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Abstract, Hilgardia, 1936

Rhizoctonia solani Kuhn has been reported from various localities as the most important fungus associated with the damping-off disease of citrus seedlings.(5)(19)(20)(21)(25) During the last four years this fungus has been constantly isolated from diseased seedlings of sweet orange, grapefruit, and sour orange which were collected in seed beds from various sections of southern California. In the present experiments, the high virulence of the pathogen in sterilized and natural soils has been repeatedly verified. There is an indication from field and greenhouse observations that sour orange is not quite so susceptible to the attack of the fungus as sweet orange.

The disease is readily recognized in the seed bed. The dead seedlings with withered leaves usually remain standing about the margins of the bare areas which mark the centers of infection. At the margins of these areas may be found plants with lesions and apparently healthy seedlings with strands of the fungus hanging like spider webs around the base of the stem. In general, nurserymen are able to avoid damping-off or to keep it within reasonable limits by good cultural practices, such as are discussed by Fawcett and Lee.(5) In some seasons and in some sections, however, the losses due to damping-off have been excessive. In 1934, for instance, 50 to 60 per cent of the seedlings in some beds were affected.