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Effect of Soil Salinity and Water Content on Stem Rot Caused by Phytophthora citrophthora and Accumulation of Phytoalexin in Citrus Rootstocks. L. Sulistyowati, Department of Botany, La Trobe University, Bundoora, Victoria 3083, Australia, Permanent address: Faculty of Agriculture, Brawijaya University, Malang, Indonesia; P. J. Keane, Department of Botany, La Trobe University, Bundoora, Victoria 3083, Australia. Phytopathology 82:771-777. Accepted for publication 4 March 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/Phyto-82-771.

Both high soil salinity and water content increased the severity of stem rot caused by P. citrophthora in the citrus rootstocks troyer citrange, sour orange, and rough lemon. As high salinity did not stimulate growth of the pathogen in vitro, the increase in disease under saline conditions probably was not attributable to a direct effect of salt on the fungus but to a reduction in the resistance of the host. Severity of stem rot was positively correlated with relative stem water content (θs) as affected by soil water content but was negatively correlated with θs as affected by soil salinity. Therefore, although soil water content may affect disease directly through an effect on θs, salinity probably does not increase disease through an effect on θs. High salinity apparently causes reduced accumulation of the phytoalexin, 6,7-dimethoxycoumarin, and therefore increases susceptibility of plant tissues to invasion by the fungus. Concentration of 6,7-dimethoxycoumarin was not affected by soil water content.

Additional keywords: resistance mechanisms, environmental stress, stress physiology.