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An Epidemiological Study of Mummy Berry Disease of Highbush Blueberry. D. C. Ramsdell, Assistant Professor, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824; J. W. Nelson(2), and R. Myers(3). (2)Director of Research, Michigan Blueberry Growers Association, Grand Junction 49056; (3)Laboratory Technician, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824. Phytopathology 64:222-228. Accepted for publication 5 September 1973. DOI: 10.1094/Phyto-64-222.

A Burkard recording volumetric spore trap was operated in a highbush blueberry field in Michigan from 19 April to 15 June 1972. Ascospores of Monilinia vaccinii-corymbosi (incitant of mummy berry disease) were trapped from 28 April to 11 May, a period during which the bushes progressed from green tip to pink bud prebloom stage. Ascospore discharge occurred mostly during daylight when relative humidity (RH) was <100%; very few spores were trapped at night when RH was 100%. Ascospore discharge was inversely correlated with RH and wind speed (P = <0.001 and <0.01, respectively). Nocturnal periods of continuous leaf wetness ranged from 5.5 - 12.0 h which, according to germination studies on glass slides, could allow primary infection to occur. Bushes had a mean of 92 visible leaf and shoot infections by 23 May. Triarimol sprays applied 72 h after the first major ascospore discharge on 4 May, while bushes were still at the green tip stage, and again on 15 May, were much more effective in reducing primary infection than sprays applied only on 15 May. Trapping of conidia commenced 19 May, when bushes were at ca. 20% pink bud prebloom stage, and ended 3 June at petal fall. Large numbers of conidia were trapped both during day and night. Release of conidia was inversely correlated with leaf wetness (P = <0.1) and directly correlated with wind speed (P = <0.1). Continuous leaf wetness at night ranged from 1.5 - 12.0 h during the period conidia were trapped.