Carrot Country

Cavity Spot Disease of Carrot

Casual Organism
Cavity spot is a disease caused by several species of fungal like organisms belonging to the genus Pythium. Members of this genus can directly penetrate fine roots and do not require a wound as a point of infection. Disease development is encouraged by overly wet soil conditions. Members of this genus produce thick walled spores called oospores that can survive in soil for long periods. Characteristic to this genus is the ability to produce swimming spores when conditions are favorable, called zoospores. They can follow a chemical gradient from roots and cause infections.

Symptoms
Cavity spot rarely reduces carrot yield but instead causes root blemishes making the carrots unacceptable for the fresh market. Initial symptoms are sunken, elliptical lesions that form underneath the periderm of the root. As the lesions elongate, the periderm ruptures and becomes black. Small vertical cracks are often associated with cavity spot lesions. Cavities can provide a point of entry for secondary infection by other microorganisms further damaging the root (Figure 2). Since there are no foliar symptoms associated with cavity spot, the disease is undetectable until the carrots are harvested and washed.

Management
Since Pythium sp. can persist in the soil for years and has a wide host range, avoid fields with a history of severe cavity spot disease. However, if a field with a history of cavity spot is to be used, be certain to practice crop rotation and rotate out of carrots for at least three years.

Plant carrots on raised beds in well drained soils and utilize proper irrigation management practices to avoid long periods of soil saturation, which encourage cavity spot.

Harvest carrots in a timely fashion since the severity of cavity spot increases as the root matures. Once in cold storage, incidence of cavity spot does not increase.

Mefenoxam is currently used to control cavity spot in carrots. However, due to concerns over mefenoxam resistant Pythium sp., current research is focused on evaluating different chemical control options for cavity spot control.

Source
Nick David and Philip Hamm, Oregon State University, Hermiston Agricultural Research and Extension Center, Hermiston Ore.

Published
Spring 2005 issue of Carrot Country.