Pest and disease management - Powdery mildew in edible field crops

Powdery mildew in field crops, by Professor Geoffrey Dixon

Powdery mildew on marrow
Powdery mildew on marrow

Powdery mildew 

Carrot: Erysiphe heraclei; brassicas: E. cruciferarum; cucurbits: E. cichoracearum; beans: E. polygoni; beet: E. betae. Powdery mildews are normally characterised by sporulating on the upper surface of leaves (cf downy mildews), but this is not an absolute rule.

Crops affected

Carrot powdery mildew affects angelica, carrot, celery, chervil, coriander, dill, fennel, parsley and parsnip. Brassica powdery mildew attacks all brassicas. Cucurbit powdery mildew attacks all cucurbits. Bean powdery mildew attacks Phaseolus beans. Beet powdery mildew is found on table (red) beetroot and Swiss chard.

Cause 

Erysiphe species are a common cause of fungal diseases in vegetable crops, seen as a dense white crustation on leaves. There are suggestions of host specificity in E. heraclei, which attacks umbellifer crops, with some strains specialising on herbs.

In beetroot, symptoms first appear as small white colonies, usually affecting older leaves first.

These pathogens can overwinter in diseased debris as mycelium and in some cases as sexual resting bodies. Recently Oidiopsis taurica has been reported causing powdery mildew on allium crops.
 
During the cropping season they spread in air currents and can travel considerable distances. 

Symptoms

These start as star-like white lesions on upper leaf surfaces where conidia land on leaves and start penetrating into the cells from where nutrition is taken. A mycelia mat is established from which further generations of asexual conidia are formed and released. This superficial mycelium coalesces across the leaf, covering the entire surface in an off-white mass.

Some species form sexual stages but in others these are rare. The mycelium can cause damage to crop quality. In Brussels sprouts it darkens in cold conditions, reducing the visual appeal of the crop. Two forms of infection have been identified on cabbage and cauliflower — necrosis of the outer leaves with obvious powdery mildew lesions accompanied by much reduced curd size and disease lesions on the outer leaves and necrosis of the inner wrapper leaves, starting at the tips and progressing towards the curd, that begin to rot due to invasion by secondary bacteria.

In carrots the pathogen E. heraclei usually infects older foliage first and is seen as scattered white fungal colonies. Infections spread to younger leaves until the entire plant is affected. In severe cases the leaves become twisted and deformed, leading to early senescence. In herb crops grown for seed, infection of the flowers and fruits retards their development and yields. Seed quality, colour and number are reduced.

Similarly, severe E. polygoni results in pea pods that are small, deformed and low-yielding. This is preceded by heavy colonisation of leaves, tendrils and stems, which become covered in off-white to grey mycelium that carries huge numbers of conidia.

Dense colonisation of older leaves is also typical of infection by E. cichoracearum attacking cucurbits. Leaves turn yellow then necrotic and the whole blade is infected. Infected plants are stunted, bear smaller misshapen fruit with reduced yields and early senescence. The early loss of foliage can lead to sun-scorching of the fruit, reducing quality and value.

Oidiopsis taurica causes light yellow blotches on allium leaves.

Weather effects

Powdery mildew development is favoured by warm, dry days and cooler nights that encourage dew formation. High relative humidity is required for colonisation and invasion of the hosts.

Soil conditions

Powdery mildew infection tends to develop as crops mature. It is normally less evident on younger crops or developing tissues. Crops grown on fertile, well-drained soils and given moderate but not excessive fertiliser applications are less likely to be damaged.

Integrated disease management

Husbandry Field hygiene including the removal or ploughing-in of infected residues after harvest is an essential step in combating powdery mildews. This ensures that overwintering sexual stages of the fungus are diminished by attack from natural antagonists. Growers should ensure that crops are not placed close to agricultural crops such as oil seed rape or sugar beet, which will be sources of massive release of airborne spores capable of causing disease on brassicas and beetroot respectively.

In some crops such as brassicas it is suggested that late planting avoids presenting the pathogen with mature foliage at the time of maximum spore release. For crops such as green Phaseolus beans, early sowing is advised to avoid infection. Carrots become more susceptible to powdery mildew after week seven or eight as the foliage starts maturing. At these stages disease severity is lessened by rain or overhead irrigation.

Resistance

There are suggestions that forms of resistance may be available in some brassica crops such as Brussels sprouts, cabbage and swede. This appears to involve some suppression of fungal development or reducing the rate of asexual and sexual reproduction. Similar resistance mechanisms may also exist in green Phaseolus beans.
 

Fungicides

Brassicas Leaf and flower head: 
Azoxystrobin + difenoconazole, Prothioconazole, Tebuconazole, Tebuconazole + trifloxystrobin.
 
Brassicas Root: 
Azoxystrobin (off-label EAMU), Azoxystrobin + difenconazole (off-label EAMU), Fenpropimorph (off-label EAMU), Isopyrazam (off-label), Prothioconazole, sulphur, Tebuconazole, Tebuconazole (off-label EAMU), Tebuconazole + trifloxystrobin (off-label).
 
Brassicas Salad greens:
Tebuconazole (off-label EAMU).

Cucurbits:
Azoxystrobin (off-label EAMU), Bupirimate (off-label EAMU),Boscalid + pyraclostrobin (off-label), Bupirimate (outdoor only), Cyflufenamid, Penconazole (off-label), Potassium bicarbonate (commodity substance, off-label), Proquinazid (off-label).
 
Beans (Phaseolus):
Tebuconazole (off-label EAMU).
 
Beet (off-label especially aimed at applications for fresh produce):
Azoxystrobin + cyproconazole, , Cyproconazole + trifloxystrobin, Cyproconazole + trifloxystrobin (off-label EAMU), Cyproconazole + picoxysrobin. Cyproconazole + picoxystrobin off label), Difenconazole + propiconazole, Epoxiconazole, Epoxiconazole + pyraclostrobin, Epoxiconazole + pyraclostrobin (off-label), Fenpropimorph (off-label), Isopyrazam (off-label),  Quinoxyfen, sulphur, sulphur (off-label).
 
Carrots/parsnips: 
Azoxystrobin, Azoxystrobin (off-label EAMU), Azoxystrobin + difenconazole, Azoxystrobin + difenconazole (off-label), Boscalid + pyraclostrobin, Fenpropimorph (off-label EAMU), Prothioconazole, sulphur (off-label EAMU), Tebuconazole, Tebuconazole + trifloxystrobin, Tebuconazole + trifloxystrobin (off-label).
 
Stem and bulb vegetables onions, leeks and garlic:
Azoxystrobin (off label),Tebuconazole (off-label). 

Warning

For use on all fresh-produce vegetable crops growers must in advance of use ensure that a particular commercial agrochemical formulation is legally acceptable for their particular crop/husbandry regime and also accepted by the intended purchaser’s crop-quality standards specification as agreed with the relevant crop technologist.


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Powdery mildew in field crops, by Professor Geoffrey Dixon