Fig. 16. Discharge of pycnidiospores from pycnidium of Phoma betae on a sugar beet seedling. (Courtesy L. D. Leach)
Fig. 17. Alternaria leaf spot of sugar beet. (Courtesy E. D. Kerr; reprinted, by permission, from Franc et al., 2001)
Management
Cultivars have not been developed with resistance to any of the diseases caused by P. betae. Susceptible beet crops should be planted in a 4-year rotation with nonhost crops. Weed populations, particularly C. album, should be properly managed. Fungicide seed treatments help reduce disease incidence. Processing seeds to remove cortical tissues assists in eliminating the pathogen from this often localized area of the seed. Selected References Bugbee, W. M., and Soine, O. C. 1974. Survival of Phoma betae in soil. Phytopathology 64:1258-1260. du Toit, L. 2004. Diseases of vegetable seed crops: Identification, biology and management. Proc. Organic Seed Growers Conf., 2004. Organic Seed Alliance, Port Townsend, WA. www.seedalliance. org/uploads/pdf/VegSeedDiseases.pdf. Nyvall, R. F. 1979. Diseases of sugar beets (Beta vulgaris L.). Pages 295-314 in: Field Crop Diseases Handbook. AVI Publishing, Westport, CT. Pool, V. W., and McKay, M. B. 1915. Phoma betae on the leaves of sugar beet. J. Agric. Res. 4:169-177. Walker, J. C. 1952. Diseases of Vegetable Crops. McGraw-Hill, New York.
(Prepared by R. M. Harveson)
Alternaria Leaf Spot Alternaria brassicae and A. alternata (syn. A. tenuis) cause leaf lesions in Beta spp. Both A. brassicae and A. alternata are globally distributed and are common wherever sugar beet is grown, especially where cruciferous crops are grown in close association with sugar beet. Alternaria leaf spot also can affect table beet and Swiss chard.
Symptoms
Both fungi are necrotrophic pathogens and may cause conspicuous spotting of all aerial plant parts in an interaction dependent on host reaction and environmental conditions. These spots start as pinheadlike structures that expand into uniform or concentrically zonate lesions of various sizes. On sugar beet, lesions caused by either fungus are circular to irregular (usually 2–10 mm in diameter), are gray (Fig. 17) to dark brown to almost black, and may be surrounded by a chlorotic area. Lesions can develop a velvety black appearance with heavy sporulation. Foliar infection may cause loss of photosynthetic area, accelerated senescence, and defoliation. Infection by A. alternata 12
Fig. 18. Alternaria leaf spot symptoms on sugar beet infected with Beet western yellows virus. (Courtesy R. T. Lewellen)
is believed to be secondary in nature, with symptoms only occurring on old leaves already weakened by senescence, poor nutritional status, or other plant stressors. The presence of A. alternata is also commonly observed in association with Beet western yellows virus (Fig. 18) or Fusarium yellows infections. Although A. brassicae also infects weakened or stressed leaves, this fungus is a primary pathogen able to initiate infection of otherwise healthy leaves. Infection by A. brassicae may cause an incipient green-island effect in the foliar lesions, presumably because of the production of cytokinins by the fungus. Differential diagnosis is further aided by the fact that conidia are usually present on field-infected material. If not present, sporulation can be induced by incubating infected material for 24 h in a moist chamber at 20°C.
Causal Organisms
Sporulation of A. brassicae is sparse to moderate on the host plant, with conidia usually solitary and found near the center of the lesions. However, chains of two to four conidia are common in culture. Primary conidiophores are single or in groups of 2 to 10 or more emerging through stomata; they are brown, usually simple, frequently geniculate, smooth, and 5–11 × 140–170 µm, and they have zero to seven septa and bear one to several small but prominent conidial scars. Conidia are acropleurogenous, straight or slightly curved, obclavate, beaked with 5–19 transverse and 0–8 longitudinal or oblique septa (Fig. 19), light to medium olivaceous brown, usually smooth, rarely very inconspicuously warted, and 11–42 µm wide in the broadest part and 75–350 µm long. Beaks are one-quarter to one-half of the mature conidium length, 4–9 µm wide, sturdy, never fragile or