During March of 2006, in Famailla, Province of Tucuman, Argentina, a new foliar disease was observed on Vaccinium corymbosum L. cv. O'Neal causing severe premature leaf drop. Symptoms consisted of circular to irregular reddish brown necrotic spots varying from specks to mature lesions of 10 to 15 mm in diameter. Larger spots frequently exhibited a zonate pattern with lighter centers and darker margins that are surrounded by a black, oily halo that is consistent with target spot (3). Lesions often coalesced to form irregularly shaped necrotic areas that reached the petioles. Leaf abscission occurred soon after >50% of the foliar area was affected. No stem lesions were observed. Conidiophores were erect, brown, single or in clusters, one to seven septa, 6 to 9 × 70 to 684 μm (shorter in culture), and arose on the abaxial surface of diseased leaves; conidia were borne singly or in chains of 2 to 5, varying from cylindrical to broadest at the base and tapering toward the apex, straight to slightly curved, 3 to 20 pseudosepta, 75 to 330 × 5 to 9 μm (mean of 153 × 7 μm, shorter in culture), with conspicuous hilum. On potato dextrose agar (PDA), the rate of growth was moderate (colonies reach 25 mm in diameter after 7 days grown at 27°C) with conidia production 5 days after the culture was started. Microscopic characters and cultural patterns conformed to the description of Corynespora cassiicola (Berk & M. A. Curtis) C.T. Wei (2,4) and were identical to isolates of C. cassicola from lesions of soybean target spot. A conidial suspension (104 conidia/ml) of C. cassicola was prepared from a 2-week-old culture grown on PDA and sprayed on surface-disinfected excised blueberry stems, each with 10 healthy leaves per stem. The stems were placed in hydroponia and incubated at 27 ± 2°C with permanent light. All sprayed leaves showed symptoms within 3 days following inoculation. Within 5 days, leaves were conspicuously infected, soon followed by defoliation. C. cassicola was reisolated from the inoculated-leaves lesions and was identical morphologically to the original isolate sprayed on test leaves. Another Corynespora species, C. arctespora (Cooke & Ellis) Carris (1), was described on Vaccinium spp. stems with a distinctive feature of a phialidic synanamorph formation. This synanamorph character was not found in our isolate. C. cassiicola has a broad hosts range, but to our knowledge, has not been recorded before on V. corymbosum as the causal agent of target spot.
References: (1) L. M. Carris. Mycotaxon 30:127, 1987. (2) L. S. Olive et al. Phytopathology 35:822, 1945. (3) J. B. Sinclair. Target spot. Page 27 in: Compendium of Soybean Diseases. G. L Hartman et al., eds. The American Phytopathological Society. St. Paul, MN, 1999. (4) J. A. Spencer and H. J. Walters. Phytopathology 59:58, 1969.
