LEXINGTON, Ky. — Potatoes are often one of the first crops planted into the vegetable garden. However, growers and gardeners alike should be on the lookout for blackleg disease. This disease can occur in both commercial and homegrown potatoes. “Seed” potatoes may be infected from the previous season and, once planted, fail to emerge or result in stunted plants with bacterial disease symptoms. Losses from blackleg can reach 100% since there are no management options after plants are infected. Clean seed along with proper planting and maintenance practices can reduce disease incidence.
Cause and Disease Development
Multiple species of Pectobacterium and Dickeya bacteria can cause blackleg of potato. Initial infections likely come from seed potatoes that were infected during the last field season, in storage, or during preparations for planting. Infected seed pieces may not have symptoms since the bacteria can remain latent until conditions are appropriate for disease development. Blackleg symptoms develop during wet, cool conditions, then become more obvious when temperatures warm in subsequent weeks. Bacteria moved through soil by rain or irrigation may infect additional seeds and plants. Secondary infections from the same pathogens can occur throughout the season on stems (commonly called aerial stem blight) or tubers (also known as soft rot).
Symptoms
Blackleg begins in seed potatoes. Infected seed pieces may yield plants that are weak or stunted (Figure 1) in appearance, or plants may not emerge at all.
Water-soaked lesions may be present at the base of the plant, and over time darken and expand up the stem (Figure 2). These lesions may be soft and wet under moist environmental conditions, or drier under dry conditions. As the lesion expands, vascular tissues break down resulting in the wilting, chlorosis, and/or necrosis of leaves. Eventually, the plant may collapse and die.
Management
Management of blackleg is dependent on the prevention of disease introduction via cultural practices. Once seed pieces or plants are infected, in-season management of blackleg is limited to removal of obviously infected plants. The following methods can be used to mitigate blackleg.
- Plant certified disease-free potato seed pieces. Blackleg disease has been identified in numerous different varieties from different sources, so carefully inspect the seed certification for any positive identifications of Dickeya or Pectobacterium spp. in the source farm’s history. Avoid planting seed from these sources.
- If preparing your own seed pieces, use only the best quality potatoes. If splitting seed pieces, sanitize tools prior to each cut using a solution of 1 part bleach to 9 parts water. Spread seed pieces in a single layer for at least 24 hours to completely dry the cut sides.
- Plant into well drained, warm (greater than 50°F) soils.
- Increase plant spacing to reduce dense canopies.
- Water plants at the soil line. Do not water seed pieces before shoot emergence.
- In areas where blackleg has been a problem, rotate potato crops to a different field or area of the garden for a least 2 years.
- Harvest under cool conditions (less than 70°F). Try to limit tuber injuries to prevent the spread of blackleg bacteria on tools or stored tubers.
Resources
- Potato, Blackleg (University of Massachusetts-Amherst, January 23, 2018; Link)
- Home Vegetable Gardening (ID-128)
- Vegetable Production Guide for Commercial Growers (ID-36)
Click here to visit the University of Kentucky Pest News blog.
— Kim Leonberger, Plant Pathology Extension Associate and Emily Pfeufer, Extension Plant Pathologist, University of Kentucky
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