Zinnia acerosa (D.C.) Gray, is a Southwest native flowering plant frequently observed in the Trans-Pecos desert and desert grasslands of Arizona, Texas, and New Mexico. The plant is valued for producing an abundance of distinctive 2.0 cm diameter white flowers on greenish and sparsely leaved stems. Selections of Z. acerosa from West Texas are under evaluation as a water conserving plant species for use in arid landscapes of the Southwest. During April through June, Z. acerosa plants in TAMU, Dallas field plots were observed with small, brown flower spots that enlarged to include whole petals, causing conspicuous flower blighting. Microscopic examination of lesions from infected flower blossoms demonstrated the presence of short beaked, cylindrical spores near the smaller lesions on flower petals. Isolations from symptomatic flower petals consistently yielded cultures of an Alternaria sp. with long chains of conidia. Typical conidia contained 3 to 5 transverse walls and 1 to 2 longitudinal walls and measured 43 μm length by 15 μm width. The fungus was identified as A. alternata (Fries) Keissler consistent with the description in Ellis (1). Pathogenicity tests were conducted on plants maintained on a greenhouse bench by spraying spore suspensions obtained from 16-day-old A. alternata cultures on potato-dextrose agar. Inoculations produced light brown lesions on blossoms typical of field disease symptoms of the disease. This disease is important as a flower blight, however, infections on the leaves are also apparent but limited. Outbreaks of the disease are frequently observed during periods of rainfall during the summer months. This is the first report of Alternaria alternata causing a floral blight on Z. acerosa.
References: (1) M. B. Ellis. 1971. Dematiaceous Hyphomycetes. Commonwealth Mycological Institute, Kew, Surrey, England.