Effects of calcium salts in controlling Phytophthora pistaciae, the causal agent of pistachio gummosis

Abstract

The ability of calcium chloride and calcium sulfate to control pistachio gummosis caused by Phytophthora pistaciae was evaluated both in vitro, on solid and liquid media, and in vivo, on broad bean, a potential annual host of the pathogen, and seedlings of three pistachio cultivars. In vitro, all concentrations of the salts (1.5, 4, 8 and 15 mM) reduced mycelial growth, dry weight of mycelial mats, and sporangial production of the pathogen. Calcium chloride reduced the size and volume of sporangia, antheridia and oogonia. Calcium sulfate also reduced the size and volume of sporangia, but it had less impact on the volume of oogonia and increased the size of antheridia and the volume of oospores. Moreover, concentrations of both calcium salts higher than 1.5 mM reduced zoospore discharge and induced cyst germination. In vivo, application of calcium salts before and after inoculation of sterilized soil with P. pistaciae reduced the mortality of broad bean plants and different cultivars of pistachio trees compared to the respective controls. Analyses using atomic absorption showed calcium concentrations to increase in the foliage of Ca-treated broad bean and pistachio plants. Based on the results, the application of calcium chloride to P. pistaciae infested soils could be more advantageous than calcium sulfate to control P. pistaciae gummosis in commercial pistachio crops.