Detection of Myrothecium Leaf Spot, A New Disease of Watermelon

Abstract

Leaf spots were first observed on watermelon (Citrullus vulgaris Schrad) under polyethylene film-covered green-house in November 2002. Symptoms appeared as dark-brown circles or large irregular spots on the leaves of watermelon. Occasionally, zonal growth of the lesions was observed. Under humid conditions, small black sclerotium-like bodies (sporodochia) were produced on the surface of the lesions. The sporodochia on leaf lesions were sessile, polymorphic, variable in size, 35-850 <TEX>$\mu\textrm{m}$</TEX> in diameter, and 30-470 <TEX>$\mu\textrm{m}$</TEX> in depth. Conidia in sporodochium were black in mass, one-celled, rod-shaped, with rounded ends, hyaline, guttulate, and measured 6-8<TEX>$\times$</TEX>1.6-2.2 <TEX>$\mu\textrm{m}$</TEX> in size. The pathogen was identified as Myrothecium roridum Tode ex Fr. This is the first report of Myrothecium leaf spot on watermelon naturally occurring in commercial greenhouses.