Induction of Systemic Resistance to Anthracnose in Cucumber by Phosphates

Publisher Website
Google Scholar
Abstract
Solutions of K3PO4, K2HPO4, NA3PO4, and NA2HPO4 sprayed on the undersides of the first and second true leaves of cucumber [Cucumis sativus L.] induced systemic resistance in leaves 3 and 4 to anthracnose caused by Colletotrichum lagenarium. Solutions of KH2PO4, NAH2PO4, CaHPO4, (NH4)2HPO4, and NH4H2PO4 were less active, and a suspension of Ca3PO4 was inactive. Induced resistance in leaves 3 and 4 depended on the concentration of K3PO4 applied to leaves 1 and 2. Spraying leaves 1 and 2 each with 1-2 ml of a solution of K3PO4 at concentrations of 100, 50, 10, 5, and 1 mM protected leaves 3 and 4 99, 96, 78, 54, and 15%, respectively. The level of protection was based on the total necrotic lesion area of plants sprayed on leaves 1 and 2 with water. A pH greater than 7.0 was required for high activity of potassium phosphates, and activity of di- and tripotassium phosphates was markedly reduced at lower pHs. Induction of systemic resistance, however, was not solely a result of an alkaline pH, because 50 mM potassium hydroxide (pH 11.7) was inactive. Induced systemic resistance was associated with the gradual appearance of chlorotic and necrotic stippling on leaves 1 and 2. The lack of stippling or rapid death of leaves 1 and 2 was associated with little or no induced systemic resistance. Induced systemic resistance in newly developing leaves above leaves 3 and 4 was apparent for at least 5 wk in greenhouse and outdoor tests. These data suggest that induced systemic resistance to disease caused by infection is not due to a specific component of the pathogen, but rather to the persistence of a low level of metabolic perturbation. One cause of such perturbation may be the sequestering of calcium ions.