Variation in the Severity of Mummy Berry Disease among Lowbush Blueberry Clones
Lowbush blueberry production is important for the economy of Maine and the Maritime provinces, and mummy berry disease, caused by Monilinia vaccinii- corymbosi, often reduces plant stand vigor and fruit production. The severity of mummy berry disease among clones of lowbush blueberry were measured in four fields over a two year period (2001 and 2002). In each of 37 clones,lO flowering stems with and without symptoms of blight were randomly selected at flowering to represent "diseased" and "healthy" stem populations, respectively. The severity of leaf and flower blight and fruit mummification was measured for each of these stems, and the relationships between blight and fruit mummification, adjusted fruit set, and berry weight was examined. There were significant differences in the severity of leaf blight among clones within each field, but significant differences in the severity of flower blight and fruit mummification among clones were observed in only half of the fields. The average severity of leaf blight per clone (the average proportion of leaves with symptoms of blight on "diseased" stems) was consistently correlated with the average incidence of blight (the average proportion of blighted stems within a clone). However, there was not a consistent relationship between the average severity of flower blight and the average incidence of blight. Furthermore, there was no relationship between the severity of leaf blight and the severity of fruit mummification. In some fields, as the severity of leaf blight increased, adjusted fruit set and average berry weight decreased, which may justify current attempts to reduce leaf blight through chemical and cultural controls. In order to examine the possible relationship between pathogen virulence and the severity of leaf blight, isolates of M. vaccinii-corymbosi were obtained from four clones of lowbush blueberry with different amounts of blight, and in vitro pectinase activites of these isolates was examined. Significant differences in pectinase production were observed among isolates, but pectinase activity of isolates did not account for differences in blight severity among clones. To examine whether host factors contribute to the severity of mummy berry blight, ten "phenology" stems were randomly selected in each of 27 clones prior to bud break in 2002. The development of leaf and flower buds on each "phenology" stem was examined weekly until Julian day 145, and the height of each stem was recorded. The average severity of leaf blight per clone decreased as the average height of stems within the clone increased, but the relationship was not significant in all fields. However, the severity of leaf and flower blight on "phenology" stems increased with the developmental stage of their leaf and flower buds, respectively, on Julian days 131 through 139. Clones with less susceptible tissue during ascospore release had less blight than clones with more susceptible tissue, which suggests that avoidance or escape may account for differences in the severity of blight among clones. However, differences in inoculum density and host biochemical resistance may have also been involved.