre.
L'échaudure des feuilles ¡le a f scald) de la canne à sucre est un e m a la d ie b a c té rie n n e v a s c u la ire p ro v o q u é e p a r X a n th o m o n a s a lb ilin e a n s (A s h b y ) Dow son. Les conséquences économiques peuvent être graves lorsque des variétés sensibles sont atteintes dans les zones de production. La sym p tom ato lo gie com porte une form e chronique, une forme aiguë ainsi que des phases dites de latence et d 'é d ip s e qui re nd e nt le diagnostic délicat. Actuellement, cette m aladie est présente dans au moins 61 zones de prod uc tion de la canne à sucre. Elle se propage d'autant plus rapidem ent qu'elle est transmise par les boutures. Les méthodes de lutte sont préventives : sélection v a r ié t a le de pla n te s résistan tes ; co ntrôle des échanges de m a té rie l v é g é ta l pa r une station de quarantaine ; production de boutures parfaitement saines à partir des pépinières ; techniques de thermothérapie des boutures. La maîtrise de l'échaudure des feuilles de canne à sucre ne p e u t ê t r e possible sans un sc h é m a de m u lt ip lic a t io n et de d iffu s io n du m a t é r ie l v é g é ta l rigoureux, depuis la production de vitroplants pour la mise en place de la pépinière mère à la fourniture de boutures saines pour la plantation des champs commerciaux. M o ts -c lé s : é c h a u d u r e des fe u ille s , c a n n e à sucre,
Xanthomonas albilineans, quarantaine, sélection variétale,
therm othérapie, multiplication végétative, Guadeloupe, zone tropicale.
Whitening on a leaf blade due to albicidine, a toxin produced by Xanthomonas albilineans.
Photo P. Rott
