Palti, J., Agricultural Research Organization, Bet Dagan, 50250, Israel

The effect of environmental factors on the development of each stage of Peronospora destructor (Berk.) Caspary on onions is reviewed. For sporulation to take place, a period of light must precede the period of darkness and high humidity in which spores are formed. Spores are discharged when the relative humidity (RH) is increasing or decreasing, and over a wide range of temperatures. Their discharge is triggered by exposure to red-infrared radiation and by vibration of the leaf. Dissemination of spores follows a daily periodic cycle and spores can be blown by wind over long distances. Duration of spore survival depends on temperature, RH and, especially, the absence of strong radiation. The rate of spore germination is highest at 10°C and declines with the rise in temperature. Germ tubes develop in liquid water, and a continuous period of wetness is required for infection to be completed. Systemic infection is common in cooler climates, where necks of onion bulbs are slow to dry. The principal sources of downy mildew infection by wind-borne spores are systemically infected propagation material, onion volunteer plants, and neighboring older crops. Development of mildew epidemics depends primarily on moisture, but the different stages in the infection cycle are favored by various kinds of moisture (water, high RH, or changes in RH levels). The temperature optimal for epidemics is 10-12°C, but temperatures up to approx. 22°C can support rapid disease development. Light is essential to build up photosynthates for subsequent sporulation. Shade plays a complex role, especially where dew supplies most of the moisture required. Where this is the case, under semi-arid and rainless conditions, favorable early morning temperatures may compensate the pathogen for the limited hours of high humidity, and thus promote severe mildew outbreaks. Reliable methods of predicting onion mildew are not currently available, but disease-free periods can be defined empirically by multi-annual observations. Control recommendations include securing propagation material free from systemic infection, destruction of volunteer plants and onion refuse, avoiding proximity of infected crops, choosing sites with relatively short dew periods (e.g. southern slopes), avoiding excessively dense stands, and irrigation preferably by methods other than overhead sprinkling. Onion mildew can be effectively controlled by fungicides; considerations for correct timing of their application are discussed. © 1989 Springer Science + Business Media B.V.