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First Report of Ramulispora sorghicola in the United States Causing Oval Leaf Spot on Johnsongrass and Sorghum in Texas

Oval leaf spot (OLS) caused by Ramulispora sorghicola Harris was observed on grain sorghum, Sorghum bicolor (L.) Moench, and johnsongrass, S. halepense (L.) Pers., near Beeville, TX during August 2002. Symptoms were first observed on several sorghum lines and hybrids in a field nursery including a bulk planting of the line ATx623. Highest incidence of OLS occurred in rows adjacent to johnsongrass with symptoms of OLS. Average lesion size (mm) was 1.3 × 2.8 with a range from 0.5 to 2.5 × 1.5 to 5.0. Lesions had a straw-colored sunken center and on red- and purple-pigmented sorghums, lesion borders were highly pigmented. Cone-shaped conidial masses and superficial sclerotia (subglobose, black, 80 to 190 μm in diameter × 50 to 70 μm high, with spiny setae) were sometimes present or readily produced on lesions within 48 to 72 h after placement in humidity chambers. Conidia were branched, filiform, tapered, and 1.1 to 2.4 × 20 to 75 μm. The pathogen, R. sorghicola, was isolated from conidia and sclerotia. A water suspension of culturally derived conidia of R. sorghicola (3 × 10⁴ conidia per ml) was spray inoculated (5:30 p.m., October 11, 2002) onto four or more upper leaves per plant of six grain sorghum plants (ATx623) and approximately nine johnsongrass plants (three tillers each of three plants) at a Corpus Christi field location where OLS was absent. Three grain sorghum and one johnsongrass plant were sprayed with a water control. Cloudy, wet, and cool conditions after inoculation and increasingly cooler nights probably delayed symptom expression until 3 to 4 weeks after inoculation. Typical lesions were observed simultaneously on both hosts with symptoms restricted to inoculated plants. Lesions from both hosts were placed onto water agar at 25°C for 24 h, and the pathogen was reisolated from field-produced conidia of rehydrated conidial masses. Through 2004, OLS was observed on sorghum hosts in 29 counties from central Texas to the Lower Rio Grande Valley. During the growing season, OLS was predominantly absent in grain and forage sorghum fields and absent or often difficult to detect in johnsongrass. In all 3 years, OLS was most common after the normal growing season from August through December with occurrence primarily on johnsongrass but also on late-planted and feral S. bicolor hosts, especially when proximal to symptomatic johnsongrass. Presence and incidence of OLS was highly variable between and within stands of johnsongrass with incidence ranging from a few to most plants. Incidence in forage or grain sorghum fields was highest at field borders adjacent to johnsongrass with OLS. Disease severity was low except on johnsongrass at a few locations. The pathogen appears to pose low economic risk to any sorghum host in Texas at any time of the year although highly susceptible lines and hybrids should be identified and possibly avoided. The previous most proximal report of R. sorghicola in the Western Hemisphere was in Honduras (1). The widespread distribution of OLS across southern Texas and its pattern of occurrence in johnsongrass suggest that the pathogen may have been unobserved in Texas for several years. Presence of OLS near the Rio Grande indicates probable occurrence in johnsongrass at least in some areas along this river in northeastern Mexico.

Reference: (1) G. C. Wall et al. Trop. Pest Manag. 35:57, 1989.