INVOLVEMENT OF FLUORESCENT PSEUDOMONADS AND OTHER MICROORGANISMS IN INCREASED GROWTH-RESPONSE OF PLANTS IN SOLARIZED SOILS

Enhancement of plant growth in the absence of known pathogens was studied in solarized soils and, to a limited extent, in methyl bromide- and in metham-sodium-treated soils. Increased growth response, expressed by increased dry weight of tomato plants in treated over untreated soils in the greenhouse, was evident in most of the 24 tested soils from various locations in Israel. Increased growth also was evident in field experiments and in an artificially heated soil. Regression analyses showed a significant, inverse relationship between soil pH and increased growth and between soil pH and population densities of fluorescent pseudomonads in the rhizosphere. Solarization reduced bacterial and fungal population densities to a depth of 90 cm, whereas actinomycetes were less affected. Thermotolerant bacteria and fungi also were reduced by solarization. Population densities of fluorescent pseudomonads were increased up to 130-fold in the rhizosphere of plants in solarized soils, although these bacteria are heat sensitive. Solarization drastically reduced population densities of total fungi in the rhizosphere and roots, especially Penicillium pinophilum, which causes plant stunting in greenhouse tests, and Pythium spp. Isolates of fluorescent pseudomonads, identified as Pseudomonas putida, P. fluorescens, and P. alcaligenes, which were recovered from solarized soil, stimulated growth of tomato plants. Solarization increased the frequency of recovery of bacteria showing antagonistic activity from the rhizosphere and roots of tomato plants.