Symptomology and epidemiology of Exobasidium leaf and fruit spot of blueberry

During the past decade, the state of Georgia has become one of the largest producers of cultivated blueberries in the United States. As production has expanded and intensified, several new blueberry diseases have emerged, with Exobasidium leaf and fruit spot, caused by the newly described fungal pathogen Exobasidium maculosum, being one of the most recent. The disease affects both rabbiteye and southern highbush cultivars, causing leaf spotting early in the season as well as green spots on ripe fruit that can result in the rejection of the fruit at the packinghouse. The pathogen produces both basidiospores and yeast cells, but its life cycle, including overwintering biology and primary and secondary inocula, is currently unknown. To fill these knowledge gaps, epidemiological field studies that included trap plants, leaf spot demographic surveys on naturally infected field plants, and epiphytic population monitoring were initiated in 2014. Monitoring of surface populations showed that E. maculosum is capable of overwintering epiphytically on all blueberry tissues tested during dormancy. Shoot demography and trap plant data suggested that dispersal of primary inoculum occurs shortly after leaf and flower emergence in the spring, and that infection occurs primarily on young and tender tissue and is favored by prolonged rainy periods. Disease progress curves indicated that the disease is active from March through late May, and that initial fruit and leaf infection may occur simultaneously. Additional observations revealed a novel symptom type, lesions on emerging shoots causing girdling and blighting. Initial results suggest that the disease is monocyclic, but further confirmatory studies are needed to fully elucidate disease epidemiology and pathogen life cycle.