Phythophthora cactorum was recovered from irrigation water from late June to September 1989 and early June to September 1990.  Apple (cv. Golden Delicious) and pear (cv. Bartlett) fruit inoculated with a zoospore suspension (10,000/ml) of P. cactorum were used to determine the effect of wetness duration and temperature on disease severity and incidence.  In controlled environment studies, incidence and severity increased with increased wetness duration (1–12 h) at temperatures between 10 and 30 C on pears and 7 and 30 C on apples.  On pears, the loss threshold of one lesion per fruit required wetness durations of 5, 4, and 3 h at 15, 20, and 25–30 C, respectively.  At 20–30 C, a ≥3-h wetness duration resulted in 100% infection on pears.  On apples, the loss threshold of one lesion per fruit required 11-, 7-, 6-, 5-, and 3-h wetness durations at 10, 15, 20, 25, and 30 C, respectively.  Wetness durations of 6–7 and 3–4 h were required for 100% infection at 15–20 and 25–30 C, respectively.  Multiple regression equations using temperature and wetness duration as independent variables adequately described disease incidence and severity on both hosts.  In orchard studies on pears, infection increased with increased wetness duration (1–20 h) and temperatures up to about 28 C, and then declined slightly at longer wetness durations at 20–31 C.  Infection of apples in the field increased with increased wetness duration (1–20 h) up to 20–25 C and then declined.  Multiple regression equations using temperature and wetness duration, and temperature, wetness duration, and increasing day of year as independent variables adequately described disease incidence and severity on pears and apples, respectively.  The predicted loss threshold of one lesion per fruit on orchard-inoculated pears required wetness durations of 1, 3, 6, and 6–11 h at 25, 20, 15, and 10 C, respectively; on apples the loss threshold ranged from 6 h at 10 and 27.5 C to 3.5 h at 20 C.  Susceptibility of pear fruit remained nearly constant from about 60 days after petal fall until harvest; apple fruit susceptibility increased as harvest approached.  Results indicate that fruit infection can occur throughout the summer fruit development period and management of the disease may be improved by reducing the duration of overtree irrigation and applying water during cooler periods (i. e., at night).

Additional key words: irrigation management, Malus domestica, Pyrus communis, and quantitative epidemiology.