Blueberries rank as the second most important berry crop in North America with a total area of 96,869 ha (Strik, 2006). As blueberry acreage increases, pathogen diversity and diseases become a more important issue. Stem blight, caused by the species complex of Botryosphaeria Moug.:Fr and Neofusicoccum, Crous, Slippers and A. J. L. Phillips, is considered one of the most common diseases limiting the establishment of blueberry plantings in the southeastern United States (Ballington et al., 1993; Wright and Harmon, 2010). In addition to blueberry, these causal fungi infect other woody plants, thus increasing the likelihood that field-grown blueberry plants will be exposed to inoculum of these pathogens. Species of Botryosphaeria and Neofusicoccum infect blueberry plants through wounds and spread from infected plants to healthy plants by wind, rain, and field equipment (Milholland, 1972). Disease symptoms include reddening and drying of leaves, wilting of infected shoots, and plant death as the pathogen spreads (Polashock et al., 2017; Wright and Harmon, 2010). Incidence of stem blight has been increasing in the United States, and the disease has been reported in many other countries, including Mexico (Boyzo-Marin et al., 2016), New Zealand (Tennakoon et al., 2018), and China (Yu et al., 2012). Various management strategies have been recommended to reduce the severity of the disease, including irrigation management (Creswell and Milholland, 1988), use of resistant cultivars and clean planting stock, and selective pruning of infected parts (Weaver, 1978). Information about fungicide sensitivity could be used to reduce the severity of the disease when applied after pruning or injury to plants. Efficacy of fungicides to control the growth of B. dothidea was determined in a previous study using a detached stem assay. Stems treated with cyprodinil, fludioxonil, and pyraclostrobin developed lesions shorter than those of nontreated control (Smith and Miller-Butler, 2017b).
For U.S. producers to maintain competitiveness, new and highly productive cultivars possessing enhanced tolerance to biotic and abiotic stresses are needed. However, identifying the causal pathogen(s) is critical before developing resistant cultivars as well as implementing effective management and quarantine strategies. Breeding for disease resistance relies on accurate identification of the causal pathogens before incorporating genes for resistance. Resistance of certain half-high and lowbush blueberry cultivars to Botryosphaeria has been reported (Polashock and Kramer, 2006). Southern highbush blueberry (species complex between V. corymbosum L. 2n = 4x = 48 and V. darrowii Camp 2n = 2x = 24) breeders have used various native Vaccinium species in crosses to incorporate genes for adaptation; however, little is known about Neofusicoccum species identification and the responses of native Vaccinium species to infection. Morphological differentiation between different species of Botryosphaeria and Neofusicoccum may not be accurate because isolate pigmentation varies with temperature (Wright and Harmon, 2010). Differentiation based on multigene sequence analyses has proven to be a more reliable method of differentiation between species (Wright and Harmon, 2010). The objectives of our research were to 1) characterize six isolates of stem blight pathogens based on comparison of the DNA sequences of internal transcribed spacer (ITS) and elongation factor (EF1-α), 2) determine the optimal temperature for the growth of these isolates, 3) characterize the efficacy of several fungicides in controlling the growth of these isolates, and 4) conduct pathogenicity tests on 39 accessions from different Vaccinium species.
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