Use of a Metalaxyl Tolerant Pythium Oligandrum Isolate for Selectively Following Its Population in Soil and Its Colonization of Roots of Diseased and Non-Diseased Plants

Summary

When encysted zoospores of a metalaxyl-tolerant mutant of Pythium oligandrum were added to moist, unsterile soil at 20° C, 88% germinated within one hour. Most probable number analysis (MPN) showed that the fungus was recovered at one-half to one-third of the original level at 28 d, and it was still detected at 80 d. Growing cress plants did not affect the population, but decaying cress enabled the population to establish at a higher level.

The ability of P. oligandrum to colonize roots of diseased and non-diseased plants was compared. In the first study, the plants were 8 weeks old Exacum affine growing in peat, and the diseased plants were naturally infected by Botrytis sp. in the root collar. Four days after inoculation with P. oligandrum mycelium, the fungus was detected on 10.7% of the diseased root segments, and on 3.9% of the non-diseased root segments. Three weeks after inoculation, P. oligandrum had colonized 55.7% of the root segments of the diseased plants, and only 2.0% of the root segments of the non-diseased plants. In a second study, cress seedlings were placed in unsterile soil, and P. oligandrum was inoculated into the soil as encysted zoospores. The roots utilized were either colonized by P. ultimum or uninfected roots either attached (AR) or removed (RR) from the hypocotyl. After 64 hours, P. oligandrum had not colonized any of the roots at a detectable level. After 7 days, P. oligandrum was growing on 26.2%, 16.3% or 8.6% of the P. ultimum colonized, the RR or the AR segments respectively. This significant difference in the ability of P. oligandrum to colonize roots of diseased and non-diseased plants is discussed.