Stalk Rot of Sugar Beet Caused by Fusarium solani on the Pacific Coast

In 2006, symptoms of stalk blight (2) were observed on sugar beet (Beta vulgaris L.) plants from roots produced in Oregon that were being grown for seed production in a greenhouse in Salinas, CA using Salinas Valley soil. Symptoms included vascular and cortical browning, necrosis, and death of seed stalks. Isolations were made from the edge of stalk lesions and the crown. In addition to Fusarium oxysporum, the known cause of stalk blight (2), two isolates of F. solani were identified by morphology. For pathogenicity tests, sugar beet plants (FC606 [4]), grown in pasteurized potting mix and induced to flower by exposure at 4 to 7°C for 90 days (1) were used. Bolting plants were maintained in a greenhouse at 24 to 27°C. A 100-μl drop of a spore suspension (10⁴ spores per ml) of each Fusarium isolate was placed on the surface of the seed stalk. The plant was stabbed through the drop with a sterile 18-gauge needle so that the drop was taken into the plant by hygroscopic pressure. Positive and negative control treatments were a stalk blight isolate of F. oxysporum from an Oregon seed production field and sterile water, respectively. Three plants were inoculated per isolate. Each inoculation site was wrapped loosely in Parafilm for 1 week to maintain a high humidity level around the site of inoculation, and seed stalks were covered in cloth bags (1). After 1 week, the Parafilm was removed and plants were examined weekly for symptoms. At 4 weeks, lesion size was measured. After 5 weeks, sections were taken from the seed stalk around the site of inoculation, surface disinfested with 0.5% NaOCl, and plated on potato dextrose agar to confirm the presence of the pathogen. The experiment was done twice. One of the two isolates of F. solani caused dark brown lesions on all inoculated seed stalks. On one plant, at 4 weeks after inoculation when the bag was being removed for observation, the seed stalk broke at the site of inoculation because of a spreading, brown lesion at the site. No lesions were observed on the water control plants. Brown lesions were observed on seed stalks inoculated with the known stalk blight isolate. Lesions were significantly (P = 0.001) larger with F. oxysporum than with F. solani when measured at 4 weeks (mean of 6.3 cm versus 2.2 cm, respectively). Lesions caused by F. solani showed a dark discoloration through the cortical tissue, as opposed to those caused by F. oxysporum, for which most of the initial discoloration was in the vascular bundles and epidermis. Fusarium isolates recovered from inoculated plants were morphologically similar to the isolates used for inoculation. Fusarium spp. were not isolated from the water control plants. While some F. solani isolates cause seedling or mature root disease in sugar beet (3), to our knowledge, this is the first report of a Fusarium species other than F. oxysporum causing a rot of sugar beet stalks. References: (1) E. Biancardi et al. Genetics and Breeding of Sugar Beet Science Publishers, Inc., Enfield, NH, 2005. (2) A. N. Mukhopadhay. Handbook of Diseases of Sugar Beet, Vol. 1. CRC Press, Boca Raton, FL 1987. (3) E. G. Ruppel. Plant Dis. 75:486, 1991. (4) G. A. Smith and E. G. Ruppel. Crop Sci. 19:300, 1980.