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Influence of NO3/NH4 Ratio, N, K, and pH on Root Rot of Viola x wittrockiana Caused by Thielaviopsis basicol . W. E. Copes, Graduate Student, University of Georgia, Athens 30602-7274. Floyd F. Hendrix, Professor, Department of Plant Pathology, University of Georgia, Athens 30602-7274. Plant Dis. 80:879-884. Accepted for publication 17 April 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0879.

Fertility treatments were applied every 2 days in three factorial experiments: (i) N (52, 105, 158 g of N per ml) ? NO3/NH4 ratio (1:0, 3:1, 1:1, 1:3); (ii) NO3/NH4 ratio (3:1, 1:3) x K (113, 213, 313 g of K per ml); and (iii) pH range (4.6 to 4.8 and 6.2 to 6.4) x NO3/NH4 ratio (1:0, 2:1, 1:3) to pansy plants grown in sand culture. Inoculum (noninoculated and inoculated with T. basicola) was a factor in all three experiments. The NO3/NH4 ratio was the dominant nutritional factor that reduced disease. Disease incidence, presence on root segments, was lowest with high NH4 (1:3 NO3/NH4 ratio) and a low level of K (113 g/ml) when the NO3/NH4 ratio was 1:3, compared with other NO3/NH4 ratios. pH had no effect on disease incidence in sand medium (cation exchange capacity [CEC] = 0.225), presumably because element availability from sand surfaces into substrate solution was low. High disease incidence corresponded with reductions in plant growth. Plant growth of noninoculated plants was greatest at the 3:1 NO3/NH4 ratio. Plant growth of inoculated plants was greatest at the 3:1, 2:1, or 1:1 NO3/NH4 ratios with disease being moderate at the 2:1 and 1:1 ratios, compared with other ratios. Plant growth was not affected by pH or K at the levels tested.

Keyword(s): black root rot