During recent growing seasons, a new leaf blight was observed on potatoes (Solanum tuberosum L.) in various production regions in South Africa. Symptoms were observed before early blight, from 50 to 60 days after emergence of the potato plants. Typical leaf symptoms were small, circular, brown lesions, first visible on the abaxial sides of leaves. Lesions resembled those of early blight, but were smaller and did not show concentric rings. During favorable environmental conditions, severe infections were seen as coalesced lesions and blighted leaves and stems. Such severe infections occurred in seasons when high humidity, leaf wetness, and warm temperatures were present. Yield losses as much as 40% were reported on approximately 50 20-ha pivots in various potato-growing regions, particularly Ceres, Eastern Free State, KwaZulu Natal, and Mpumalanga, due to this leaf blight because conventional fungicidal spray programs did not adequately control the disease. Isolations from leaf lesions were made on V8 juice agar under aseptic conditions. The fungus, Alternaria alternata (Fr.) Kreissler, was consistently isolated and preliminarily identified on the basis of morphological characteristics. Dark brown conidia were produced in chains on conidiophores. Conidia had short beaks and ranged from 20 to 60 × 9 to 18 μm. Morphological identification was confirmed by amplification of the internal transcribed spacer (ITS) region. Primers used were AAF2 (5′-TGCAATCAGCGTCAGTAACAAAT-3′) and AAR3 (5′-ATGGATGCTAGACCTTTGCTGAT-3′), specifically designed for identification of A. alternata (4). PCR products were sequenced and the identity of isolates confirmed by a BLAST search on the GenBank database. Koch's postulates were conducted by inoculation of healthy potato leaves of cv. BP1. Spores at a concentration of 106 spores per ml were suspended in an oil/surfactant mixture and sprayed onto leaves until runoff. Control plants were sprayed with a sterile oil/surfactant mixture until runoff. Plants were covered by polyethylene bags for 2 days to achieve high humidity levels and maintained in a greenhouse at 25 ± 2°C. Three days after inoculation, plants were exposed to a moisture regimen simulating that of in-field irrigation. Plants were placed in a fogging chamber twice a week for 1 h at a time. Leaf blight symptoms similar to those observed on diseased potato plants in the field began to develop 3 weeks after inoculation. Isolations made from these lesions consistently yielded A. alternata. Control plants did not develop any symptoms. Five plants were used for each treatment and the experiment was repeated twice. Leaf blight on potatoes caused by A. alternata has previously been reported in Israel, (2), Brazil (1), and North America (3). To our knowledge, this is the first report of A. alternata causing leaf blight on potatoes in South Africa. Future research will focus primarily on management of this disease.
References: (1) L. S. Boiteux and F. J. B. Reifschneider. Plant Dis. 78:101. 1994. (2) S. Droby et al. Phytopathology 74:537, 1984. (3) W. W. Kirk et al. Plant Dis. Manage. Rep. 2:V065:1, 2007. (4) P. Konstantinova et al. Mycol. Res. 106:23, 2002.