Occurrence and Development of Mycosphaerella laricina on Larch in the North Central United States. M. A. Palmer, Research Plant Pathologist, North Central Forest Experiment Station, USDA Forest Service, St. Paul, MN 55108. M. E. Ostry, Research Plant Pathologist, North Central Forest Experiment Station, K. Robbins, Plant Pathologist, Northeastern Area State and Private Forestry, and T. H. Nicholls, Research Plant Pathologist, North Central Forest Experiment Station, USDA Forest Service, St. Paul, MN 55108. Plant Dis. 70:921-923. Accepted for publication 10 April 1986. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1986. DOI: 10.1094/PD-70-921.

Needle cast caused by Mycosphaerella laricina resulted in premature defoliation of European larch (Larix decidua) and hybrid larch (L. × eurolepis) in 31 plantations in Michigan, Wisconsin, and Iowa. Trees from Austrian, French, and Italian seed sources were more severely diseased than those from Poland and Czechoslovakia. The first symptoms developed in early summer. Disease intensified as the season progressed, and trees became completely defoliated as early as July. Spores were dispersed throughout the growing season. Most ascospores were trapped from May through July, and most conidia were trapped in August and September. In a preliminary host range study, European larch was more severely diseased than tamarack (L. laricina) and Japanese larch (L. kaempferi).