External Infection Mechanism of Hypocotyls and Cotyledons of Cowpea Seedlings by Macrophomina phaseolina. C. J. deMooy, Department of Agronomy, Colorado State University, Fort Collins 80523. D. W. Burke, Department of Agronomy, Colorado State University, Fort Collins 80523. Plant Dis. 74:720. Accepted for publication 24 May 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0720D.
Infection of cowpea (Vigna unguiculata (L.) Walp.) seedlings by Macrophomina phaseolina (Tassi) Goid., known as ashy stem blight or charcoal rot, was studied in replicated pot experiments. Plants were grown in steam-pasteurized soil drenched with suspensions of M. phaseolina microsclerotia before planting. Infection occurred underground in emerging cotyledons and hypocotyls. Cotyledons showed dark, necrotic lesions upon unfolding after emergence. Shallow, pinhead size hypocotyl lesions became necrotic 2-3 days later. Roots were colonized completely but appeared healthy. Cross sections through hypocotyls at 1-cm intervals and radial thin sections made down to the pith between lesions were plated out on Rose Bengal rice agar. No evidence was found of internal growth of the fungus from the cortical lesions or from the roots. Mycelium of M. phaseolina was not detected in microscopic examinations of stem pith, phloem, and xylem. The absence of mycelial interconnections supports direct external infection at the site of the lesions. Most lesions remained dormant up to 1 mo. Some lesions developed later, and plants died. Blight symptoms that appear under field conditions when plants are in drought or are approaching maturity may be due to activation of dormant hypocotyl lesions or internal growth of the fungus from infected roots.