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Melanconium fuligineum and the Bitter Rot Disease of Grape. W. H. Ridings, Graduate Assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27607; C. N. Clayton, Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27607. Phytopathology 60:1203-1211. Accepted for publication 16 March 1970. DOI: 10.1094/Phyto-60-1203.

Melanconium fuligineum infections result in bitter rot of maturing berries and small necrotic flecks on succulent plant parts of muscadine grape (Vitis rotundifolia). Mycelial growth and spore germination occurred at temp from 8 to 36 C and at pH values as low as 2.3. Spores germinated poorly in water unless yeast extract or casein hydrolysate was added. Mycelial growth occurred in media containing dl-malic and d-tartaric acids at concn as high as 2.4%. Isolates differed somewhat in rates of growth in liquid media and in symptoms and sporulation on infected berries. No perfect stage of M. fuligineum was found. Inoculations in the greenhouse with spores in distilled water resulted in heavy flecking of immature plant parts and very light flecking of mature plant parts. However, flecking and necrosis of mature leaves resulted from inoculations with spores in yeast extract (0.4%). Penetration of immature leaves by germinating spores appeared to be direct by means of an appressorium. Necrotic areas developed on immature or mature leaves wounded and inoculated with mycelial discs or spores. In laboratory and field inoculations, a wound was necessary for the infection of nearly ripe or ripened bunch and muscadine berries using spores or mycelial discs. The rate of bitter rot development in wound-inoculated berries at temp from 12 to 36 C was most rapid at 28 C. No consistent differences in bitter rot development were noted among isolates or between varieties. M. fuligineum rotted fruits of grape, apple, cherry, peach, strawberry, blueberry, and banana. It produced the enzymes polygalacturonase and trans-eliminase in vitro. Early season flecking of immature berries was implicated in the subsequent bitter rot development of mature berries. All fungicides except Benlate were more effective in inhibiting spore germination than in suppressing mycelial growth. Maneb, Dithane M-45, and ferbam were the more effective fungicides in inhibiting mycelial growth and spore germination.