Damping Off

This article appeared in “The Tomato Magazine,”
April 2001, page 22.

Causal Agents:

Pythium species and Phytophthora species
Rhizoctonia solani
(teleomorph: Thanatephorus cucumeris)

Distribution:

Worldwide

Symptoms:

Seeds may rot before germinating, and seedlings may decay before emergence (pre-emergence damping-off), giving the appearance of poor germination. After emergence (post-emergence damping-off), the seedlings develop lesions at the base of the stem; the tissue becomes soft, constricted, and the plants wilt and fall over.

Pythium species and Phytophthora species.
Pre-emergence damping-off commonly occurs with these fungi; typical symptoms are a soft mushy rot and dark brown-to-black, water-soaked lesions that rapidly spread over the entire seedling. Post-emergence damping-off is characterized by dark-colored, water-soaked lesions that start on the roots and spread up the stem to above the soil line. The lesions continue to expand above the soil line, eventually girdling the stem, causing the plant to wilt and die.

Rhozoctonia solani
Pre-emergence damping-off is characterized by tan-to-reddish-brown lesions developing on the seedling and by the death of the growing tip. Post-emergence damping-off symptoms include reddish-brown-to-black lesions developing on the roots and the lower stems at or below the soil line. The stem becomes constricted, and infected seedlings quickly wilt and die. Older plants can become infected, however. As the plant matures, it becomes more tolerant to infection, and the lesions are usually restricted to the cortical zone.

Conditions for Disease Development:

These fungi generally survive for long periods in soil, and may persist in plant debris or on roots of weeds. Damping-off tends to be most severe under conditions of high soil moisture, overcrowding, compaction, poor ventilation, and cool damp cloudy weather. In greenhouses, damping-off can be more common when improperly pasteurized soil or previously used seedling trays are used for planting. Water splash can move infested soil from diseased to healthy plants and spread this disease.

Control:

Crop rotation with cereal crops and soil fumigation or solarization may help reduce damping-off in fields. Improving soil drainage by using raised beds, and regulating soil moisture by avoiding excessive irrigation help to reduce disease. In greenhouses, good sanitation, including using sterilized planting trays and proper soil pasteurization, reduces damping-off. Some fungicide seed treatments or soil drenches can help prevent serious damping-off.