Long-term Effect of Temperature and Triadimefon on Proliferation of Uncinula necator: Implications for Fungicide Resistance and Disease Risk Assessment

Abstract

Triadimefon has been used in California to control Uncinula necator, causal agent of grape powdery mildew, since 1982. Instances of unsatisfactory control have occurred mainly in the cooler coastal areas of California. The effect of temperature and application of triadimefon was investigated over a 53-day-period on two U. necator isolates, sensitive and resistant to triadimefon. At 15°C, 25°C, or temperatures fluctuating between 15 and 25°C, in absence of triadimefon, the isolates continued to produce high numbers of conidia for the entire duration of the experiment. Sporulation declined at daily maximum temperatures of 32°C for 6 h, 36°C for 3 h, and 40°C for 1 h, but was detectable when the experiment was terminated. At these temperature regimes, sporulation of the triadimefon-treated sensitive isolate ceased after 23 days. When treated with triadimefon, sporulation of the resistant isolate was comparable to that of the water-treated control. At daily maximum temperatures of 32°C for 11 h, 36°C for 6 h, and 40°C for 3 h, sporulation of both isolates generally ceased after 23 days, regardless of triadimefon application. Triadimefon resistance is most likely to manifest itself under high disease pressure, which is in part a function of temperature. The duration of daily maximum temperatures may be a valuable addition to disease risk assessment models.

Effect of High Temperature and Exposure Time onErysiphe necatorGrowth and Reproduction: Revisions to the UC Davis Powdery Mildew Risk Index

“…In our controlled-environmental experiments, we found that E. necator continued to germinate, grow, and sporulate at higher temperatures than previously reported (10,11,36). Erom 30 to 34°C, we showed that temperature is not lethal to three of the major stages of the pathogen's asexual life cycle, although long exposure times (up to 24 h) at these temperatures reduced growth and reproduction.…”

“…As an obligate parasite, E. necator cannot be grown on defined media but it responds well to a variety of leaf and leaf disk systems with and without agar (30,36). Grapevines {Vitis vinifera) of the highly susceptible 'Carignane' were raised in a glasshouse to provide a year-round source of young, disease-free leaves for controUed-environment experiments and to maintain E. necator isolates.…”

“…The effects of eight temperatures (30,32,35,36,38,40,42, and 44°C) at each of 12 exposure times (0.25, 0.5, 1, 1. 5,2,4,6,8,12,18,20, and 24 h) were measured for three life stages of the fungus (colony growth, spore production, and spore germination).…”

“…Both high-temperature exposure and acute cold temperatures reduce E. necator growth and infection efficiency (23,35). The maximum temperature exposures at which E. necator conidia germinate has been variously reported as 33.5°C for 24 h and 40.5°C for 4 h (10), 34 or 36°C for an unspecified exposure time (35), and 40°C for 3 h (36). In the presence of UV light, existing colonies are damaged by exposure to 35°C for 0.25 h (16).…”

Tri-Trophic Interactions Among Grapevines, a Fungal Pathogen, and a Mycophagous Mite

“…For polycyclic fungal pathogens, abiotic conditions (i.e., temperature, rainfall, surface wetness, and relative hu-midity) are generally thought to drive the rate at which an epidemic develops (Agrios 1997). For grape powdery mildew, temperature and humidity are particularly important (Delp 1954, Ypema and Gubler 1997, Carroll and Wilcox 2003. Our results illustrate a heretofore unappreciated impact of predation on fungal population dynamics.…”