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Grey Mould Biology

Grey Mould (Botrytis cinerea)  is a devastating disease with high economic impact in production. B. cinerea infects the flowers and the fruits close to maturity.

The fungal pathogen has a very broad host range, infecting more than 200 different hosts. Fungal growth exists saprophytically and parasitic.

Symptoms

On Sunflowers the pathogen causes a grey mould on the head and stem. Therewhile the leaves start to dry out. These symptoms occur during maturation of kernels on the head. Brown spots on the back side are seen. These spots are covered by the fungal mycelium and spores, giving the appearance of a powdery.  Spores are able to be spread during wet weather conditions.

Black sclerotia deprived of medulla appear on the crop debris after harvesting or directly on the plants, if they are harvested too late.

The fungus overwinters during winter on the soil surface or in the soil as mycelium or sclerotia. In springtime the overwintering form starts to germinate and produce conidia. These conidia are spread by wind and rain and infect new plant tissue.

Germination is possible at relative humidity over 85%. The optimal germinating temperature is 18°C. The fungal pathogen can reproduce multiply times.

Control options: Seed control can protect plants of damping- off. Chemical control is difficult due to the resistance of the pathogen. Therefore attempts are made for natural control strategies with Trichoderma harzianum.

Conditions for modeling infection

B. cinerea infections are related to free moisture. Therefore in open field production leaf wetness, which is a good indicator, is determined.
Bulger et al. (1987) studied the correlation of leaf wetness periods during flowering and the occurrence of grey mould on the fruits. They found that for a higher risks of infection at 20°C a time periode of  longer than 32 hours of leaf wetness is needed.  At lower temperatures the leaf wetness periods have to be longer for infection of the disease.

FieldClimate is indicating risk of Botrytis cinerea on base of leaf wetness periods and the temperature during these periods. 

The graph below shows the duration of wet leaves in dependence of the actual temperature needed for a Botrytis infection. If the risk is higher than 0 every leaf wetness period longer than 4 hours will increase the risk by the same relation.
A day with a leaf wetness period shorter than 4 hours is assumed to be a dry day and will reduce the risk by 20% of the actual value.

Practical use of the Grey Mould Model: The model indicates periods with a risk of a Botrytis infection. This risk periods during the bloom of strawberry will lead to infected fruits. As longer the risk period lasts and as higher the risk is as higher is the propability and the number of infected fruit. The risk, which can be taken unter consideration, depends on the market. Growers, which are selling their fruits to the super market will not take any risk, knowing that they are not able to sell infected fruits. While growers, who sell their fruits directly to the people are able to take a higher risk.

Literature

Bulger M.A., Ellis M. A., Madden L. V. (1987): Influence of temperature and wetness druation on infection of strawberry flowers by Botrytis cinerea and disease incidence of fruit originating from infected flowers. Ecology and Epidemiology; Vol 77 (8): 1225-1230.

Sosa-Alvarez M., Madden L.V., Ellis M.A. (1995): Effects of temperature and wetness duration on sporulation of Botrytis cinerea on strawberry leaf residues. Plant disease 79, 609-615.