First Report of Fusarium Yellows of Sugar Beet Caused by Fusarium oxysporum in Michigan

Fusarium yellows of sugar beet (Beta vulgaris L.), caused by Fusarium oxysporum Schlechtend.Fr. f. sp. betae (Stewart) Snyd & Hans., has been a long-term problem in the western United States (3) and recently was reported in Minnesota and North Dakota (4). This disease is typified by interveinal yellowing and wilting of the foliage. Roots have no external symptoms but show internal vascular discoloration. In 2005, 12 sugar beet roots from Michigan with yellows-type symptoms were received by the author. Isolations were made from the cortical and vascular tissue of the crown and tap root. Fusarium spp. isolates were obtained from 10 of the beets, and 16 isolates were identified as Fusarium oxysporum on the basis of morphology and pigmentation on potato dextrose agar and spores and phialides on carnation leaf agar (2). F. oxysporum isolates were tested for pathogenicity by dipping roots of 5-week-old susceptible sugar beet plants (FC716) in a suspension of 10 spores per ml for 8 min, 10 plants per isolate. Two known pathogenic isolates of F. oxysporum f. sp. betae, Fob13 and Fob216c (4), were used for comparison. For a negative control, plants were dipped in sterile water. Beets were planted in Cone-tainers (3.8 cm diameter × 21 cm) containing pasteurized potting mix. Plants were placed in a greenhouse at 24 to 27°C and fertilized with 15-30-15 fertilizer every 2 weeks to avoid chlorosis from nutrient deficiency. Plants were rated weekly for foliar symptoms for 6 weeks using a Fusarium yellows rating scale of 0 to 4 in which 0 = no disease and 4 = complete plant death (1). After the final rating, plants were removed from soil and the tap root examined for root symptoms. Root segments were surface disinfested with 0.5% sodium hypochlorite and cultured on potato dextrose agar to confirm presence of the pathogen. The experiment was done twice. Seven F. oxysporum isolates tested caused typical Fusarium yellows symptoms including interveinal yellowing, stunting, and wilting of inoculated plants. Pathogenic isolates were obtained from 7 of the 10 beets that yielded F. oxysporum. Symptoms were indistinguishable from those caused by Fob13 (average ratings ranged from 1.8 to 2.4) and milder than those caused by Fob216c (average rating 3.1). No interveinal chlorosis or wilting was observed on the control plants. Isolations from inoculated plants provided F. oxysporum cultures morphologically similar to those used in inoculation by the methods of Nelson et al. (2). No F.