Blachinski, D., Dept. of Plant Pathology, ARO, Volcani Center, Bet Dagan 50250, Israel
Shtienberg, D., Dept. of Plant Pathology, ARO, Volcani Center, Bet Dagan 50250, Israel
Dinoor, A., Dept. of Plant Pathology, Hebrew University of Jerusalem, Faculty of Agriculture, Rehovot 76100, Israel
Kafkafi, U., Dept. of Field Crops, Hebrew University of Jerusalem, Faculty of Agriculture, Rehovot 76100, Israel
Sujkowski, L.S., Dept. of Plant Pathology, Cornell University, Ithaca, NY 14853, United States
Zitter, T.A., Dept. of Plant Pathology, Cornell University, Ithaca, NY 14853, United States
Fry, W.E., Dept. of Plant Pathology, Cornell University, Ithaca, NY 14853, United States

The hypothesis that enrichment of the foliage with nitrogen and potassium may enhance host resistance to Alternaria and thus reduce disease severity, was examined for potato and tomato (A. solani) and lot cotton (A. macrospora). First, the activity of urea (CO(NH2)2) and the salts NH4NO3, (NH4)2SO4, KNO3 KCl, K2SO4 and KH2PO4 against A. solani and A. macrospora was determined in vitro; each of the compounds tested had a weak inhibitory effect on spore germination of A. solani (ED50 ≤ 1000 μg/l) and on mycelial growth of both A. macrospora and A. solani (ED50 ≤ 10,000 μg/l). Next, the effect of foliar application of selected nutrients on A. solani and A. macrospora was evaluated in vivo on detached leaves of tomato and cotton. The diameter of A. solani lesions on leaves sampled from tomato plants treated with KNO3 was significantly smaller (by 56.5%) than that recorded on leaves sampled from untreated plants. A. macrospora severity on leaves sampled from cotton plants treated with urea was significantly lower than that observed on untreated leaves (70.8% reduction) but KNO3 did not affect disease severity significantly. The following step was to examine the effects of foliar application of urea and KNO3 on Alternaria development in field experiments, two with potato and one with cotton. Foliar application of both nutrients (8-10 sprays in total) did not affect Alternaria severity as compared with the untreated control in any of the experiments. The fungicides chlorothalonil and tebuconazole, on the other hand, significantly suppressed the disease throughout most of the growing season. A mixture of urea or KNO3 with the fungicides did not further improve the effects of the latter when applied alone. Based on these results, it was concluded that foliar application of urea or KNO3 does not affect host response to Alternaria.