How to Manage Pests
UC Pest Management Guidelines
Root and Crown Rots and Damping-off Diseases
Pathogens: Phytophthora capsici, Pythium aphanidermatum, and other Pythium spp.
(Reviewed 12/09, updated 11/12)
In this Guideline:
SYMPTOMS AND SIGNS
Damping-off is primarily caused by Pythium spp. Seedlings affected by damping-off fail to emerge or fall over and die soon after emergence. Stems usually have a dark, shriveled portion at the soil line. Damping-off is generally limited to areas where drainage is poor or where soil is compacted, but whole fields can be affected, especially in early plantings exposed to rain.
Root and crown rot is primarily caused by Phytophthora capsici. Symptoms on affected pepper plants include rapid wilting and death of pepper plants. Close examination of the roots and stems is necessary to confirm the cause of disease. The disease can develop at any stage of pepper plant growth. Taproots and smaller lateral roots show water–soaked, very dark brown discoloration of surface, cortical, and vascular tissues. Very few lateral roots remain on diseased plants and the tap roots may also be shorter compared to those of healthy plants. The most striking difference between healthy and diseased plants is the total amount of root tissue. Stems are usually infected at the soil line. Stem lesions first become dark green and water-soaked, followed by drying and turning brown. A lesion can girdle a stem, resulting in wilting of plants above the lesion and subsequent death. Under some conditions, P. capsici can also cause a brown foliar blight on pepper.
Another important soilborne disease of pepper in California, Verticillium wilt, causes foliar symptoms similar to those seen in root and crown rot diseases; however, Verticillium wilt does not cause any browning or rotting of the root surface and cortex. In contrast, the xylem tissues of main stems and roots of Verticillium dahliae-infected pepper show brown to black streaking and discoloration.
COMMENTS ON THE DISEASE
Disease caused by P. capcisi can develop at any stage of growth, but once pepper seedlings reach the 2- or 3-leaf stage, they generally are no longer susceptible to infection by Pythium spp. However, Pythium spp. can cause root rot when peppers are grown on plastic mulch.
"Damping-off" is a general term for the death of seedlings in damp conditions, either before or after emergence. It is mainly an early-season problem, causing greatest losses in cool, wet soils. Because almost all California pepper production uses transplants, this disease is of more concern in the greenhouse if potting soil is contaminated or overwatered.
Although infection is most common under cool conditions, P. capcisi can infect seedlings in warmer soils. Damping-off due to Pythium spp. may increase where green manures such as volunteer grain are worked into the soil just before planting. Damping-off does not necessarily carry over from one season to another in the same places, but appears only when and where conditions favor infection.
Neither P. capcisi nor Pythium spp. is seedborne. However, both can survive in soil for long periods as thick-walled oospores. Contaminated transplants or soilborne inoculum are sources of primary infections. Irrigation water often disseminates fungal propagules from infested areas to other parts of the field. Thus, irrigation can significantly increase the incidence and severity of root and crown rot in pepper. Increased frequency and duration of irrigation favor disease development.
Water, temperature, and soil texture are the major factors affecting the development of damping–off and root and crown rot. The presence of water is mandatory; soil saturation for as little as 5 to 6 hours can result in infection, and susceptible varieties can become severely diseased in as little as 5 days. Optimum temperature for plant infection is 75° to 92°F (24° to 33°C). Symptoms usually appear following a warm, wet period. The disease is severe in fine-textured (clay) soils that drain slowly and in highly compacted soils. Severely infected fields may have nearly complete loss of plants.
Infections that occur late in the season may reduce vigor and yield of plants without killing them. In addition, if the foliage wilts during the hottest time of the day and exposes fruit to the open air, such fruit can become sunburned and therefore unharvestable.
Factors that influence the development of damping–off and root and crown rot diseases in peppers in a given season include varietal susceptibility, amount and frequency of irrigation, and soil compaction and drainage. Crop rotation, proper irrigation, and clean transplants are critical in managing this disease. Fields that have a history of root and crown rots may need fungicide treatments at planting.
The disease can be effectively prevented by:
Soil and bed preparation
Practices that reduce or alleviate soil compaction may improve control; for example, growing plants on raised beds. If possible, avoid planting when the soil is cool. Seeds germinate faster and seedlings are more vigorous when the soil is warm: thus they are less likely to be damaged.
Use sprinklers during germination to better control irrigation and lessen the chance of infection. In heavy soils that are poorly drained, root and crown rot may be reduced by irrigating every other row on each irrigation or by carefully managing drip irrigation.
Commercial cultivars with acceptable levels of resistance to the disease are available. However, in general peppers are very susceptible to these diseases.
Fungicides are sometimes used preventively in fields with histories of root rot or problems with drainage. Use a fungicide seed treatment to prevent damping-off.
UC IPM Pest Management Guidelines:
S. T. Koike, UC Cooperative Extension, Monterey County