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Abstract
In North America, spring black stem and leaf spot, caused by Phoma medicaginis, is one of the most important diseases of alfalfa (Medicago sativa). Symptoms appear initially as small, dark brown to black spots that eventually enlarge and on leaves the spots are associated with chlorosis and defoliation. The infection process of P. medicaginis was previously investigated on susceptible alfalfa genotypes, but little is known about this process with alfalfa plants with differing degrees of resistance. We used light and scanning electron microscopy to follow infection and development of P. medicaginis from pycnidiospore deposition on leaf surfaces through pycnidia formation on selected susceptible and resistant alfalfa genotypes from the USDA core collection. In both old and young leaves, spore germination, penetration, development of mycelium, and pycnidia formation were delayed by approximately 24 h on moderately resistant alfalfa genotypes compared with susceptible genotypes. A similar delay in visible symptom development occurred until about 192 h after inoculation, after which the amount of chlorosis in all genotypes was similar.