Non-target effects of agrichemicals applied in the fall or during the dormant season on subsequent development of Exobasidium leaf and fruit spot of blueberry

Abstract

The basidiomycete Exobasidium maculosum causes Exobasidium leaf and fruit spot, an early-season disease of blueberry in the southern United States. Previous research suggested that the pathogen oversummers and overwinters epiphytically on the surface of blueberry bark and buds, most likely in its yeast stage. This is supported by the finding that a single dormant application of lime sulfur provides substantial control of the disease the following spring. As a surface resident for much of the year, E. maculosum is thus exposed to various horticultural and pest management practices which may affect its epiphytic population dynamics and impact subsequent disease development. The magnitude and consistency of such non-target effects was evaluated in multi-year field trials on susceptible ‘Premier’ and ‘Tifblue’ rabbiteye blueberry in southern Georgia. One fall application of stylet oil (timed to mimic sprays for blueberry bud mite control) or two fall applications of captan (timed to coincide with the management window for fungal foliar diseases) did not suppress Exobasidium leaf and fruit spot the following spring, whereas two fall applications of lime sulfur reduced the disease significantly (by 74.3% on average), albeit not to the same level as the late dormant lime sulfur standard (close to 90% average reduction across leaf and fruit spot intensity). A single late dormant spray of the growth regulator hydrogen cyanamide (a caustic compound with antimicrobial activity) suppressed disease intensity in most trials, but disease reduction was only partial (averaging 56.2% for leaf lesion density and 39.3% for fruit spot incidence). In contrast, a late dormant application of dormant oil consistently increased both leaf lesion density (by 80.7% on average) and fruit disease incidence (by 78.6%). The pronounced non-target effects associated with these off-season applications could explain variations in Exobasidium leaf and fruit spot risk among blueberry sites and years.