Induction of Growth Increase and Systemic Resistance to Exserohilum turcicum in Maize by Foliar Spray of Phosphates

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Abstract
A single foliar spray of 0.1 M solution of phosphate salts on the upper side of maize (cv. ‘jubilee') leaves 1, 2 and 3 at the 5–6 fully–expanded leaf‐stage, 2‐4 h before inoculation induced systemic resistance to northern leaf blight caused by Exserohilum turcicum. Nine days after challenge, protection was demonstrated by 91% reduction in the size and 69% in the number of E. turcicum lesions developing on leaves 4, 5, 6 and 7. Appropriate mixing of phosphate solution with KOH revealed that the level of protection was not necessarily dependent on the pH of the solution. The size and number of lesions decreased to 62% and 56%, respectively, 12 days after challenge. There was no damage or chlorotic stipling on the induced leaves (1, 2, and 3) as a result of the phosphate spray. There were no significant differences in the reduction in the number or size of the lesions obtained when the foliar spray was applied 2‐4 h or 1, 3, 6, 8, or 10 days before inoculation. One foliar spray ofK2HPO4 on leaves 1, 2 and 3 in these intervals before inoculation, remarkably stimulated growth of inoculated plants, which was expressed by several parameters. The possible dual use of phosphate salts as foliar fertilizers and as resistance‐inducing agents is discussed