Myrothecium leaf spot of cotton caused by myrothecium roridum

Abstract

Myrothecium leaf spot caused by Myrothecium roridum Tode ex Fr. was observed on cotton fields in Maranhão State, Brazil, causing yield reduction of up to 60%. Disease symptoms are lesions with concentric necrotic rings, with salient structures (sporodochia) irregularly distributed. Symptoms were observed on petioles, bracts, leaves and bolls of cotton cultivars Deltapine Acala 90, Fibermax 966 and Sure Grow 821. The pathogen was isolated and cultivated on potato-dextrose-agar (PDA) medium. The pathogenicity was evaluated on detached healthy bolls from cotton cv. Fibermax 966 (R6 vegetative stage), previously disinfested. Thirteen isolates of M. roridum were tested, which included 8 from cotton and 5 from soybean. Fungal structures were evaluated by using light microscope with ocular micrometer. The isolates infected cotton bolls and the MA-75 was the most aggressive strain. In average lesion diameter is 1,3cm long at 7 days after inoculation, and 2,7cm at 14 days. They also formed sporodochia concentrically disposed on PDA. Conidia are hyalines to dilute olivaceous, single-celled, abundantly produced in a dark green to black mass. Spores from cotton isolates are 5,1µm x 1,5µm in size, and from soybean isolates are 5,8µm x 1,5µm. This is the first report of Myrothecium leaf spot in Brazilian cotton fields.