Damping-off and Root Rots
A number of species of soil-dwelling fungi, including Fusarium, Rhizoctonia, Pythium and Phytophthora spp., infect vegetables, especially legumes and solanaceous crops. If the infection occurs either before or just after emergence, and development of a lesion at the soil line results in collapse and shriveling of the plant, the disease is called 'damping-off'.
The symptoms of this disease are brown, water-soaked areas around the lesion that shrivel and pinch the seedling off at the base. In beans, Pythium spp., Rhizoctonia or a number of other diseases or mechanical and fertilizer injuries can cause symptoms similar to damping-off. In tomato seedlings, damping-off is usually caused by Pythium or by Rhizoctonia solani but can also result from Phytophthora parasitica or P. capsici infection. In general, infection by Pythium is restricted to young or succulent plants. Thus seedlings, the feeder roots or root tips of older plants and watery fruits or stem tissues are most vulnerable to attack.
If part or all of the root system is destroyed by a pathogen such as Phytophthora megasperma, the plant is said to have a root rot. Root rots attack crucifers, carrots, potatoes, spinach and beet in many parts of the world. If the rot progresses so that the entire root system is destroyed, the shoot then collapses as well. In peppers, root rot is caused by Phytophthora capsici and in tomatoes it is caused both by P. parasitica and by P. capsici.
Root rots in beans can be caused by a complex of soil fungi, including Fusarium solani, Rhizoctonia solani, Pythium spp. and Thielaviopsis basicola. Symptoms vary with the disease and environment but root rots should be suspected in beans whenever plants are stunted, wilted, yellow or are losing leaves. Diseased plants appear in patches in the field, with the location and size depending on weather and soil conditions.
Both damping-off and root rots occur in areas with poor drainage or areas with a previous history of the disease. Diseases will be reduced in young potato plants if fields are bottom plowed at least 30 days be-fore planting to allow time for old crop and weed residues to decompose. Slowly emerging seedlings are the most susceptible.
Thus deep planting, planting into cold, wet, or poorly prepared soils, low soil fertility and use of old or poor quality seeds can all increase the incidence of damping-off. Root damage from salts and soil compaction can also lead to increased loss due to root rots. If a field has symptoms of root rot, the plants that survive are probably also damaged and may have lower yield or show disease symptoms when stressed later in the season.
Soils high in organic matter, especially fresh organic matter, may have higher populations of damping-off fungi. On the other hand, populations of Gliocladium virens, a beneficial fungus that controls damping-off, are also higher in soils high in organic matter. Gliocladium is naturally present in most soils, but is available commercially as 'Soil Guard' for control of Pythium and Rhizoctonia. Optimizing drainage and growing conditions and using vigorous seed and transplants are still the best preventatives since damping-off fungi rarely attack transplants in the field or established seedlings.
bridgesj@unity.ncsu.edu