Factors Affecting the Response of Pomegranate Fruit to Alternaria alternata, the Causal Agent of Heart Rot.

Heart rot, caused by Alternaria alternata, is a major pomegranate disease that impacts production worldwide; most fruits in orchards are colonized by A. alternata but, nevertheless, symptoms are apparent on only a small proportion of the colonized fruits. During the years of our previous research it was noticed that within individual orchards, the incidence of pomegranate fruits exhibiting heart rot symptoms was related to the visual appearance of the trees: trees that appeared visually frail bore more diseased fruits than robust trees. Furthermore, it was noticed that the disease responses of different pomegranate accessions and possibly of different variants of the same cultivar varied markedly. The specific objectives of the present study were: (i) to characterize the relationship between the visual appearance of pomegranate plants or individual stems and the incidence of heart rot and their vulnerability to heart rot; and (ii) to elucidate factors affecting the response of pomegranate fruit to A. alternata. Analysis of heart rot incidence in four orchards in 2014 revealed large differences in heart rot incidence among trees growing side by side in the same orchard; these differences were related to the visual appearance of the pomegranate trees. There were significant differences among germination rates of A. alternata spores in juice prepared from asymptomatic fruits originating from these trees, and comparable differences were found among the acidity levels (pH) of the juices. These differences may reflect differences among the physiological responses of pomegranate trees to heart rot. Fruits collected from the pomegranate collection located in Newe Ya'ar, which comprised 95 accessions in 2015 and 110 accessions in 2016, were also examined. There were differences among the acidity levels (pH) of the juices produced from these fruits and among the germination rates of A. alternata spores in the juices. These differences may reflect variances among the genetic responses of pomegranate accessions to heart rot. Results of studying the relationship between the acidity levels of pomegranate juice and the germination rates of A. alternata spores supported the hypothesis that, apart from pH, a compound(s) present in the juice regulates the germination of A. alternata spores in the juice.