Rice (Oryza sativa L.) plants in experimental plots in Beaumont, TX developed symptoms on leaf blades similar to white leaf streak caused by Mycovellosiela oryzae (Deighton & Shaw) Deighton (synonym Ramularia oryzae Deighton & Shaw) (1,3,4) during the late summer of 2009. Symptoms were observed on several rice cultivars and breeding lines including Cocodrie, Cypress, and Lemont. Lesions usually appeared on lower leaves and were approximately 2 to 7 mm long and linear with whitish or grayish centers surrounded by a narrow brown-or-dark brown margin. Symptoms were similar to narrow brown leaf spot caused by Cercospora janseana (Racib.) O. Const. (synonym C. oryzae Miyake) (3,4), but the centers of lesions were slightly wider (up to 2 mm). Symptoms were visible on the upper and lower leaf surfaces. Some lesions on heavily infected leaves were long (10 to 15 × 0.5 to 2 mm) whitish streaks parallel to the midrib. Leaves with typical symptoms were collected and incubated in a petri dish lined with moistened filter paper for 3 to 4 days at room temperature under a 12-h fluorescent photoperiod. Conidiophores were produced on external mycelium growing out through stomata on the lesion surface. Conidiophores were hyaline, straight, and 7 to 22 μm long and 2 to 3 μm wide with conidial scars. Conidia were washed from the lesions, diluted in sterilized distilled water, and placed on acidified potato dextrose agar. After 6 to 7 days of incubation at room temperature, slow-growing colonies were transferred onto potato dextrose agar (PDA). Three isolates were obtained from single-spore cultures. The colonies of these isolates grew similarly and very slowly on PDA and their radial growth averaged 0.8 mm/day at room temperature. The colonies were dense, grayish, and did not produce any pigments. Conidia were formed singly or in chains and measured 12 to 30 μm long. They were hyaline, straight, cylindrical, typically with no or one septum, a few with two to three septa, and had a hilum and tapered ends. Pathogenicity of these three isolates was assessed in greenhouse tests by spraying a conidial suspension (105 conidia/ml) onto 12 plants of each of the cvs. Cocodrie, Cypress and Lemont at the late tillering stage. Inoculum was obtained by harvesting conidia from the colonies grown on PDA for 3 weeks at room temperature under a 12-h fluorescent photoperiod. Plants sprayed with sterilized distilled water served as the controls. All plants were maintained in a humid chamber for 2 days and then grown in a greenhouse at 20 to 31°C. After 15 days, early lesions appeared on inoculated leaf blades; after 4 weeks, typical symptoms similar to those observed in the field developed. Control plants did not exhibit any symptoms. M. oryzae was reisolated from symptomatic plants, confirming that the disease was caused by this pathogen. To our knowledge, this is the first report of white leaf streak on rice in Texas and the second report after Louisiana (2). This disease has been reported in Papua New Guinea, the Solomon Islands, North Borneo, Sierra Leone, and Nigeria (3,4).
References: (1) F. C. Deighton and D. Shaw. Trans. Br. Mycol. Soc. 43:515, 1960. (2) A. K. M. Shahjahan et al. Plant Dis. 82:1282, 1998. (3) B. C. Sutton and A. K. M. Shahjahan. Nova Hedwigia 25:197, 1981. (4) R. K. Webster and P. S. Gunnell. Compendium of Rice Diseases. The American Phytopathological Society, St. Paul, MN, 1992.
