Pansy leafspot, caused byCentrospora acerina, host range, and control

Abstract

A serious leafspot disease of pansies and violas has been prevalent in the San Francisco Bay region since 1928. The disease develops only during the cold, wet, winter months. The causal organism has been identified as Centrospora acerina (Hartig) Newhall. On pansy and viola leaves, small, blue-black, spherical to elliptical lesions, which may have a wet, greasy appearance on the lower side of the leaf, are found between, adjacent to, and on the veins and veinlets. Lesions also occur on petioles and inflorescences. Within a day or two after initial infection, the centers of the lesions may become necrotic, exhibiting a light to dark brown color, surrounded by a blue-black ring. Infected plants die promptly. In the greenhouse, infection of pansies and violas was obtained by placing drops of a spore suspension from a pansy isolate on the upper surface of the leaves, atomizing with sterile distilled water, and covering the inoculated plants with glass jars. The incubation period ranged from 24 to 48 hours. Infected plants died within a week. The fungus was reisolated in pure culture, and the reisolates proved pathogenic. In addition to pansy and viola, other ornamental bedding plants and vegetables were both naturally and artificially infected. Susceptible and nonsusceptible plants are listed. The disease on pansy, viola, other susceptible ornamental bedding plants, French endive, narrow-leaf dandelion, and lettuce may be controlled by frequent spraying of the leaves during the rainy season with a copper oleate solution. Bedding plants listed as nonsusceptible may be grown in nurseries and outdoor plantings and require no spraying.