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First Report of Alternaria alternata Causing Leaf Spot on English Walnut

A foliar disease of English walnut (Juglans regia L.) was observed in 1998 in an orchard in northern Italy. Symptoms consisted of circular, necrotic spots bordered with concentric zones of darker tissue. Average lesion diameter was 20 mm, although lesions could extend to half of the leaflet lamina. A fungus identified as Alternaria alternata (Fr.:Fr.) Keissl. (1) was observed on the necrotic tissue and was consistently isolated from the margins of the necrosis. Conidia from leaves were brown, ellipsoid to ovoid (primary conidia 28 to 60 × 8 to 15 μm; secondary conidia 10 to 30 × 7 to 13 μm), with walls often ornamented, 1 to 6 transversely septate and 0 to 3 longitudinally septate. For pathogenicity tests, three isolates were grown on potato dextrose agar for 2 weeks. Inoculations were performed on detached, surface-sterilized, healthy J. regia leaflets. Four drops (5 μl each) of a sterile water suspension of 1 × 10⁵ conidia per ml were placed on each leaflet; three leaves per isolate were used. Leaves were incubated in a moist chamber. After 10 days, leaf spots similar to the original symptoms developed on all the inoculated points for all three isolates, and the pathogen was reisolated. Control leaflets inoculated with sterile, distilled water remained symptomless. The experiment was performed three times and the results were similar. Alternaria alternata is a well-known pathogen on many crops but a few records report this fungus as a causal agent of leaf spot on deciduous trees. This is the first report of A. alternata on English walnut.

Reference: (1) E. G. Simmons. Mycotaxon 37:79, 1990.