[Image: Picture of post emergent damping off]
Symptoms
Seedling diseases include the decay of seeds in the soil, death of seedlings before they emerge from the soil (preemergent damping off), death of seedlings after they emerge from the soil (post-emergent damping off) and lesions on the roots and lower stems of young plants.
The most obvious results of seedling disease include skips in the planting row caused by seed rot or death of seedlings. Infected seedlings may be stunted and yellow. Dark, necrotic lesions on the stem are referred to as “soreshin” if they occur on one side of the stem, or as “wirestem” if they encircle the entire stem. On some seedlings, the tap root may be killed, causing a condition referred to as “nub-root” that results in a proliferation of secondary roots above the point where the tap root is killed . Black root rot causes the outer layer of the root to be black, but the inner tissue remains white. Root and stem lesions that result in post-emergent damping off are visible as dead, shriveled seedling in the planting row. Infected seedlings may survive, but are less vigorous, prone to drought stress and may produce lower yields.
Severe stand losses can result in yield losses directly or in losses imposed by replanting and delay in crop development. Minor stand losses reduce the uniformity of the stand and result in problems associated with crop development, weeds and insect management. Nonlethal root and lower stem injury contribute to losses by reducing seedling growth and delaying plant development.
Causal Agents and Disease Development
Seedling diseases are caused by a complex of fungi acting either independently or together. Rhizoctonia solani, Pythium spp. and Fusarium spp. are the primary organisms associated with seedling disease in Louisiana. Thielaviopsis basicola, which causes a seedling disease called black root rot, is more common in other areas of the Cotton Belt, but it has recently been identified in Louisiana. Exact identification of the causal agent often requires isolation and laboratory identification. Except for T. basicola, these fungi are common in most cotton fields. The composition of fungi involved in the seedling disease complex are affecte[Image: Cotton plant exhibiting "nub root "]d by soil temperature and moisture. Rhizoctonia solani and Fusarium spp. are more common under higher (75-90°F, 24-32°C) temperatures. Pythium spp. and T. basicola are more common in cool (59-68°F, 15-20°C), wet conditions.
Seedling diseases are more common under cool (temperatures below 65°F,18°C), wet conditions, but they may occur under any conditions that retard germination and early seedling growth. Poor seed quality, soil compaction, herbicide injury and early season insect and nematode feeding all retard early season growth and increase seedling diseases.
Control
Management starts with the planting of high quality seed treated with fungicides. Delaying planting until soils are 65°F or higher and favorable weather is forecast after planting will reduce the incidence of seedling diseases. Avoid compacted and crusted soils. Plant at the proper depth based on soil and moisture conditions and on raised seedbeds that adequately drain water. Apply herbicides and fertilizers properly, and control early season insects and nematodes. All of these practices will encourage rapid germination and seedling growth and allow the seedlings to escape infection.
The applica[Image: Picture of cotton field showing stand loss]tion of in-furrow or hopper-box fungicides is used to supplement seed treatment fungicides and is recommended in situations where seedling diseases are expected. Virtually all cottonseed that is purchased in Louisiana is commercially treated with a combination of fungicides to control seedling diseases. In addition, infurrow or hopper-box fungicides may be applied at planting. These materials are recommended in situations where seedling diseases are likely to occur. Hopper-box materials are generally not as effective as in-furrow fungicides. These fungicides often contain more than one active ingredient to control both Rhizoctinia and Pythium. See the appendix for labeled fungicides.