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

Abstract P-containing nutrient solutions at concentrations of 5, 20 and 40 ppm, applied through a hydroponics system, provided induced systemic resistance (ISR) against Sphaerotheca fuliginea in young cucumber plants. Protection was expressed as a significant reduction (up to 92% compared with control) in the mildewed leaf area. ISR was also expressed as a 53–91% reduction in the number of S. fuliginea conidia per infected leaf area as determined on leaves 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 hydroponics solution was found to be optimal for ISR. The optimal P concentration enhanced the uptake and increased the content of Ca in the leaves by 50% compared with the controls. Once the fungus had been established, root-applied P did not affect well-developed colonies, but, foliar application of 1% solution of mono-potassium phosphate (MKP) effectively protected the foliage against powdery mildew, regardless of the P concentration in the nutrient solutions. This treatment was persistent up to 21 days after inoculation, it significantly inhibited powdery mildew development and caused a reduction of 72.3% in sporulation of the fungus as compared with the control. The results suggest different mechanisms for the two phenomena and highlight the role of P in ISR.