PREDICTING THE OCCURRENCE OF FIRE BLIGHT IN THE SAN JOAQUIN VALLEY OF CALIFORNIA

Abstract

Fire blight is a serious disease of apple in the San Joaquin Valley of California. Applications of streptomycin sulfate usually begin after 10% bloom and continue every 5 days until flowering is complete, especially if humid and wet conditions persist. Several models relate the presence of Erwinia amylovora on blossoms to temperature and other meteorological factors to establish thresholds above which disease can be expected to occur. Three fire blight predictive models, Mean Temperature Line, Cougarblight, and MARYBLYT, were compared in 1999 and 2000. All models accurately predicted low fire blight potential during the susceptible bloom period when daily average temperatures were below 15.5(0)C. In 1999, rain was associated with cold temperatures and some growers eliminated several antibiotic sprays without observing disease. Later in the season, and without rainfall, disease was observed in association with a 4-day period with more than 5 h of leaf wetness per day and average daily temperatures above 15.5(0)C. Twelve and 14 h of leaf wetness were recorded on 2 of the 4 days. In 2000, fire blight was severe and infections were observed in orchards before rain occurred. Multiple infections were associated with a period of 15 days during bloom when leaf wetness developed at average daily temperatures above 15.5(0)C. Symptoms were already apparent when two rain events occurred during this time. The models accurately predicted infections based on rainfall and temperature in the San Joaquin Valley but need modification to include hours of leaf wetness. The duration of leaf wetness, which can quantify wetting periods of both rain and dew, may provide a more reliable basis for disease prediction in the relatively warm arid climate of California.