Effect of SoilbornePseudomonasspp. on the Biological Control Agent,Trichoderma hamatum,on Pea Seeds

Abstract

In some soils, T. hamatum, applied as conidia to peas, colonizes seed coats and protects them from seed rots caused by Pythium spp. However, it fails to protect seeds in New York [USA] soils with low Fe (< 4 .mu.g/g of soil) availability. This failure is caused by antagonism by fluorescent soil-borne pseudomonads that colonize seed coats rapidly. Short, rod-shaped bacteria were observed in association with lysed germlings of T. hamatum on treated seeds after planting in soil. Addition of pseudomonads to seeds treated with T. hamatum caused T. hamatum to be ineffective as a biocontrol agent when these seeds were planted in steamed soil containing Fe at 1 .mu.g/g of soil and amended with P. ultimum. Conversely, planting seeds treated with both T. hamatum and pseudomonads in quartz sand with 7 .mu.g of extractable Fe/g of soil resulted in no decrease in efficacy of T. hamatum. Increasing Fe levels in field soil to 8 .mu.g of Fe/g of soil by the addition of ferrous oxalate resulted in effective control of seed rots by T. hamatum. Ferric EDTA was ineffective because it was toxic to T. hamatum. Various inorganic Fe salts were ineffective when added as seed treatments; extraction of these seeds 5 days after planting gave low Fe levels (< 4 .mu.g/g). Growth of T. hamatum together with pseudomonads on a medium with little available Fe (King''s B) resulted in fluorescent zones around pseudomonad colonies that were inhibitory to T. hamatum. Growth of the fungus was inhibited by addition of partially purified fluorescent pigment to cultures in King''s B broth; this inhibition could be overcome by addition of 100 .mu.M Fe2+ or Fe3+. Seed-colonizing pseudomonads grow well even if seeds are treated with antibacterial antibiotics. Seeds treated with the T. hamatum and germinated in the absence of pseudomonads for 12-18 h and then transplanted to field soils were protected against seed rot.