Effects of chemicals and microbial antagonists on nematodes and fungal pathogens of citrus roots

Abstract

In pot and field experiments in naturally infested soil, commercial products formulated with microbial antagonists, applied to soil, performed poorly against the nematodes Tylenchulus semipenetrans and Paratrichodorus lobatus, and the root rot fungi Phytophthora nicotianae and Pythium ultimum on citrus, compared with the conventional chemicals aldicarb, cadusafos and metalaxyl. Prosper Nema did not reduce nematode levels even when combined with a sawdust mulch over the soil. Nutri-life 3/20 inhibited citrus growth and caused leaf chlorosis. Tri-D25 did not reduce root rot or isolation frequency of Phytophthora nicotianae and Pythium ultimum. However, a commercial formulation (Actizyme) of Bacillus subtilis stimulated citrus growth, and the herbicide oryzalin both stimulated citrus growth and reduced isolation frequency of Pythium ultimum. Potassium silicate did not reduce isolation frequency of Phytophthora nicotianae and Pythium ultimum or root rot, however, it did reduce levels of T. semipenetrans in soil. Cadusafos and aldicarb were equally effective in controlling T. semipenetrans on roots but cadusafos was more effective in soil and against Paratrichodorus lobatus. Aldicarb stimulated citrus growth, while cadusafos plus metalaxyl was more effective than metalaxyl alone in stimulating citrus growth and preventing leaf chlorosis in soil infected with Phytophthora nicotianae. Results suggested that Phytophthora nicotianae was much more damaging to citrus than Pythium ultimum, and that nematodes (in association with Fusarium solani) were also more damaging than P. ultimum. Relatively low initial combined levels of T. semipenetrans and Paratrichodorus lobatus depressed citrus growth; reproduction rates for P. lobatus exceeded 23-fold the initial level. Cadusafos (3 applications at 3 g/m2 over 6 months) reduced T. semipenetrans to very low levels, but did not eliminate the nematode; mechanical incorporation of nematicide granules improved the level of nematode control after the first, but not succeeding, applications.