Pest & Disease Management - Grey mould

This disease compromises brassica, pea and bean field crop quality, Professor Geoffrey Dixon warns.

Grey mould botrytis on cabbage - image: Geoff Dixon
Grey mould botrytis on cabbage - image: Geoff Dixon

Botrytis cinerea = Botryotinia fuckeliana is a ubiquitous disease that has a severe impact on all brassica, pea and bean crops in the field, as well as being a post-harvest problem.

Cause

The fungus Botrytis cinerea is weakly pathogenic, invading through delicate or damaged tissues or as a secondary invader of leaves, stems and flowers, following other pathogens. This is the asexual stage of Botryotinia fuckeliana. It produces huge numbers of conidia on the surfaces of infested tissues, forming grey felt-like masses, hence the common name, grey mould.

Mature infections form black oval or irregular resting bodies, sclerotia, which are obvious to the naked eye (4-10mm or larger in size). The pathogen remains dormant as sclerotia in soil or on crop debris in soil, and in stores that have not been thoroughly cleansed after use.

Symptoms

Under favourable conditions, disease progress is rapid, quickly destroying crop quality and saleability.

Alternatively, infection may remain latent until after harvest. Normally, grey mould is seen in the field, initially as brown blotches on leaves and stems. These expand into a mushy rot. Harvested heads of infected cauliflower show brown expanding areas of infection and rotting, and become worthless. Similar symptoms develop on stored cabbages, with the wrapper leaves being progressively destroyed. In severe infections, the entire head rots and sclerotia may be seen.

In legume crops, especially green beans and peas, the developing leaves are particularly prone to infection, especially if bruised by wind damage or where fallen petals have lodged on them. Botrytis cinerea colonises the delicate nutrient-rich floral tissues and then transfers to the leaves. Brown, expanding, water-soaked lesions develop on the leaves. The most serious damage is found where lesions form on developing pods. Infection can spread from pod to pod by direct contact, especially where leaf canopies are dense.

Botrytis cinerea = Botryotinia fuckeliana is a ubiquitous disease that has a severe impact on all brassica, pea and bean crops in the field, as well as being a post-harvest problem.

Weather effects

Cool temperatures (15-20°C), free moisture and high humidity (90-95 per cent relative humidity) favour the development of grey mould disease. There are some suggestions that sporulation is encouraged by exposure to light. Physical damage caused mechanically or by poor handling in harvesting, scorching from fertilisers or pesticides, environmental stresses (cold or heat) and damage from insects provides entry portals for grey mould.

Soil conditions

Healthy crops grown in fertile, well-cultivated soil with balanced nutrition and adequate but not excessive irrigation are less likely to suffer from this disease. Poorly timed or badly adjusted irrigation delivering overlarge droplet size damages foliage and developing heads of cabbage or cauliflower, allowing infection by grey mould. Similarly, rank over-vigorous growth in beans is prone to grey mould infection, especially where the crop density is too high.

Integrated disease management

Husbandry

Site selection is particularly important as a means of disease avoidance. Healthy cropping is encouraged by good air movement, which dries the foliage, flowers and legume pods or brassica heads more quickly after rain or irrigation. Drilling or planting rows parallel to the prevailing wind encourages air movement.

Avoidance of excessive nitrogen fertiliser use reduces the likelihood of unrestricted canopy growth, which leads to high humidity and disease development.

Harvesting mature but not over-mature crops reduces the likelihood of infection — crops should preferably be dry at harvest. Crop harvesting and post-harvest handling should avoid damaging or abrading produce. Even minor grazes to surface tissues are quickly exploited by grey mould for entry and colonisation. Stored cabbage should be regularly inspected and the outer wrapper leaves trimmed as soon as signs of grey mould are evident.

Biological control

The fungus Gliocladium catenulatum is an antagonist against other fungi, providing moderate control of grey mould for use off-label (EAMU) on brassicas and legumes and growers may use Bacillus subtilis (off label) on stem and bulb vegetables.

Resistance

This pathogen is highly genetically variable, so plant breeders have not yet produced resistant cultivars. Forms of tolerance may be identified in the field, but these are most likely to result from good husbandry. Crop architecture can be an important factor in the control of grey mould. Green bean cultivars that carry their pods well above soil level are less likely to be infected. Consequently, the choice of cultivar is a critical means of avoiding disease.

Fungicides

Brassicas (root)
• Boscalid + pyraclostrobin
(off-label, EAMU).

Brassicas

Cyprodinil + fludioxonil (off label)

Fenhexamid (off label + baby leaf production)

Pyrimethanil (off label)

Beans (Phaseolus and Vicia)

• Azoxystrobin (off-label, EAMU).
• Boscalid + pyraclostrobin
(off-label, EAMU).
• Cyprodinil + fludioxonil (moderate control)

Legumes (peas)

Chlorothalonil + cyproconazole

Cyprodinil + fludioxonil (moderate control)

Iprodione (off label)

Metconazole (reduction)

Onions / leeks/ garlic

Azoxystrobin +chlorothalonil

Cyprodinil+ fludioxonil (off label)

Fludioxonil

Fludioxonil (off label)

Fluoxastrobin + prothioconazole (useful reduction)

Iprodione (off label)

Warning

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


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