Effects of relative humidity and high temperature on spore germination and development of tomato powdery mildew (Leveillula taurica)

The effects of relative humidity (RH) and temperature on tomato powdery mildew (Leveillula taurica) were studied in controlled environments to define conditions that affect disease development in fresh market tomatoes in California. Gradients of RH (20-90%) at constant temperatures (20-30degreesC) were generated in single growth chambers to determine their effect on mildew development on mature tomato plants. Temperatures of 30degreesC and above were deleterious for spore germination, germ tube elongation, and disease development. Lesion growth and rate of disease progress were significantly higher at 20degreesC than at 25degreesC. Low RH levels (20-40%) reduced spore germination and lesion growth, accelerated host tissue death and reduced disease progress. Intermediate RH levels (50-70%) increased spore germination and optimized disease development, provided temperatures were maintained within favorable limits. High RH levels (80-90%) were favorable for spore germination but continued exposure to these conditions led to a limited lesion growth and disease progress. Short daily periods (two or three daily exposures of at least 2h) of high temperatures (35degreesC) suppressed disease development by 70-92%. (C) 2003 Elsevier Ltd. All rights reserved.