Pythium species and Phytophthora species Rhizoctonia solani (teleomorph: Thanatephorus cucumeris)
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, and 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 and 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.
Rhizoctonia solani -Preemergence damping-off is characterized by tan to reddish-brown lesions developing on the seedling and by the death of the growing tip. Postemergence damping-off symptoms include reddish brown to black lesions developingon 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.
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, helps reduce damping-off. Some fungicide seed treatments or soil drenches can help prevent serious damping-off.