Author : Sombat Srichuwong

Summary : Colletotrichum truncatum had been recognized as a soybean seed pathogen for over 40 years, causing pre- and post-emergence dampiing-off of seedlings and seedling blight. Although much is known of the effects of this pathogen on soybean seed quality and vigor, fundamental studies on the pathogen, its seed-borne nature, seed transmission and control were rather limited. Present studies were thus focused on these issues. Detection and isolation of this pathogen was carried out in order to understand the nature of the seed-borne pathogen. In the examination of seed-borne microorganisms of soybean (Glycine max (L.) Merill cultivar Palmetto, SJ4 and SJ5 collected from Malaysia and Thailand, nine genera composed of 13 fungi species were associated with the seeds. Among these fungi, Collototrichum truncatum incidence was recorded in the range of 2.5-6.5 percent. The blotter method proved more suitable for detecting C. truncatum than agar plate method. It was confirmed that C. truncatum infection reduced seed quality and seedling vigour. Infestation of soybean seeds with conidial suspension or culture suspension significantly reduced percentage of seed germination, seedling emergence, and seedling height in in vitro and glasshouse studies. The fungus caused seed rot, pre- and post-emergence damping-off and lesions on the hypocotyls and cotyledons of young seedlings. Seeds infected by C. truncatum contained more soluble protein but less soluble carbohydrate than that of healthy seeds. Although the electrophoretic patterns were qualitatively identical between infected and healthy seeds, the relative amount still varies quantitatively. This supported the observation that C. truncatum resulted in seed deterioration and reduced germinability. In addition, infected seeds had lower activity of soluble isozymes such as esterase, peroxidase, acid phophatase, and alkaline phosphatase than healthy seeds. Soybean plants were found to be more susceptible to C. truncatum during the maturing stage.