| Abstract: | The effect of overhead irrigation on plant disease can conveniently be discussed under the following headings. Spore dispersal: irrigation effects splash dispersal and provides the necessary moisture. Under semi-arid conditions, spore dispersal takes place during the morning hours. Irrigation early in the day will aid spore dispersal. Spore resistance: irrigation will modify microclimatic conditions and reduce the stress of exposed spores to radiation, heat and dryness. Moisture period required for infection: leaf surface temperatures will determine the moisture period required for infection. Irrigation timing in relation to moisture provided by rainfall or dew may be critical. The macroclimate: the less favourable the macroclimate, the greater is the importance of microclimatic modifications, and its effect on epidemiology. Microclimatic modifications: the lower the relative humidity during the day and the shorter the period of leaf wetness (dew), the greater will be the impact of sprinkling. Foliar density interacts in determining the microclimate. The later the spore dispersal during the morning, the smaller will the effect of sprinkling be. The greater the resistance of the spores to stress conditions during the day, the smaller the effect of sprinkling. The shorter the period required for host penetration, the more pronounced will be the effect of overhead irrigation. |
