First Report of Alternaria Species Groups Involved in Disease Complexes of Hazelnut and Walnut Fruit

Abstract

During the last 5 years, two new diseases, brown apical necrosis (BAN) and gray necrosis (GN), were observed on English walnut (Juglans regia) and hazelnut (Corylus avellana), respectively (2,3). Both diseases caused severe fruit drop resulting in yield loss often exceeding 30%. Previous work demonstrated that BAN and GN are disease complexes caused by several fungi (Alternaria spp., Fusarium spp., and a Phomopsis sp.) (2,3). In both diseases, preliminary identification of Alternaria spp. revealed they were a complex of small-spored catenulate taxa related to A. alternata. To further characterize these taxa, additional pathogenicity tests and morphological examinations were conducted with isolates obtained from each host. Single-spored isolates were prescreened for pathogenicity by inoculating detached, surface-disinfested hazelnut leaves or walnut leaflets (1). Only isolates that produced foliar lesions after 5 days were used in subsequent fruit inoculations. From this screening, 35 isolates were selected (19 from walnut and 16 from hazelnut). For each isolate, attached fruit of respective hosts were inoculated at bloom by placing 10 μl of a conidial suspension (1 × 106 conidia per ml of H2O + 0.26% agar) onto the stigmas (150 fruit per isolate). Controls (150 fruit) were treated with agar solution only. After 15 days, fruit were examined for development of disease symptoms, and examination continued until fruit maturation (late July). Approximately 20 to 50% of the inoculated fruit displayed discoloration or necrosis of internal tissue, particularly the pericarp and the embryo, although symptoms were more limited than those typically seen in fully expressed BAN and GN. No differences in symptoms were evident among the isolates tested. The controls showed no symptom development initially, although 5% began to develop discoloration at fruit maturity. Fungal isolates used as inoculum were reisolated from all symptomatic fruit by surface disinfesting tissue from the margins of necrotic lesions. For each isolate, the conidial characteristics were described from cultures grown under defined conditions (4). Three distinct groups of isolates were identified. Alternata sp. group isolates produced conidial chains (8 to 20 spores) with numerous secondary and occasionally tertiary chains branching from apical and median cells. Conidia were typically ovate and often possessed a one-celled apical extension. Tenuissima sp. group isolates developed conidial chains (10 to 22 spores) with occasional branching forming secondary chains from apical and median cells. Conidia were ovate to obclavate, often with long apical extensions (10 to 35 μm). Arborescens sp. group isolates developed conidial chains (5 to 12 spores) with numerous secondary, tertiary, and quaternary short chains branching from apical cells. Conidia were typically ovate with minimal apical extensions. Of the walnut isolates, 12, 4, and 3 were from the arborescens, alternata, and tenuissima sp. groups, respectively. Of the hazelnut isolates, 7, 6, and 3 were from the arborescens, alternata, and tenuissima sp. groups, respectively. The finding that Alternaria from several distinct sp. groups can cause similar disease on a single host is consistent with previous work on pistachio, almond, and pear (4). References: (1) A. Belisario et al. Plant Dis. 83:696, 1999. (2) A. Belisario et al. Plant Dis. 86:599, 2002. (3) A. Belisario et al. Inf. Agrario 59:71, 2003. (4) B. M. Pryor et al. Phytopathology 92:406, 2002.