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Corky Root of Lettuce1
L. E. Datnoff and R. T. Nagata2Corky root is a serious disease of lettuce. It has been reported in the states of California, Florida, New York and Wisconsin; and the countries of Canada, Great Britain, Greece, Italy, Netherlands and Spain. Yield losses for fresh and marketable head weights caused by corky root have been reported to range from 37 percent to 53 percent in Florida.
SYMPTOMS AND CAUSAL AGENT
Symptoms of corky root appear initially as yellow lesions, or bands on the tap and/or lateral root ( Plate 1 ). As the disease progresses, the taproot becomes corked and brittle ( Plate 2 ), and may exhibit vascular discoloration. Development of tap and lateral roots in infected plants are severely reduced or completely destroyed. Above ground, infected plants appear chlorotic and stunted. Uneven growth patterns are evident in the field. Heads are not formed at all or maybe unmarketable because of poor size ( Plate 3 ).
a)
Plate 1.
Plate 2. The etiology of this soilborne disease was unknown for many years. Its presence was attributed to numerous abiotic and biotic factors. However, in 1984, a slow-growing bacterium was isolated from diseased roots and, in 1988, was finally proven to be the causal agent of the disease. The name Rhizomonas suberifaciens has been proposed for this gram-negative bacterium.
Plate 3. EPIDEMIOLOGY
Little information is known about the biology and epidemiology of this pathogen such as the survival mechanisms and the influence of soil environment on disease development. This is due to the lack of a truly selective medium and the slow growth rate of this bacterium in culture. However, R. suberifaciens has been isolated from lettuce grown in fields recently brought into production after sugarcane. Similar observations have been made on lettuce grown in soil after pasture or forest. This organism also has been isolated from bean, melon, rye, and tomato, but only members of the Compositae closely related to lettuce (endive, common sowthistle, and prickly lettuce) are susceptible. Evidently, R. suberifaciens can survive in association with a number of crop and weed species, especially in root zones of these plants.Soil type probably has little effect on disease development since corky root has been reported to occur on most soils used for growing vegetables. The pH of soils also seems to have little effect on corky root development, since in the laboratory R. suberifaciens has been reported to grow at pHs ranging from 5.7 to 8.2.
Severity of corky root will increase with applications of nitrogen fertilizer, especially with side dressings of N such as urea. High soil moistures and temperatures also appear to favor disease development.
CONTROL
Fumigants such as dazomet, metam sodium and methyl bromide + chloropicrin are very effective for controlling corky root. However, the application of these materials probably are cost prohibitive on a commercial scale.Host resistance also is very effective for managing this disease. Resistance is conferred by a single recessive gene. In Florida, several commercially resistant crisphead, romaine and buttercrisp lettuce types are available (Table 1 ).
Transplanting of corky root susceptible lettuce cultivars either 3 to 5 weeks old also is effective for managing this disease. This practice allows the use of susceptible types that otherwise would be lost.
Tables
Table 1.
Table 1. Commercial corky root resistant lettuce cultivars grown in Florida. Crisphead Romaine Buttercrisp Florical 50011 Augustus Florida Buttercrisp Greenlake Floriglade Florida 202 Montello Tall Guzmaine Raleigh South Bay
Footnotes
1. This document is PP50, one of a series of the Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date June 1992. Reviewed April 2003. Visit the EDIS Web Site at http://edis.ifas.ufl.edu.2. L.E. Datnoff, associate professor, Plant Pathology, Everglades Research and Education Center (EREC), Belle Glade, Florida; R.T. Nagata, associate professor, Plant Breeding, EREC, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville FL 32611.
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U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.
Copyright Information
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