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Managing Quarantine-Significant Postharvest Diseases in Pacific Northwest Apple and Pear Orchards

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Dr. Parama Sikdar, WSU-Tree Fruit Research and Extension Center (TFREC), Wenatchee, is currently conducting research on the project “Managing Quarantine–Significant Postharvest Diseases in Pacific Northwest Apple and Pear Orchards” to control Speck rot, caused by Phacidiopycnis washingtonensis, and Sphaeropsis rot caused by Sphaeropsis pyriputrescens. These fungal pathogens were reported more than a decade ago, and since then found to cause postharvest storage rots. Both diseases infect tree tissues, causing dieback in limbs and cankers in crabapples. These postharvest diseases start in the apple orchard, although fruit symptoms appear later in cold storage. Dr. Chang-Lin Xiao, former plant pathologist with WSU-Tree Fruit Research and Extension Center in Wenatchee, discovered that ‘Manchurian’ crabapples, used as pollinizers in apple orchards, are highly susceptible to these fungi. In addition, they serve as hosts to these diseases and sources of inoculum that can spread the diseases to other fruit trees. Fruiting bodies containing millions of spores (inoculum) form on infected dead tissues of apple and crabapple trees, and mummified crabapple fruit left on trees. The spores are spread by water (rain, overhead irrigation and cooling) and infect apple fruit, which later develop symptoms in cold storage.  In 2012 the export of Red Delicious apples to China was stopped due to the occurrence of postharvest fungal decay. Export of apples to China was reopened in December, 2014, under strict conditions that fruit will be clean and disease free and that orchard sanitation practices will be managed to ensure that the trees are disease free. Orchardists should remove diseased twigs with dieback and cankers and mummified crabapple fruit to help reduce fungal inoculum in orchards. Preharvest fungicides applied near harvest reduce Speck rot. A postharvest fungicide drench is effective at controlling these diseases on apple fruit. Postharvest treatments are more effective than preharvest treatments. Dr. Sikdar will continue field trials that began in 2014 to elucidate the efficacy of pruning and postharvest fungicide applications to control the diseases. Her work is part of a larger team USDA funded TASC grant including ARS scientist Mark Mazzola, and WSU faculty including Stefano Musacchi and Karen Lewis. Others collaborating in the project are Mike Willett (Northwest Horticulture Council), Richard Kim (Pace International) and Tom Auvil (WA Tree Fruit Research Commission).

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