Epidemic Development of Hawthorn Leaf Blight (Monilinia johnsonii) on Mayhaw (Crataegus aestivalis and C. opaca) in Georgia.

Mayhaws are small trees and shrubs in the hawthorn genus, Crataegus. They are native to the southern United States, where their fruit is highly valued for use in jellies and preserves. Since 1997, symptoms of hawthorn leaf blight, caused by Monilinia johnsonii, have been observed in mayhaw orchards in southwestern Georgia. We studied epidemic development of the disease in a mixed planting of Crataegus aestivalis (eastern mayhaw) and C. opaca (western mayhaw) between 2000 and 2002. Apothecia of M. johnsonii were first observed in early to mid-February on overwintered, mummified fruit of C. aestivalis; no apothecia were detected in plots underneath C. opaca trees. Both mayhaw species exhibited moderate to severe leaf blighting beginning in early March, although some genotypes within each species apparently escaped primary infection via delayed leaf bud break or a slower rate of leaf expansion. On a per-tree basis, leaf blight incidence was positively correlated with mean leaf length during the period when apothecia were most numerous (r = 0.7225, P = 0.0003, n = 20). Fruit mummification, which results from secondary infection of open flowers by conidia, was widespread by late March to early April and was significantly (P < 0.05) more severe on C. aestivalis, most likely because trees of this species were at an earlier bloom stage when conidia-bearing blighted leaves were first observed. By contrast, C. opaca advanced through bloom earlier, thereby partly escaping secondary infection. On a per-tree basis, there was no relationship between incidence levels of leaf blight and fruit infection for either species; indeed, some trees with the lowest incidence of leaf blight had the greatest incidence of fruit mummification and vice versa. Thus, in a mixed planting of different mayhaw genotypes, conidia appear to be dispersed readily from heavily blighted trees, leading to high levels of fruit infection even in trees with negligible incidence of leaf blight. There are genotypes within both mayhaw species that almost completely avoid either primary infection or secondary infection; therefore, planting such genotypes in pure stands may aid in minimizing losses due to the disease.