Effect of heat-stress predisposition on the development of Scytalidium wilt of ‚Star Ruby’ grapefruit, caused by Scytalidium lignicola

Abstract

Scytalidium wilt, caused by Scytalidium lignicola, has become prevalent on ‚Star Ruby’ grapefruit in orchards in the Jordan Valley, an area with a warm climate in the north of Israel. It occurs in the summer in certain years, soon after extreme hot and dry weather conditions have prevailed for several consecutive days, but not in other years with regular summer temperatures. The effect of temperature conditions before and after inoculation with S. lignicola on disease development on ‚Star Ruby’ was studied in greenhouse chambers with three day/night temperature regimes: ‚Very Hot’ (47 °C/34 °C); ‚Hot’ (36 °C/28 °C); and ‚Moderate’ (30 °C/20 °C). Among the pre-inoculation regimes, ‚Very Hot’ was most conducive to infection, whereas the ‚Hot’ regime sustained canker development only when followed by a ‚Very Hot’ post-inoculation regime. The moderate pre-inoculation conditions appeared to have a negligible, if any, effect on canker development, even with a ‚Very Hot’ post-inoculation regime. Wilt developed in infected saplings if they were exposed to the ‚Very Hot’ temperature regime either pre- or post-inoculation, but did not develop under the cooler conditions. Saplings of ‚Star Ruby’ exposed to a ‚Very Hot’ regime developed heat-stress symptoms, similar to those observed on ‚Star Ruby’ in the Jordan Valley. Under a constant ‚Very Hot’ regime, both canker expansion and subsequent foliar wilt developed on ‚Flame’, but not on ‚Marsh Seedless’ or ‚Rio Red’ grapefruit. The study confirmed an hypothesis that predisposition induced by extremely hot temperature is a prerequisite for infection of susceptible hosts by S. lignicola.