Pythium irregulare Associated with Pinus Seedling Death on Previously Cultivated Lands. C. LINDE, Department of Microbiology and Biochemistry, University of the Orange Free State, P.O. Box 339, Bloemfontein, 9300, South Afric. G. H. J. KEMP, and M. J. WINGFIELD, Department of Microbiology and Biochemistry, University of the Orange Free State, P.O. Box 339, Bloemfontein, 9300, South Africa. Plant Dis. 78:1002-1005. Accepted for publication 23 February 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-1002.

A serious root disease of Pinus patula seedlings has occurred in the northeastern Cape forestry region of South Africa during the past 3 yr. Mortality as high as 100% was experienced 4-5 mo after seedlings were planted on previously cultivated agricultural lands. No mortality, however, occurred in plantings on virgin lands. Where seedlings survived on previously cultivated lands, their growth compared poorly with those on virgin lands. Pythium irregulare was consistently isolated from diseased roots of Pinus patula, as well as from the soil of previously cultivated lands. P. irregulare was highly virulent when artificially inoculated onto 4-mo-old Pinus patula and 2-mo-old Eucalyptus grandis seedlings. P. irregulare, therefore, appears to be an important factor associated with deaths of Pinus patula on previously cultivated agricultural lands.