The influence of germinating seeds and developing roots on re-establishment of micro-organisms in fumigated soil

A laboratory study was made of re-invasion of soils fumigated in closed containers at rates of 3 ml./sq.ft. 10 in. deep with chloropicrin, methyl bromide and a 67:33 mixture of the two materials (MBC–33), using the dilution plate method of determination. Counts after aeration, 3 weeks later, showed that all three fumigants eliminated practically all fungi. Chloropicrin and MBC–33 also killed almost all bacteria and actinomycetes, whereas fumigation with methyl bromide markedly increased numbers of bacteria without materially altering numbers of actinomycetes. Subsequently, in soils fumigated with chloropicrin or MBC–33, fungi recolonized rapidly, but up to 93 % of the colonies isolated were Trichoderma viride. Increase of fungi in soils fumigated with methyl bromide was slower and Penicillium spp. predominated. Bacteria at first declined in numbers, then increased rapidly in soils fumigated with all three materials. In soils fumigated with methyl bromide, actinomycetes, usually considered to include good antagonists, increased but did not exploit their initial advantage over fungi. In soils fumigated with chloropicrin and MBC–33 actinomycetes did not recolonize in appreciable numbers. The greatest variety of fungous genera occurred in isolates from rhizo-spheres of red beet, but the greatest numbers of colonies of both fungi and bacteria were isolated from rhizospheres of pea and pumpkin. None of the crops tested greatly stimulated recolonization by actinomycetes. Compared with the drastic changes brought about by fumigants the influence of seed and root exudates on either the magnitude or the composition of re-invading micro-organisms was slight.