First Report of Sclerotium rolfsii Causing Collar Rot of Guar (Cyamopsis tetragonoloba) in Mexico.

Abstract

Cyamopsis tetragonoloba (Fabaceae), also known as guar or cluster bean, is an annual legume grown mainly for industrial purposes and also as an ingredient for animal feed. In October 2021, collar rot symptoms were observed in five guar fields located in Guasave, Sinaloa, Mexico. Abundant white mycelium, and later brown and small sclerotia were observed at the base of the stems. Diseased plants showed reduced growth, wilting, and drying of the entire plant. Disease incidence ranged from 15 to 40%. Samples were collected from each field at two phenological stages (vegetative and reproductive). For fungal isolation, symptomatic stems pieces were surface sterilized with 2% sodium hypochlorite for 2 min, rinsed in sterilized distilled water two times, placed on PDA medium and incubated at 28°C in darkness for 3 days. Sclerotium-like colonies were consistently obtained and five isolates from five different fields were purified by the hyphal-tip method. Fungal colonies were white, cottony, and often forming fans. Sclerotia (1 to 2 mm diameter) were white at first and then gradually turned dark brown. Microscopic examination showed septate hyphae with some cells having clamp connections. A representative isolate was deposited in the Culture Collection of Phytopathogenic Fungi of the Faculty of Agriculture of Fuerte Valley at the Sinaloa Autonomous University under Accession no. FAVF647. For molecular identification, genomic DNA was extracted, and the internal transcribed spacer (ITS) region was amplified by PCR and sequenced using the primer pair ITS5/ITS4 (White et al. 1990). The sequence was deposited in GenBank (accession no. OM510466). BLASTn searches in GenBank showed 99.21 to 100% identity with the available sequences of Sclerotium rolfsii (accession nos. MK926446, MH854711, and KY175225). A phylogenetic analysis using the maximum Likelihood method placed isolate FAVF647 in the same clade as S. rolfsii. Pathogenicity tests were performed by inoculating 10 healthy guar seedlings (15-day-old) grown in pots. Four sclerotia were directly placed on the stem base of each plant. Five uninoculated guar seedlings were used as control. All plants were placed in a moist chamber at 25°C with a 12-h photoperiod for 2 days. Collar rot symptoms appeared on inoculated plants after 3 days, whereas control plants remained symptomless. Pathogenicity test was performed twice with similar results. The fungus was reisolated from the artificially inoculated plants, thus fulfilling Koch's postulates. Sclerotium rolfsii has been reported on guar plants in Australia, Brazil, Fiji, India, and the United States (Farr and Rossman 2022). To our knowledge, this is the first report of Sclerotium rolfsii causing collar rot of guar in Mexico. The disease is very common in guar fields in Sinaloa, Mexico, therefore additional studies are needed to develop effective disease-management strategies.