ISHS Acta Horticulturae 51: Symposium on Protected Cultivation of Flowers and Vegetables
HUMIDITY CONDITIONS AND THE DEVELOPMENT OF FUNGUS DISEASES IN GLASSHOUSES |
| Authors: | L. Bravenboer, Th. Strijbosch |
| Abstract:
All crops grown under glass can be attacked by a great number of fungus diseases. Except the fungi causing powdery mildew, all the fungi need water for the germination of the spores, for sporulation etc. In heated glasshouses with a good climate control system, these diseases need not to be a problem, especially if the system is used to prevent the plants from becoming or staying wet. In non-heated glasshouses or plastic houses, on the other hand, the crops can be very badly attacked. In the last two years we have had two striking examples of the occurrence of this type of diseases in Holland. In 1973 the cucumbers were very badly attacked by downy mildew (Pseudoperenospora cubensis). Although it was incidentally found in heated glasshouses too, the fungus mainly caused losses in air-heated and non-heated glasshouses. The second example is downy mildew (Bremia lactuceae) in lettuce. In the Western part of The Netherlands this disease seldom causes problems in glasshouse grown lettuce. In the autumn and winter of 1974 downy mildew was found on many holdings and in some cases caused a complete loss of the crop. Furthermore in air-heated and non-heated tomato crops ghost spot (Botrytis cinerea) of the fruits is of economic importance practically every year. In all these cases even frequent application of fungicides usually is not sufficient to give adequate control. Basically the cause of the occurrence of these diseases is the outside climate respectively the glasshouse climate. In principle the plants become and/or stay wet in two different ways. In the first place by watering. If the plants dry quickly again, the time will be too short for spores of fungi to germinate. However if this is not the case, the conditions for an attack of fungi become favorable. This was the situation in lettuce growing in the autumn and winter of 1974 as to Bremia lactaceae. Due to the very wet season combined with relatively high temperatures, it was with regard to the growing techniques impossible to put enough heat into the glasshouse to dry the plants. Consequently downy mildew caused much greater losses than in other years. As to ghost spot of tomato fruits and downy mildew in cucumbers the situation is different. In air-heated and non-heated glasshouses with these crops condensation on the plants occurs quite easily and frequently. This especially takes place early in the morning, when the air temperature rises quickly through the influence of the sun, whilst the plants stay cool. Depending on the air humidity and the difference in temperature between the air and the plants, the crop will stay more or less wet during | |
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