Damping-off of Sweetgnm by Pythium sylvaticum

During the spring of 1964, preand postemergence damping-off killed many sweetgum (Liquidambar styraciftua L.) seedlings in a nursery at Stoneville, Miss. Fusarium solani (Mart.) Appel & Wr., Sclerotium rolfsii Sacc., Talaromyces spiculisporum Benjamin, and Pythium sylvaticum Campbell & F.F.Hendrix were isolated from the diseased seedlings. At soil temperatures similar to those prevailing when damping-off occurredat or slightly below 20 C-only P. sylvaticum appeared to be virulent in preliminary tests. Therefore, its pathogenicity on sweetgum was evaluated. MATERIALS AND METHODs.-Twelve pairs of 36 X Sl-cm metal flats containing soil 10 cm deep were autoclaved at 121 C. Five hundred sweetgum seeds were then sown in rows in each flat, and 7.5 g of inoculum were introduced into one flat in each pair. The other served as a check. Inoculum was prepared by growing a pure culture of P. sylvaticum on sterilized oat seeds for 1 month. It was mixed with the soil in the rows of planted sweetgum seeds at 2.S-cm intervals. Sterile oats were distributed in the check flats. . In March, August, and September 1965, pairs of flats were placed in a greenhouse where temperatures varied from 17 C at night to 40 C in the daytime. Relative humidity varied from 40 to 100%. In September, an additional two pairs of flats were placed in a growth chamber maintained at 20 C. Twelve hr of light/day at 800 ft-c were provided. In all, there were 12 replications (pairs of flats). After 21 days in the growth chamber or greenhouse, the surviving seedlings were counted. Survival was expressed as a percentage of the number of seeds sown. Reisolations of the fungus from tissues of dead seedlings were attempted. RESULTS AND DISCUSSION.-P. sylvaticum caused damping-off in the seedlings. Over all the tests, survival averaged 33% in flats inoculated with the fungus and 66% in the check flats. The difference was significant (P = < .05). The original fungus was consistently isolated from diseased tissues of dead seedlings. This is the first report of pathogenicity for the recently named heterothallic species of Pythium (1).