Management of Exobasidium leaf and fruit spot disease of blueberry

The southeastern states currently account for more than one-third of the cultivated blueberry production area in the United States. The increase in acreage and production intensity in the region has been accompanied by the emergence of new fungal, bacterial, and viral diseases. For example, Exobasidium leaf and fruit spot, caused by Exobasidium maculosum, only rarely affected blueberries in the past, but has now become a common problem. This fungal disease causes green or white lesions on the fruit, rendering them unmarketable. Symptomatic fruit are not readily detected on the packing line, slowing down the packing process and sometimes causing fruit loads to be downgraded or rejected. Management options for this disease were unknown. In 2012, an initial fungicide timing trial was conducted to determine the efficacy of early (pre-bloom and bloom), mid-season (petal fall and early cover), late (during fruit maturation), or full (all three periods combined) spray programs. Early and full fungicide schedules provided the best control, whereas mid-season applications provided some disease suppression and late applications did not reduce disease; this indicated an early infection period by the fungus. Captan provided the best efficacy of the materials tested. In 2013, an additional trial further supported the early use of fungicides, and a single late-dormant application of calcium polysulfide was determined to be equivalent to multiple in-season applications of Captan. Calcium polysulfide efficacy when applied at the late-dormant stage was further confirmed as a critical management tool in 2014 multi-county demonstration trials. It is now recommended that Exobasidium-active fungicides be applied during late-dormant and bloom, followed by two to three applications post-bloom in conditions of high disease pressure to further reduce disease levels. These recommendations have been rapidly adopted by blueberry producers throughout the Southeast, and Exobasidium incidence has been reduced significantly as a result.