Abstract
Fungi in the genus Colletotrichum have been detected on sugar beet in several regions. In many areas, no symptoms have been reported. However, in recent years we have observed a variety of symptoms caused by different Colletotrichum species on beet. A seedling damping-off was first found in Michigan in 2016, with plant symptoms primarily of stunting and discoloration. Few beets died, but growth was delayed. This has continued to be present, with samples found around the Michigan production region every year that seedling samples were collected since that time; one of two Colletotrichum species has been confirmed for each sample. In addition to seedling damage, unusual foliar symptoms were observed in 2016, with oblong lesions on the petiole of beets. From these, Colletotrichum incanum was isolated consistently. While this symptom has only rarely been observed, in foliar inoculations it was observed that symptoms similar to those caused by Phoma betae formed on the blades of leaves. The lesions were large, round with irregular borders, and often had at least one darker ring, but showed less distinct browning than generally observed with Phoma leaf spot. Two species of Colletotrichum could be isolated from lesions on the leaf blade, C. incanum or C. lineola. Both caused similar lesions in leaf inoculations. A third Colletotrichum has been isolated from beet roots. Colletotrichum coccodes was first isolated in Idaho but a similar Colletotrichum sp. has now been found on beet in Michigan. This causes shallow lesions on the root that have many small “black dots” (leading to the disease name on potato). It has been reported to increase susceptibility to some other fungi in potato but its impact in beet is unknown. Anthracnose caused by Colletotrichum dematium had been reported in Japan, with similar symptoms to the foliar and seedling damage observed in the US. This pathogen also could cause loss of flowers in seed production as well as infesting seed. It is unknown whether any of the recently identified Colletotrichum can be associated with seed production issues and their overall impact is still being determined.