| Summary: | The annual legume crop clusterbean (Cyamopsis tetragonoloba (L.) Taub.) is a promising crop for cultivation in the south of Russia. In 2018, phytosanitary examinations of clusterbean fields (13 collection accessions) were conducted in five VIR branches (Krasnodar Territory, Dagestan, Astrakhan and Volgograd Regions). At all points, several types of leaf spots were observed on clusterbean leaves and symptoms of plant damage by fungi of the genus Alternaria Nees prevailed. Using microbiological methods and Next-Generation Sequencing (NGS) of the nuclear ribosomal internal transcribed spacer two (ITS2), the species composition of micromycetes was identified. It was found that the micromycetes Alternaria spp. in different ecological and geographical conditions of the south of Russia cause two main types of leaf spots: the typical (beige and brown round spots, usually accompanied by concentric zonality) and brown spot (small brown bulging merging spots). Overwhelmingly the damage to leaf tissues is caused by the fungus A. tenuissima (Nees & T. Nees : Fr.) Wiltshire. A quite numerous accompanying mycoflora (first of all, fungi of the genus Fusarium Link) was also detected. A. cyamopsidis Rangaswami & A.V. Rao, one of the most harmful guar pathogens in the countries where the main acreage is located (India, Pakistan, USA), was not found on clusterbean fields in Russia. The accessions of clusterbean differ in degree of damage by A. tenuissima that causes Alternaria leaf blight. Differential interaction of parasite and plant host was revealed. Therefore, to prevent epiphytotics, varieties protected by non-identical resistance genes should be grown. At all VIR branches, accessions k-52568 (Argentina) and k-52569 (Pakistan) were weakly damaged by Alternaria leaf blight, and some accessions were resistant only in the environmental conditions of one or two experimental stations. The accessions studied were heterogeneous in pathogen resistance, which allows selecting disease-resistant lines from most of the collection accessions. Thus, in various VIR branches, plants without symptoms of disease were selected from accessions k-52571, k-52573 and k-52580, and seeds were collected to create new donors of disease resistance.
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