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Fungi and Nematodes in South African Citrus Orchard Soils in Relation to the Citrus Replant Problem¹

ABSTRACT

Trichoderma 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.

¹ Contribution from the Department of Soils and Plant Nutrition, University of California Citrus Exp. Sta., Riverside, and Citrus and Subtropical Horticultural Research Sta., Nelspruit, East Transvaal, Union of South Africa. Paper No. 1229, University of California Citrus Exp. Sta.

² Chemist, Department of Soils and Plant Nutrition. The work reported was carried out while on sabbatical leave in South Africa. The author expresses his appreciation to the South African Department of Agriculture and Dr. R. H. Marloth, Director of Nelspruit Research Station, and Mr. O. T. Van Niekerk and the South African Co-operative Citrus Exchange for the invitation to spend a sabbatical leave in South Africa; to L. Rosselet, F. le Roux, C. J. Barnard, I. Mathews, T. Coates, and others who provided assistance when needed; to the field officers of the South African Citrus Exchange who gave generously of their time in showing the author the citrus areas of the Union; and to E. C. Calavan and S. D. Van Gundy of the University of California Citrus Exp. Sta. who identified Phytophthora species and additional nematode species.

Received for publication March 11, 1960. Accepted for publication April 4, 1960.