Sclerotium Germination and Histopathology of Monilinia vaccinii-corymbosi on Highbush Blueberry

MILHOLLAND, R. D. 1977. Sclerotium germination and histopathology of Monilinia vaccinii-corymbosi on highbush blueberry. Phytopathology 67: 848-854. Germ tubes from mature ascospores penetrated succulent primary tissue through which the fungus entered the ovary. blueberry leaves either directly through the epidermis or Sclerotium morphology, germination and apothecium indirectly through stomates. The fungus hyphae in the leaf, development also were studied. Mature sclerotia required flower, and fruit tissues grew both intraand intercellularly chilling below 7 C for a minimum of 900 to 1,200 hr for and caused cellular collapse and necrosis. The stigmatic normal apothecium development. tissue that surrounds the stylar canal appeared to be the Additional key words: Vaccinium corymbosum, histopathology. Monilinia vaccindi-corymbosi (Reade) Honey (6) is a 2,160 at a Bladen County farm during the winter of 1968pathogenic fungus of considerable economic importance 1969 to 1,280 in Pender County during the winter of 1973in most highbush blueberry (Vaccinium corymbosum 1974. Since apothecium development and ascospore L.) -producing areas (4, 12, 13). The principal damage on discharge normally coincide with bud break and new highbush blueberry is a fruit rot resulting in mummified shoot growth, the chilling requirements appear to be fruit. However, blighting of twigs and leaves in similar for the host and pathogen. southeastern North Carolina is more devastating on These studies were conducted (i) to determine the rabbiteye blueberry (V. ashei) than is the fruit rot. chilling requirements necessary to initiate sclerotium Longyear (9) first reported the mummy berry disease germination and development of normal apothecia; and occurring on wild Vaccinium species in Michigan and (ii) to examine the penetration and infection of blueberry identified the causal organism as Sclerotinia vaccindi leaves, flowers, and fruits by Monilinia vacciniiWor. Later Reade (16) reported that the mummy berry corymbosi. fungus differed from S. vaccinii in host range and sizes of ascospores and conidia, and named the fungus S. MATERIALS AND METHODS vaccinii-corymbosi. Honey (6) established the genus Monilinia to include those members of the genus Overwintered sclerotia of M. vaccinii-corymbosi were Sclerotinia commonly possessing a monilioid collected on 21 November, and 22 December 1975,froma macroconidial stage and a pseudosclerotium, and highbush blueberry farm located in Bladen County, reclassified the organism as Monilinia vaccinii-corymbosi North Carolina, where a planting of the cultivar Croatan (Reade) Honey. was severely infected with the fungus the previous year. In southeastern North Carolina, overwintered sclerotia Two hundred dormant sclerotia were selected for of M. vaccinii-corymbosi break dormancy around the 1st germination studies each month. A 300-g sample of a wk in February and develop mature apothecia about 1 mo nonsterile highbush blueberry soil with a moisture later (11). Growth of the apothecium initial(s) from the content of 31% was placed in a 150 X 25 mm glass petri sclerotium is favored by a cool temperature (5 C), the dish. Soil was oven-dried and the percent moisture mature apothecium develops at a higher temperature (16 content (SMC) was determined on a dry weight basis (5) C). and recorded. Half of the sclerotia collected each month Highbush blueberry plants require a minimum of 800 were soaked in distilled water for 72 hr, the other half to 1,200 hr below 7 C during the winter in order to bloom were not soaked. All the sclerotia (in lots of 25), were and leaf normally in the spring (3). According to placed in eight dishes and sealed in a 0.025-mm (1-mil) Mainland (10), the number of hours below 7 C in recent polyethelene bag to hold the SMC constant. One dish years in southeastern North Carolina has ranged from containing soaked sclerotia and one dish with nonsoaked sclerotia were placed in a Sherer-Gillett CEL 25-7HL Copyright © 1977 The American Phytopathological Society, 3340 constant temperature chamber at 16 C with a 12 hr Pilot Knob Road, St. Paul, MN 55121. All rights reserved. daylength at a light intensity of 9.6 X l03 lx. The