Infection of alfalfa pollen by Sclerotinia sclerotiorum

Blossom blight, caused by Sclerotinia sclerotiorum, has become an important disease of alfalfa (Medicago saliva L.) in seed production areas of western Canada. Studies using light microscopy and scanning and transmission electron microscopy revealed that pollen grains of alfalfa are susceptible to infection by S. sclerotiorum. Ascospores of S. sclerotiorum germinated readily in water with or without pollen grains. Examinations of ascospore-pollen mixtures incubated at room temperature (20-22 degrees C) for 5 days revealed that numerous pollen grains were infected by S. sclerotiorum by direct hyphal penetration through the equatorial germinative pores or through the exine and intine layers of the pollen wall without the formation of infection cushions or appressoria. After penetration, hyphae ramified within the pollen grains, causing plasmolysis of the cytoplasmic membrane and eventual disintegration of the pollen cytoplasm. The study suggests that alfalfa pollen may play a role in the epidemiology of blossom blight in alfalfa.