Plant Pathogens

Dans le document The Economic Cost of Invasive Non-Native Species on Great Britain (Page 66-72)

The importance of plant pathogens (fungi, nematodes, bacteria, viruses etc) has long been recognised in Great Britain (Ainsworth, 1969) and the economic impact of plant diseases probably first became apparent when plants were cultivated as crops (Carlile, 1995). Many of the fungal pathogens causing plant diseases in Great Britain today are non-native or exotic species that have been introduced either accidentally or deliberately from other countries. The most historic and infamous example of a non-native pathogen causing significant crop loss is late blight of potato caused by Phytophthora infestans. During the 1840s, potato crops were completely destroyed by this oomycete pathogen and gave rise to the great Irish famine.

Invasive non-native species are currently receiving increasing attention in the literature (Manchester and Bullock 2000). However, non-native plant pathogens are far less well-studied than plants, vertebrates, insects, etc. and are under-represented in the scientific literature (Desprez-Loustau et al. 2007).

A recent, comprehensive analysis of available data for introduced non-native plant pathogens in Great Britain by Jones and Baker (2007) found that 234 pathogens were recorded between 1970 and 2004. Of these, 67% (157) were fungi, 11.5% (27) were oomycetes, 11% (26) were viruses, 10% (23) were bacteria and <0.5% (1) were phytoplasmas. Approximately 53% of these were first recorded on ornamentals, 16% on horticultural crops, 15% on wild native plants, 12% on agricultural crops, 2% on pasture plants and 2% on forestry tree species. Interestingly, 47% of these non-native pathogens introduced into Great Britain could be traced back to the Netherlands, 16.7% to New Zealand, 13.9% to France and 11.1% to the USA. This study also analysed the regional distribution of non-native pathogen introductions within Great Britain: 81% in England, 15%

in Scotland and 4% Wales. Jones and Baker (2007) believe that the number of non-native

pathogens introduced into Great Britain does not appear to be increasing and consider 45 of the 234 (19%) non-native pathogens to be of importance because of economic or environmental losses (Table 5.7).

Table 5.7. Plant pathogens numbers affecting selected crops.

Plant Number of important

non-native pathogens Tomato (Lycopersicon esculentum) 6

Lettuce (Lactuca sativa) 5

Camellia (Camellia japonica) 5

Sugar beet (Beta vulgaris) 4

Potato (Solanum tuberosum) 4

Wheat (Triticum aestivum) 3

Barley (Hordeum vulgare) 3

Onion (Allium cepa) 3

Cucumber (Cucumis sativus) 3

Blackberry (Rubus fruticosus) 3 Pelargonium (Pelargonium spp.) 3 Corsican pine (Pinus nigra subsp. laricio) 3

Sixteen of the 157 fungi and 10 of the 27 oomycetes listed affecting ornamentals, agricultural crops and horticultural crops are considered to be important (Table 5.8). Most important of these are the oomycete pathogens Phytophthora kernoviae and P. ramorum which are currently affecting the native flora of Great Britain.

Table 5.8. Economically important non-native pathogens (fungi and oomycetes) introduced into Great Britain between 1970 and 2004 (adapted from Jones and Baker 2007).

Pathogen Disease Host


Ciborinia camelliae Petal Blight of camellia Camellia japonica Coniothyrium lavendulae Dieback of lavender Lavandula angustifolia Cylindrocarpon parva Stem rot of pelargonium Pelargonium sp.

Cylindrocarpon buxicola Box blight Buxus microphylla Discula destructive Anthracnose of dogwood Cornus florida Erysiphe azalea Powdery mildew of rhododendron Rhododendron sp.

Fusarium oxysporum f.sp.


Fusarium crown rot of tomato Lycopersicon esculentum

Pathogen Disease Host

Fusarium trichothecioides Dry rot/gangrene of potato Solanum tuberosum

Kabatiella zeae Eyespot of maize Zea mays

Oidium neolycopersici Powdery milder of tomato Lycopersicon esculentum Podosphaera verbenae Powdery mildew of verbena Verbena lasiostacys Pseudocercosporella


Eyespot of wheat Triticum aestivum

Puccinia distinct Rust of daisy Bellis perennis

Ramularia collo-cygni Leaf spot of spring barley Hordeum vulgare Sclerotium hydrophilum Leaf spot of giant water lily Nymphaea gigantea

Stemphylium sp. Leaf spot of hebe Hebe albicans


Peronospora hariotii Downy mildew of buddleia Buddleja globosa Phytophthora alni Root and collar rot of alder Alnus glutinosa Phytophthora infestans (A2

mating type)

Late blight of potato Solanum tuberosum Phytophthora fragariae var.


Root rot of raspberry Rubus idaeus Phytophthora idaea Root rot of raspberry Rubus idaeus Phytophthora ilicis Twig dieback of holly Ilex sp.

Phytophthora kernoviae Trunk canker of beech Fagus sylvatica Phytophthora ramorum Sudden oak death, dieback of

viburnum Plasmopara obducens Downy mildew of impatiens Impatiens balsamina Pythium tracheiphilum Stem infection of lettuce Lactuca sativa

According to Hill et al. (2005), 135 species of non-native fungi have been identified in England outside the somewhat artificial environment of the urban garden, which corresponds to 9.6% of the total number of recorded non-native species. Three of these pathogens, Ophiostoma novo-ulmi (Dutch elm disease), Phytophthora alni (dieback of alder) and P.

ramorum (sudden oak death), were noted for their major impact. A further 10 species of microbes were identified, 0.7% of the total. However, Hill et al. (2005) also note that of the 14,000 known fungi in Great Britain the origin of many of the species is unknown, and may never be known. It is possible that many plant pathogens arrived with the plant they are a pathogen of, but for species introduced a long time ago, such as cultivated cereals (e.g.

wheat, barley) or potatoes, it may not be possible to establish the origin of the pathogen. No data on the percentage of plant pathogens found in Great Britain that are non-native has

been found, but given that many crop species are non-native it is likely that their pathogens are also non-native. Therefore a figure of 40% for arable and vegetable crops and 50% for protected crops has been estimated.

In the USA at present, crop loss due to invasive non-native plant pathogens is estimated at

$21 billion per year, greater than that caused by non-native insects (Rossman, 2008). One non-native pathogen of particular economic importance is the potato (late) blight.

Phytophthora infestans (Mont.) de Bary is an oomycete pathogen that causes the serious potato and, to a lesser extent, tomato disease known as late blight or potato blight. The origin of P. infestans can be traced to a valley in the highlands of central Mexico and was introduced into Europe in 1845 (Grünwald and Flier, 2005). The disease destroyed potato crops throughout Europe and was responsible for the Irish famine that occurred between 1845 and 1852.

Despite numerous control methods (fungicides, resistant varieties, management practices), potato blight still remains an important plant pathogen today. Potato blight is estimated to cost the global potato industry £3.5 billion every year30 due to crop failure and the cost of fungicides, with outbreaks varying depending on climatic conditions: 2007 was one of the worst years for late blight in the UK, when a particularly wet summer saw an unprecedented 300 outbreaks31.

5.3.1 Control Cos ts

The main control costs of plant pathogens relate to the control of fungal diseases. Viruses and bacteria cannot be controlled through the use of chemicals, although biological control can be effective by controlling the insect vectors that spread some of these pathogens.

These costs are included in the agriculture sector.

According to Defra’s pesticide usage survey report for arable crops in Great Britain (Garthwaite et al. 2008), fungicides accounted for 38% of the total pesticide-treated area of arable farm crops grown in Great Britain in 2008. The two most extensively-used fungicide formulations applied as sprays were chlorothalonil (used on all crops with the exception of rye, triticale and sugar beet) and epoxiconazole (used on cereals). In 2009, according to


industry data, the average cost of fungicide is £76/ha32, with prices varying from £25 per hectare per rotation in sugar beet to £255 per hectare per rotation in potatoes (Nix 2009).

Based on the areas of crops grown, the prices above and assuming that 40% of fungi affecting arable and vegetable crops and 50% of fungi affecting protected crops are non-native (see above), the cost of fungicides to control non-non-native fungi was estimated to be


Table 5.9. Annual control cost of non-native fungi.

England Scotland Wales GB

Arable crops £96,788,000 £13,797,000 £1,407,000 £111,992,000 Vegetable crops £3,438,000 £273,000 £10,000 £3,721,000

Edible crops £59,000 £3,000 £1,000 £63,000

Ornamental crops £27,000 £1,000 £1,000 £29,000

Soft fruit crops £581,000 £118,000 £12,000 £711,000

Top fruit £1,617,000 £86,000 £17,000 £1,720,000

Total £102,510,000 £14,278,000 £1,448,000 £118,236,000

5.3.2 Yie ld Los s

In addition to the cost of control, plant pathogens can still cause yield loss even after control measures have been taken. Oerke et al. (1994) have estimated the percentage yield loss caused by plant diseases, without distinguishing between native and non-native diseases, at 7% in wheat 7% in barley and 9% in potatoes, after control measures had been undertaken.

Using the same cropping areas, and percentage of non-native fungi as above, and adjusting the price obtained for crops to account for the increased production levels, a cost of yield loss caused by non-native plant diseases is estimated at £281,903,000.

Table 5.10 Cost of yield loss due to non-native plant pathogens

England Scotland Wales GB

All grains £130,885,000 £21,910,000 £2,182,000 £154,977,000

Potato £28,575,000 £7,812,000 £623,000 £37,010,000

Oilseeds £3,324,000 £123,000 £18,000 £3,465,000

Sugar Beet £9,614,000 - - £9,614,000

Peas & beans (fodder) £555,000 £18,000 £4,000 £577,000 Commercial Vegetables £47,641,000 £5,305,000 £188,000 £53,134,000 Plants and flowers £4,004,000 £213,000 £43,000 £4,260,000 Hardy nursery stock £6,939,000 £386,000 £386,000 £7,711,000 Fresh fruit (orchard and soft) £7,019,000 £439,000 £3,697,000 £11,155,000 Total £238,556,000 £36,206,000 £7,141,000 £281,903,000

5.3.3 Re s e a rch

However there is considerable additional research work carried out on agricultural plant pathogens that is funded by government institutions as well as privately, in particular by the agrochemical companies.

The Scottish Crop Research Institute receives funding of approximately £15 million each year from government and commercial contracts33. This funds their four main science programmes and a further four themes, one of which focuses on plant pathogens, but no details were available as to spending on plant pathogens in particular. Rothamsted Research has spent £2.4 million over seven years on plant pathogen research (Knight and Turner 2009), which equates to approximately £343,000 per year. Again assuming that 40%

is spent on non-native pathogen research, this gives an annual spend of £137,200 at Rothamsted. Knight and Turner (2009) estimate that this forms about 33%-40% of the total independent research effort on plant pathogens, and therefore annual spend at independent research institutes is estimated at £392,000 per year. However, there is also considerable spending on agricultural pathogens by commercial companies which is said to be substantial (Knight and Turner 2009). No data on this was available due to commercial confidences, but it is assumed that agrochemical companies spend at least three times as much on research, giving a total research spend on agricultural non-native pathogens of approximately

£1,568,000 p.a.


5.3.4 Tota l Cos ts

The total cost of non-native plant pathogens, excluding some research and biological control costs is estimated at £401,707,000 per annum, though this figure is very dependent of the percentage of pathogens that are considered to be non-native. This figure excludes the costs of general quarantine and surveillance measures undertaken against plant pathogens, and therefore does not truly present the total costs of plant pathogens to the economy. It was not possible to separate the costs of plant pathogens from other plant health issues and therefore these costs are only included in the quarantine and surveillance sector and not included here.

Table 5.11. Total costs of non-native fungi to British agriculture and horticulture.

England Scotland Wales GB

Control Costs £102,510,000 £14,278,000 £1,448,000 £118,236,000 Yield Loss £238,556,000 £36,206,000 £7,141,000 £281,903,000

Research £862,000 £470,000 £236,000 £1,568,000

Total £341,928,000 £50,954,000 £8,825,000 £401,707,000

5.4 Ve rte b ra te p e s ts

Dans le document The Economic Cost of Invasive Non-Native Species on Great Britain (Page 66-72)