Beet, Red (Beta vulgaris)-Phoma Leaf Spot and Root Rot


Cause The fungus, Pleospora bjoerlingii (syn. P. betae = Phoma betae) can cause leaf spots and root rot of table beet, sugar beet, Swiss chard, and other Beta species. The fungus is seedborne. During the spring time, windblown ascospores are released from sexual fruiting bodies (pseudothecia), which developed the previous season on infected plant debris remaining on the soil surface. Asexual fruiting bodies known as pycnidia develop in the center of leaf spots as well as in lesions on petioles, stems, and storage roots. Pycnidiospores (conidia) produced by pycnidia can be moved by water splash, or equipment, tools, or workers moving through infected fields when wet. Temperatures between 57°F and 65°F with high humidity are optimum for disease. Rains close to harvest in infected seed fields will increase the occurrence of infected seeds.

Symptoms Infected seed result in damping-off or seedling blight symptoms. Leaf spots appear as light-brown lesions that may contain concentric rings of small, dark pycnidia. Wilting and dark-brown-to-black spots/rot of the root tissue develop upon infection of the storage root. Rot can also develop on beets in storage. Infected seed crops may develop grayish lesions or dark streaks on stems that can result in stem breakage; pedicels can also be infected.

Cultural control Practices that promote vigorous growth of the plant will minimize the disease in table beets.

  • Practice 3-year rotations between Beta crops.
  • Use pathogen-free seed.
  • Hot water seed treatment will control seedborne disease.
  • Apply optimal fertilizer rates, including boron to prevent heart rot.
  • Irrigate so as to avoid moisture stress or over-irrigation.
  • Plow under infected crop residues after harvest.

Chemical control Seed treatments can help reduce disease incidence.

  • Seed treatments can help reduce disease incidence.
    • Captan 4F at 12 fl oz/100 lb seed plus a dye.
    • Mertect 340-F at 0.002 mg ai/seed plus a dye is labeled for control of pathogenic Fusarium spp. and will be effective on P. bjoerlingii.
    • 42-S Thiram at 8 fl oz/100 lb seed plus a dye. See label for reentry restrictions.
  • Proline 480 SC (Group 3) at 5 to 5.7 fl oz/A is labeled for disease suppression for seed crops only in Oregon and Washington only (OR-170011, SLN WA-170011). 12-hr reentry.