Fungi and Nematodes in South African Citrus Orchard Soils in Relation to the Citrus Replant Problem

Abstract

AbstractTrichoderma viride, Aspergillus fumigatus, Penicillium javanicum, P. decumbens, and P. lilacinum constituted dominant fungus species in South African virgin soils which were present in much smaller numbers in adjacent citrus soils. The most dominant forms in old citrus soils were Fusarium solani, Fusarium spp. including F. oxysporum, Penicillium chermesinum, Pullularia pullulans, and Pyrenochaeta sp. Aspergillus niger, Mucor spp. and many others were present in about equal abundance in both old citrus and virgin soils. Phytophthora species were isolated from about a third of the old citrus soils but were not found in virgin soils. The citrus root nematode, Tylenchulus semipenetrans, was found in all but one of the old citrus soils examined but was not detected in the virgin soils. Nematodes isolated from both type soils included Trichodorus sp., Xiphinema sp., Rotylenchus sp., and numerous saprophytic species. Helicotylenchus sp. was noted in virgin soil only. The magnitude of the citrus replant problem in South Africa varied greatly but, in general, appeared to be less severe in well‐drained acid sandy soils containing relatively large numbers of the fungus, Trichoderma viride.