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Johnsongrass (Sorghum halepense (L.) Pers.), broadleaf signalgrass (Brachiaria platyphylla (L.) Beauv.), and yellow foxtail (Setaria glauca L.) are common volunteer grasses in bermudagrass (Cynodon dactylon (L.) Pers.) pastures in the southeastern United States. Johnsongrass and broadleaf signalgrass are potential forages whereas yellow foxtail is a noxious weed. In 1999 and subsequent years, necrosis and dieback of leaves, stems, and roots, stunting, and plant death were observed on all three species in bermudagrass pastures in north Mississippi (3). Symptoms on johnsongrass and yellow foxtail were most severe where bermudagrass exhibited severe symptoms of infection caused by dematiaceous hyphomycetes (2,3); symptoms on broadleaf signalgrass often occurred independently. Symptomatic leaf tissues from 15 to 33 plants of each species and stem and root tissues from 4 to 14 plants of johnsongrass and yellow foxtail were surface disinfested, plated on water agar, and examined for sporulation after 5 to 10 days (2,3). Pathogens were identified by specific morphological features of spores and sporulation as on bermudagrass (3), and axenic cultures were established by spore transfers to cornmeal agar. Bipolaris cynodontis (Marig.) Shoemaker, Curvularia lunata (Wakk.) Boedijn, C. geniculata (Tracy & Earle) Boedijn, and Exserohilum rostratum (Drechs.) Leonard & Suggs were isolated from symptomatic leaves of all three grasses and frequently also observed on stems and roots. B. stenospila (Drechs.) Shoemaker was observed only on broadleaf signalgrass (19 of 33 plants) and B. spicifera (Banier) Subr. on johnsongrass and yellow foxtail. Species most frequent on leaves (58 to 100%) were B. spicifera, C. lunata, and E. rostratum on johnsongrass and yellow foxtail and B. cynodontis, B. stenospila, and E. rostratum on broadleaf signalgrass. The three grasses were grown from seed in potting mix in the greenhouse (one plant per 375-cm3 container), and five replicates 31 to 60 days old were inoculated with a mixture of three isolates of each pathogen observed on them in two experiments. Conidia produced from infested wheat and oat grain were atomized onto foliage (1.2 to 4 × 104 conidia per ml, 20 ml per plant) as described (2). All pathogens incited similar necrotic lesions and streaks on the three grasses after 12 to 15 days, and B. stenospila also caused extensive golden yellow chlorosis on broadleaf signalgrass. All pathogens caused significant (P = 0.05) necrosis (means = 5 to 35% of foliage necrotic based on visual estimates, controls = 1 to 3%), and all were reisolated and grown in pure culture by spore transfers to cornmeal agar from surface-disinfested, symptomatic leaf tissue of each grass. When bermudagrass grown from seed was inoculated at similar spore concentrations, isolates of E. rostratum, B. cynodontis, and B. spicifera from two or all three grasses caused symptoms as severe as did isolates from bermudagrass. Results document new North American or worldwide records of occurrence and pathogenicity for B. cynodontis, C. geniculata, and C. lunata on all three grasses, B. stenospila and E. rostratum on broadleaf signalgrass, and B. spicifera on johnsongrass and yellow foxtail (1). These volunteer grasses, bermudagrass, and the six fungi all appear to represent large, interacting complexes of multiple hosts and potentially cross-infecting pathogens. Reference: (1) D. Farr et al. Fungal Databases. Systematic Botany and Mycology Laboratory. Online publication. USDA, ARS, 2005. (2) R. Pratt, Agron. J. 92:512, 2000. (3) R. Pratt. Phytopathology 95:1183, 2005. |
