INFLUENCE OF NO3 NH4 RATIO, N, K, AND PH ON ROOT-ROT OF VIOLA X WITTROCKIANA CAUSED BY THIELAVIOPSIS-BASICOLA/

Fertility treatments were applied every 2 days in three factorial expe riments: (i) N (52, 105, 158 mu g of N per mi) x NO3/NH4 ratio (1:0, 3 :1, 1:1, 1:3); (ii) NO3/NH4 ratio (3:1, 1:3) x K (113, 213, 313 mu g o f K per mi); 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 1: basicola) was a factor in all th ree experiments. The NO3/NH4 ratio was the dominant nutritional factor that reduced disease. Disease incidence, presence on root segments, w as lowest with high NH4 (1:3 NO3/NH4 ratio) and a low level of K (113 mu g/ml) when the NO3/NH4 ratio was 1:3, compared with other NO3/NH4 r atios. pH had no effect on disease incidence in sand medium (cation ex change capacity [CEC] = 0.225), presumably because element availabilit y from sand surfaces into substrate solution was low. High disease inc idence corresponded with reductions in plant growth. Plant growth of n oninoculated plants was greatest at the 3:1 NO3/NH4 ratio. Plant growt h of inoculated plants was greatest at the 3:1, 2:1, or 1:1 NO3/NH4 ra tios with disease being moderate at the 2:1 and 1:1 ratios, compared w ith other ratios. Plant growth was not affected by pH or K at the leve ls tested.