Good Fruit Grower

February 15

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30 FEBRUARY 15, 2015 GOOD FRUIT GROWER U sing a computer-based risk assessment model to forecast fire blight doesn't mean you won't get the disease in your apple or pear orchard. But it will alert you when infection potential is high enough for you to take action. Fire blight (Erwinia amylovora) is like that "crazy aunt in the attic" that nobody likes to admit they have, says Tim Smith, Washington State University Extension edu- cator for Douglas and Chelan counties. "I find out about infected orchards in the winter, after the fact, but I never hear about them during the season," he said. The rapidly evolving disease can take three to four weeks to show up and is particularly damaging to young apple and pear trees up to their fifth leaf. "It's the one thing that can take out an entire orchard— and we've had a few instances of that in the state, far more than we'd like to admit," said Smith. Computer models have been developed to forecast fire blight risk potential, and several are available, including the CougarBlight model that Smith developed in the 1980s to help growers predict blossom infections. Before the days of remote weather stations and widespread Internet use, his model initially required a spreadsheet that growers used to record leaf wetness and temperature data. Since then, a team at WSU led by Dr. Vince Jones has revised the model's risk analysis based on research that significantly changed how temperatures are measured and related to risk of flower infection. Moreover, the model has been incorporated into WSU's Decision Aid System (DAS), a web-based program that integrates phe- nology models for insects and diseases with management recommendations. "The great thing about the DAS CougarBlight is that it does all the compilations for you," said Smith. The auto- mated model uses customized, real-time weather data from AgWeatherNet and past temperatures to estimate fire blight risk levels. The model, when used properly, can help growers be more successful in preventing fire blight by allowing them to intensify control measures during the most critical days. Understanding CougarBlight To maximize use of the model, growers need to under- stand how it works. Smith provided the following in explaining the model: 1. It does not indicate that fire blight will occur in your orchard. If you don't have the bacteria in your orchard, there's nothing there to create the disease, he said. 2. When weather conditions occur that trigger fire blight, somebody is getting hit. Every time the model says that conditions are ripe, somebody is getting hit, though it might not be you. "The model never misses," said Smith, noting that eastern Getting the most out of Diseases Model helps assess and forecast fire blight risk. by Melissa Hansen "You should be thinking very cautiously about watering in the spring other than for frost control." —Tim Smith Tim Smith COUGARBLI

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