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. The corky root bacterium, Rhizomonas suberifaciens, is a soilborne pathogen that is prevalent in most coastal lettuce growing areas but may not be present in inland regions.
Corky root affects both leaf and head lettuce varieties.
Disease is typically more severe when soil temperatures are warmer. Corky root is worse in fields where lettuce is grown consecutively. High soil nitrate levels can increase disease severity. Avoid over fertilizing with nitrogen fertilizers. Some corky root resistant cultivars are now available.