First report of anthracnose on sugar beet (Beta vulgaris) caused by Colletotrichum incanum in Michigan, USA

Authors: Hanson, Linda; SCHLACHTER, EMMA - Michigan State University; MINIER, DOUGLAS - Michigan State University; BYRNE, JAN - Michigan State University; WILBUR, JAIME - Michigan State University

Submitted to: New Disease Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/17/2023
Publication Date: 2/8/2023
Citation: Hanson, L.E., Schlachter, E.M., Minier, D.H., Byrne, J., Wilbur, J.F. 2023. First report of anthracnose on sugar beet (Beta vulgaris) caused by Colletotrichum incanum in Michigan, USA. New Disease Reports. 47(1). Article e12152. https://doi.org/10.1002/ndr2.12152.
DOI: https://doi.org/10.1002/ndr2.12152

Interpretive Summary: Sugar beet anthracnose is a disease known in Japan and Canada, but not previously found in the United States. In 2016, and again in 2021 and 2022, lesions were observed on sugar beet that were not typical of the commonly occurring diseases in the Michigan growing region. Under high humidity, spore masses were produced with pointed sterile hyphae poking up through the masses, typical of fungi in the genus Colletotrichum, the causal agent of anthracnose. Isolates were identified by morphology and using molecular techniques as Colletotrichum incanum, a species recently identified causing anthracnose on soybean in the US. When inoculated on sugar beet leaves, pure cultures produced similar lesions to those seen in the field, and the fungus was re-isolated from these lesions. Older reports gave the causal agent for beet anthracnose as C. dematium, but that was before several newer species were identified in this group. Additional work is needed to determine whether the new observation is similar to older reports of beet anthracnose in other production regions.

Technical Abstract: Starting in 2016, atypical foliar lesions were observed on sugar beet in Michigan. On the petiole, lesions were elongated and brown. On the leaf blade, lesions were circular to irregular with light and dark rings, similar to those caused by Phoma leaf spot, however, at high humidity, acervuli formed in the lesions with dark setae and masses of cream-colored spores present in both lesion types. Single spore isolates showed characteristics of a curved-spore Colletotrichum sp. When inoculated on beet leaves, similar lesions were formed and a morphologically identical Colletotrichum was re-isolated from lesions. For molecular characterization, portions of three gene regions were amplified and sequenced. The internal transcribed spacer region (ITS) gave the closest match to C. spaethianum (> 98%), while the actin and beta-tubulin showed the closest matches to C. incanum (99% and 96% respectively). A maximum likelihood tree with comparison to 19 Colletotrichum species supported the identification as C. incanum. This species is known to cause disease on soybean in the US. Symptoms of the disease were similar to those reported previously for beet anthracnose in Japan and Canada. The causal agent at the time was identified as C. dematium, but new taxonomic work with the genus has shown several new species in the curved-spore Colletotrichum group. A recent study in Japan has identified an isolate from radish, previously identified as C. dematium, as C. incanum. It is possible there is a similar situation for beet anthracnose. To our knowledge, this is the first report of beet anthracnose in the United States.