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Title: Infection and development of Phoma medicaginis on moderately resistant and susceptible alfalfa genotypes

item ZEYEN, R
item Samac, Deborah - Debby

Submitted to: Canadian Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/4/2007
Publication Date: 10/1/2007
Citation: Castell-Miller, C.V., Zeyen, R.J., Samac, D.A. 2007. Infection and development of Phoma medicaginis on moderately resistant and susceptible alfalfa genotypes. Canadian Journal of Plant Pathology. 29:290-298.

Interpretive Summary: Plant pathogens that attack alfalfa foliage can cause severe losses in yields and crop quality. The disease spring black stem and leaf spot of alfalfa is one of the most damaging foliar diseases of alfalfa. The fungus causing the disease causes leaf spots and leaf yellowing followed by defoliation and also attacks stems, roots, and the crown of alfalfa plants. Most alfalfa plants are susceptible to the disease. Alfalfa plants were identified that show partial resistance to the disease. The progress of fungal growth and disease symptoms were followed by light and scanning electron microscopy from germination of spores on the leaf surface until production and release of spores resulting from completion of the pathogen's life cycle. In resistant plants all phases of fungal growth and symptom development were delayed compared to disease susceptible plants. The delay in spore germination and advance of the fungus in leaves may be due to production of chemicals by the plant that reduce pathogen growth. An understanding of the different components contributing to disease resistance is essential to developing effective methods for identifying resistant plants and developing disease resistant varieties, which will improve crop yield and quality.

Technical Abstract: In temperate regions of North America, spring black stem and leaf spot, caused by Phoma medicaginis Malbr. & Roum., is one of the most important diseases of alfalfa (Medicago sativa L.). The symptoms of the leaf phase of the disease appear initially as small dark brown to black spots, called tarspots that typically develop without chlorosis, but eventually enlarge covering larger areas that is accompanied by intensive chlorosis and defoliation. The infection process of P. medicaginis on alfalfa has been investigated on susceptible alfalfa genotypes, but little is known about interactions with alfalfa plants having differing degrees of resistance. The frequency of resistance to P. medicaginis within populations of alfalfa clones is low, and effective resistant varieties have not been released. In this study we used light and scanning microscopy to follow the infection and colonization process of P. medicaginis from the time of spore (pycnidiospore) deposition on the leaf surface to pycnidia formation. Alfalfa genotypes with differing levels of resistance were selected from the USDA core collection and used in this study. Older leaves of the moderately resistant genotypes delay spore germination and fungal development, but the initial delay in chlorosis is overcome by 8 days after inoculation.