APICAL NECROSIS OF PERSIAN (ENGLISH) WALNUT (JUGLANS REGIA): AN UPDATE

Abstract

In recent years, a new disease of Persian (English) walnut (Juglans regia L.) causing premature fruit drop and important yield losses has been observed in orchards of Mediterranean walnut-growing regions. Characteristic disease symptoms consist of an apical necrosis originating at the stigmatic end of the hull. In early stages of fruit development, differentiation of “apical necrosis” from “blight”, another walnut disease, requires a detailed observation of externally and internally affected tissues. Different approaches have been used to characterize disease symptoms in affected walnut-growing areas and to identify the causal agents and predisposing factors. Recent studies indicate that the bacterium Xanthomonas arboricola pv. juglandis is the microorganism most frequently associated with apical necrosis and may produce the initial infections in young nuts. Fusarium spp. and Alternaria spp. seem also to be involved in the induction of apical necrosis, causing secondary infections, or growing as saprophytes on bacterial- infected tissues, thus enhancing disease symptoms and severity. Nutritional problems and soil characteristics could be partially related to apical necrosis, in addition to microbial infection. Many aspects of apical necrosis are, however, still unknown and management strategies cannot be defined, so the disease is not readily controlled. Etiological and epidemiological studies on apical necrosis are discussed in this review in order to integrate information and contribute to a better understanding of the disease and its predisposing factors.