In 2004, we received beet samples from seven fields from Minnesota and Wyoming that had foliar interveinal yellowing symptoms and vascular discoloration frequently associated with Fusarium yellows. Isolations were made from the vascular and cortical tissue. Hyphal tip isolates of Fusarium were obtained from beets, including eight isolates of Fusarium graminearum. F. graminearum was isolated from beets from three fields in Minnesota and one field in northern Wyoming. F. graminearum isolates were tested for pathogenicity by dipping roots of 5-week-old sugar beet plants (FC716) in a suspension of 104 spores per ml for 8 min, 10 plants per isolate. Spore suspensions were shaken periodically to aid mixing. A known moderately virulent isolate of F. oxysporum f. sp. betae (Fob13) (3), the causal agent of Fusarium yellows of sugar beet, was used as a positive control. For a negative control, plants were dipped in sterile water. Dipped plants were planted in Cone-tainers (3.8 cm diameter × 21 cm; Stuewe and Sons, Inc., Corvallis, OR) containing pasteurized potting mix and placed in a greenhouse at 24 to 27°C. Plants were fertilized with 15-30-15 fertilizer biweekly. After 2 weeks, plants were rated weekly for 5 weeks using a 0 to 4 scale in which 0 = no disease and 4 = complete plant death (2). After the final rating, plants were removed from soil and the tap root was examined for symptoms. Root segments were surface disinfested with 0.5% sodium hypochlorite and plated on potato dextrose agar to confirm presence of the pathogen. The experiment was done twice. Three of the eight isolates of F. graminearum caused mild to moderate foliar symptoms (rating 2 to 3), including interveinal yellowing, wilting, and stunting of inoculated plants, and mild vascular discoloration was observed in some root sections. Pathogenic isolates were originally from different beets. Foliar symptoms were similar to those caused by Fob13, but the F. oxysporum f. sp. betae caused more vascular discoloration than did the F. graminearum isolates. No interveinal yellowing or wilting was observed on foliage of the control plants, and no vascular discoloration was observed in a cross section of the root. Cultures of F. graminearum or F. oxysporum recovered from inoculated plants were morphologically similar to isolates used for the inoculations. No Fusarium was isolated from the roots of plants soaked in sterile water. An interesting note is that no isolates of F. graminearum were recovered among more than 100 Fusarium isolates collected from sugar beet roots from Colorado over a 4-year period. F. graminearum was recovered in one sample from Wyoming in 2004. However, in the 2004 samples from Minnesota, this species was isolated at the same frequency as F. oxysporum. While F. graminearum has been isolated from beets in the Red River Valley (1), it has not previously been reported to cause symptoms on growing sugar beet.
References: (1) U. Bosch and C. J. Mirocha. Appl. Environ. Microbiol. 58:3233, 1992. (2) L. E. Hanson and A. L. Hill. J. Sugar Beet Res. 41:139, 2004. (3) C. E. Windels et al. Plant Dis. 89:341, 2005.
