LONG-TERM EFFECT OF TEMPERATURE AND TRIADIMEFON ON PROLIFERATION OF UNCINULA NECATOR - IMPLICATIONS FOR FUNGICIDE RESISTANCE AND DISEASE RISK ASSESSMENT

Triadimefon has been used in California to control Uncinula necator, c ausal agent of grape powdery mildew, since 1982. Instances of unsatisf actory control have occurred mainly in the cooler coastal areas of Cal ifornia. The effect of temperature and application of triadimefon was investigated over a 53-day-period on two U. necator isolates, sensitiv e and resistant to triadimefon. At 15 degrees C, 25 degrees C, or temp eratures fluctuating between 15 and 25 degrees C, in absence of triadi mefon, the isolates continued to produce high numbers of conidia for t he entire duration of the experiment. Sporulation declined at daily ma ximum temperatures of 32 degrees C for 6 h, 36 degrees C for 3 h, and 40 degrees C for 1 h, but was detectable when the experiment was termi nated. At these temperature regimes, sporulation of the triadimefon-tr eated sensitive isolate ceased after 23 days. When treated with triadi mefon, sporulation of the resistant isolate was comparable to that of the water-treated control. At daily maximum temperatures of 32 degrees C for 11 h, 36 degrees C for 6 h, and 40 degrees C for 3 h, sporulati on of both isolates generally ceased after 23 days, regardless of tria dimefon application. Triadimefon resistance is most likely to manifest itself under high disease pressure, which is in part a function of te mperature. The duration of daily maximum temperatures may be a valuabl e addition to disease risk assessment models.