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Strawberry Disease Models
Strawberries are produced in open field and under protection. Economically it is the most important soft fruit worldwide and is grown from the Mediterranean to the cool climate of Scandinavia or Southern Chile. Nevertheless of the climatic conditions, the grey mould Botrytis cinerea is the most important disease of this fruit. Nearby also powdery mildew and leather berry have some important impact. Powdery mildew occurs under warm and moist conditions with no leaf wetness and rain, where leather berry occurs under rainy conditions in open field production or under irrigation in covered production.
For the prediction of diseases in strawberry cultivation the measurement of free moisture is essentially. Therefore, the determination of leaf wetness showed to be a very good indicator for free moisture, which is caused by rain or dew. It is assumed, that rain or dew have a equal distribution to all plant parts which are air exposed.
Production in green houses or in plastic tunnels is different, because rain or dew does not occur on plants. Here, free moisture is mainly caused by sprinkler irrigation and/or by condensation. Irrigation under dry conditions will not lead to a long leaf wetness period. Whereas irrigation in a water saturated air will lead to a longer leaf wetness period. By this reason the models indicated for covered production will determine both, water saturation determined by leaf wetness and by the dew point. If the values of the actual temperature and relative humidity reach the dew point the occurrence of free moisture is assumed.
LEATHER BERRY INDICATION MODEL
The fungal pathogen P. cactorum is able to infect more than 200 species, including strawberry and woody ornamentals and fruit crops. This pathogen occurs worldwide, but is most common in temperate regions. P. cactorum forms zoospores, which originate from hyphae or germinating oospores and sporangia. In many cases, the pathogen may enter a field through infected transplants. Infection by P. cactorum usually occurs during warm periods with prolonged wetness. Zoospores are released from sporangia during saturated soil conditions and enter the plant through wounds. Once the zoospore gets in contact with a host, it enters the host and developing hyphae of the fungus colonize the host.
P. cactorum oospores form sporangia in water-saturated soil. These sporangia will release its zoospores into free water in the soil. Free water in the upper soil layer is the limiting factor for the spread of this disease. Release and mobility of zoospores can only take place if the temperature in the surrounding media is warm enough.
- 3 day average temperature higher than 12 °C,
- assessing free water in the soil:
- More than 10 mm of rain today => 1 day infecton period or
- More than 14 mm of rain on 3 rainy days or
- More than 17 mm of rain on 2 rainy days out of 3 or
- More than 20 mm of rain on 1 rainy day out of 3 => 3 day Infection period
In a high-pressure situation every infection period has to be taken seriously. In a situation with a lower disease pressure, a 3 day infection period has to be taken seriously.
In FieldClimate we calculate infection days with “yes” or “no”- so it is always no infection (0) or infection (100).
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.
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.
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 period 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 probability and the number of infected fruit. The risk, which can be taken under consideration, depends on the market. Growers, which are selling their fruits to the supermarket 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.
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.