Systemic resistance against Sphaerotheca fuliginea in cucumber plants exposed to phosphate in hydroponics system, and its control by foliar spray of mono-potassium phosphate

P-containing nutrient solutions at concentrations of 5, 20 and 40 ppm, appl ied through a hydroponics system, provided induced systemic resistance (ISR ) against Sphaerotheca fuliginea in young cucumber plants. Protection was e xpressed as a significant reduction (up to 92% compared with control) in th e mildewed leaf area. ISR was also expressed as a 53-91% reduction in the n umber of S. fuliginea conidia per infected leaf area as determined on leave s 1(first true leaf) and 2, at 9 days after inoculation, and on leaves 1-3 at 12 days after inoculation. A concentration of 20 ppm P in the hydroponic s solution was found to be optimal for ISR. The optimal P concentration enh anced the uptake and increased the content of Ca in the leaves by 50% compa red with the controls. Once the fungus had been established, root-applied P did not affect well-developed colonies, but, foliar application of 1% solu tion of mono-potassium phosphate (MKP) effectively protected the foliage ag ainst powdery mildew, regardless of the P concentration in the nutrient sol utions. This treatment was persistent up to 21 days after inoculation, it s ignificantly inhibited powdery mildew development and caused a reduction of 72.3 % in sporulation of the fungus as compared with the control. The resu lts suggest different mechanisms for the two phenomena and highlight the ro le of P in ISR. (C) 2000 Elsevier Science Ltd. All rights reserved.