Augmentation of soil with sporangia of Actinoplanes spp. for biological control of Pythium damping-off

A method was developed for applying strains of Actinoplanes spp. that are hyper-parasites of oospores of Pythium ultimum to soil for reducing Pythium damping-off of plants. The method is based on the augmentation of soil with sporangia of a strain of Actinoplanes spp. borne on clay granules. In vitro sporulation of strains K30, W57, W257 and 25844 was: (1) greater for most strains on dilute Czapek-Dox agar than on four other agar media; (2) inhibited by continuous exposure to fluorescent light of intensity 4-150 mu E m(-1) s(-1), but not by exposure to 1 mu E m(-2) s(-1) or darkness; (3) greater at 20-30 degrees C than at 10 degrees C; and (4) greater at pH 6-7 than at pH 5 or 8. On solid carriers treated with dilute Czapek-Dox broth (pH 7) and incubated in the dark at 30 degrees C for 3 weeks, strains sporulated poorly or not at all on vermiculite, perlite and rice hulls, but sporulated abundantly (10(7)-10(9) colony-forming units (CFU) g(-1) of granules)on montmorillonite clay granules. When strains 25844, W57 and W257 were applied as granules (4 X 10(7) - 4 X 10(8) CFU g(-1)) at 5% (w/w) to field plots infested with 750-1000 oospores of P. ultimum g(-1) of soil, only strain 25844 consistently increased emergence and reduced root rot of table beets at 24-28 days after planting compared with controls. Strain 25844 (10(8) CFU g(-1) of granules) at 1% (w/w) also increased the emergence of bush beans at 28 days after planting in P. ultimum-infested plots, but lower rates were ineffective. The inoculum viability of strain 25844 on clay granules declined 100-fold during 2 months of storage at 5-35 degrees C, but thereafter remained stable for another 4 months. Strain 25844 on 6-month-old granules retained a high degree of hyper-parasitic activity toward oospores of P. ultimum. Augmentation of field soil with sporangia of Actinoplanes spp. is a valid approach to the biological control of pythium damping-off.