Cucurbit Genetics Cooperative Report 2:9-10 (article 5) 1979
The Influence of Temperature on Powdery Mildew Resistance in Cucumber
H. M. Munger
Departments of Plant Breeding and Vegetable Crops, Cornell University, Ithaca, NY 14853
In studying the inheritance of powdery mildew resistance in cucumber, we have repeatedly noticed that plants in the greenhouse with any given level of resistance show much more mildew during the winter months than during May and June. Likewise, plants in the field, independent of their age, show much more mildew during August and September than they do in June and July. We have not located information about the effects of photoperiod, temperature, and light intensity on extent of mildew development that would explain the differences observed. A recent unintentional experiment seems to implicate temperature as the main factor.
In the process of adding gynoecious sex expression to 'Marketmore 76', 15 progenies were planted in the greenhouse on October 10, 1978. As a result of repeated backcrossing, these progenies were essentially isogenic with each other and with 'Marketmore 76', except for the gynoecious gene F. As standards of sex expression, progenies of constitution FF, Ff, and ff in 'Marketmore 70' background were included in the planting. Mildew developed heavily on these progenies in spite of some attempts to control it by fungicides, but 'Marketmore 76' backcross progenies which were never treated with fungicide showed no trace of mildew. In mid-December, six of the progenies were moved into another greenhouse section 40 feet away. This section was maintained at night/day temperatures of 15-21°C, while the rest of the planting remained at 21- 27°C.
Four weeks later, the plants in the cooler house showed distinct mildew on both young and old leaves, but it did not appear to be sporulating heavily. There were no susceptible plants in this cooler section to provide continuing inoculum. Meanwhile, similar progenies in the original location at the higher temperature and continuously exposed to inoculum from nearby susceptible plants remained completely free of mildew symptoms. Neither group of plants received direct supplemental light, but it happened that each group was placed almost exactly the same distance from an intervening greenhouse section in which many lights were on for 24 hours a day, giving ample light to be photoperiodically effective. Since light intensity and photoperiod were the same for both groups of plants, the lower temperature appears to have been responsible for greater mildew development on one group.
We cannot say from this experience whether a temperature regime of 15-21°C is optimum for mildew development, but it seems certain that it is much more favorable than a 21-27°C regime. Our experiences over a period of years suggest that the effect of temperature is more pronounced on cucumbers with intermediate levels of powdery mildew resistance than on susceptible ones. 'Tablegreen', which we consider to have a low intermediate level of resistance, frequently shows almost no mildew when grown in the greenhouse in late spring or in the field in early summer, while susceptible varieties have striking mildew symptoms. On the other hand, in winter months in the greenhouse, 'Tablegreen' may develop so much mildew that one would call it susceptible, except by comparison with truly susceptible varieties which have even more mildew. 'Poinsett', which we consider to have a high intermediate level of resistance, shows similar marked differences in amount of powdery mildew under different growing conditions.
This temperature effect may explain some of the contradictory reports about the level of resistance in plant introductions which have been used as sources of resistance in various breeding and genetic studies. Likewise, it may account for some of the different genetic interpretations of powdery mildew resistance as well. In studies of either sort, it seems important to report temperatures at which the studies were made and, if possible, to make comparisons under different temperature regimes.