EPIDEMIOLOGY AND PREDISPOSING FACTORS OF SOME MAJOR BACTERIAL DISEASES OF STONE AND NUT FRUIT TREES SPECIES
Despite the basic importance of studying the epidemiology of bacterial pathogens for their effective control in the field, for many pathogens causing diseases of economic importance there is still a large body of knowledge to acquire. In this minireview the main epidemiological aspects and predisposing factors of some important diseases of stone and nut tree fruit species caused by bacteria are summarized. Apricot (Prunus armeniaca L.), peach (P. persica Batsch.), sweet cherry (P. avium L.), sour or tart cherry (P. cerasus L.) and European plum (P. domestica L.) are the stone fruit trees species that can be severely attacked by two pathovars of Pseudomonas syringae worldwide: P. syringae pv. syringae (Pss) and P. syringae pv. morsprunorum (Psm). The high sensitivity of some recently introduced apricot cultivars would seem a main predisposing factor enhancing the aggressiveness of Pss. Sandy and very clayey soils as well as soils characterized by a low calcium content can also contribute to enhance the sensitivity of apricot and peach trees to Pss. Xanthomonas arboricola pv. pruni is the causative agent of bacterial spot threatening peach, apricot, Japanese plum (P. salicina) and almond (Amygdalus communis L.). This pathogen is present worldwide and is currently spreading in many European countries. A relative humidity of 100% over a period of three days has been shown fundamental for the appearance of the disease in peach leaves. Xanthomonas arboricola pv. juglandis is the causal agent of walnut blight and fruit apical necrosis often severely attacking Juglans regia L. Recent in-depth studies showed that this pathogen starts the infection process at the apex of the fruit. Fungi play a secondary role as saprophytes or opportunistic pathogens in this apical necrosis of fruits. Pseudomonas avellanae is the causal agent of bacterial canker and decline of hazelnut (Corylus avellana L.). So far, this destructive disease has been reported solely from northern Greece and central Italy. Soil of volcanic origin with pH values <5.0 can increase the susceptibility of hazelnut to bacterial canker.