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This study reports on factors affecting the development of hop downy mildew during the dry, hot season in the Sacramento Valley, as well as on factors contributing to the paucity of disease development in apparently favorable conditions. In flood-irrigated commercial hop yards, systemically infected shoots (which can be sources of inoculum) were present until late in the growing season. Dew and guttation fluid appeared on leaves and stems at the base of the plants after irrigation, but only a few lesions developed on the leaves. Data from Hirst spore traps, used to monitor the concentration of sporangia near systemically infected shoots, indicated a diurnal pattern of sporangial release. Sporangia were released with each sudden lowering of RH under laboratory conditions, and a more rapid lowering resulted in greater release. Sporangial release in the field coincided with RH drop. Most of the sporangia were released in the early morning during evaporation of dew and guttation fluid, and few sporangia were collected in the afternoon, evening, or night. Longevity of viable sporangia was dependent on RH. Relative humidity below 60 per cent killed sporangia in less than 3 hours. Even in well-irrigated yards the ambient RH remained below 60 per cent for about 10 hours each day; most sporangia do not survive throughout the day in such an environment. A few sporangia do survive on hop leaf surfaces through favorable RH conditions, and they initiate infection during the following dew period. Sporangia survived better on the lower surface than on the upper surface because of higher RH. Sporangia exposed to temperatures of 36, 39, and 42°C had delayed germination when subsequently placed in environments optimum for germination (temperatures of exposed leaves in the yards occasionally exceed 33°C during the day). The studies indicate that both inoculum and free moisture are present in the yards and can establish new infections, but that they do not occur concurrently long enough to start epidemics. In the event of summer rains, however, even these few infections can provide sufficient inoculum for an epidemic.

Ecological factors limiting epidemics of hop downy mildew in arid climates