Symptoms and Signs
Early symptoms of corky root are yellow bands on taproots and lateral roots of lettuce seedlings. These yellow areas gradually expand, taking on a greenish-brown color and developing cracks and rough areas on the surface of the root. As the disease severity increases, the entire taproot may become brown, severely cracked, and nonfunctional; the feeder root system will also be reduced and damaged. At this point, roots are very brittle and easily break off when examined. Corky root may cause internal discoloration of the root.
When the root is severely diseased, aboveground symptoms consist of wilting during warm temperatures, stunting of plants, and general poor and uneven growth. Corky root symptoms could be confused with ammonium toxicity, which causes a brick-red discoloration of the central portion of the root and wilting of lettuce foliage.
Comments on the Disease
The corky root bacterium, Rhizomonas suberifaciens, is a soilborne pathogen that is prevalent in most coastal lettuce growing areas and is present in some inland production areas. Corky root affects both leaf and head lettuce varieties. Corky root is typically more severe when soil temperatures are warmer. High soil nitrate levels can increase disease severity.
To prevent corky rot:
- Rotate crops out of lettuce, as corky root is worse in fields where lettuce is grown consecutively.
- Avoid over fertilizing with nitrogen fertilizers.
- Consider planting corky root resistant cultivars.
For corky root infected crops, add additional fertilizer and water in order to bring the crop to maturity. High, well-draining beds may sometimes reduce corky root severity.