Disease Management

Download Cotton Seedling Disease Control Guide (PDF, 97.7KB)

Seedling diseases occur on cotton in South Carolina every year. Rhizoctonia solani is the most commonly occurring pathogen with Pythium spp. occurring on early planted cotton or cotton planted on heavy or coolwet soils. One or both of these seedling disease pathogens are present in almost every cotton field. Damage to plants may vary from barely detectable to death. The seedling disease complex is estimated to reduce yields 3 to 5% in South Carolina every year. The costs to growers are even higher when the cost of replanting and delayed maturity in the second crop are included. Disease incidence and severity in a given field are determined by environmental factors such as soil temperature and moisture conditions and by other factors such as seed quality and vigor. In addition stresses on the plant such as pesticide phytotoxicity, fertilizer burn, sand blasting, or damage from other pathogens such as nematodes will increase the incidence and severity of seedling diseases.

Cotton seedling diseases can occur as:

  1. a seed rot which occurs prior to germination,
  2. a pre-emergence damping off which occurs between germination and emergence,
  3. a post- emergence damping off and
  4. Sore shin which occurs on plants greater than eight inches tall and is generally nonlethal.

Post-emergence damping off is the most commonly occurring “seedling” disease in South Carolina and over 80% of the time it is caused by Rhizoctonia solani.

Seedling disease management relies on the integration of cultural practices and the prudent use of fungicides. There are no cultivars which offer any level of resistance to seedling diseases. Crop rotation is also ineffective since both Rhizoctonia solani and Pythium spp. are capable of infecting most commonly grown rotation crops such as corn, peanut, and soybeans. Even when a susceptible host is not present both fungi are capable of surviving saprophytically on soil organic matter such as dead weeds or residue from winter cover crops. The most important cultural practice limiting seedling disease severity is to delay planting until soil temperatures at the 4 inch depth are above 68 F for three consecutive days. Planting on beds allows better drainage and creates higher soil temperatures. Other cultural practices include: 1) the use of high quality seed, 2). avoiding low pH’s (less than 6.0) which favor disease development and suppress plant growth, and 3). avoid injury from preemergence and early season herbicides, insecticides, and fungicides 2 as well as fertilizer burn.

Fungicides used as seed treatments and in-furrow sprays or granular materials can be effective in reducing infections and stand problems due to either Rhizoctonia solan or Pythium ultimum. However, most fungicides control only one of the two fungi. To control both fungi, combinations of fungicides must be used.

Almost all commercial seed sold is treated with combinations of fungicides which can help control these fungi. The most commonly used seed treatment fungicide against Rhizoctonia solani is Vitavax (carboxin) and Apron (metalaxyl) is probably the most commonly used against Pythium spp. In-furrow granulars or liquid fungicides commonly use PCNB for Rhizoctonia solani and either metalaxyl or etridiazole (also known as ethazole) for Pythium ultimum. Formulations of Terraclor Super X and Ridomil PC are very effective in preventing the seedling disease complex which occurs in South Carolina. Terraclor formulations are available for application when Pythium spp. are not believed to be present. Ridomil 2E is available for use where Rhizoctonia solani is not considered to be a problem. Using fungicides which control only one of the fungi can be risky.

As with in-furrow fungicide programs, hopper box treatments are available which are effective in reducing the seedling disease complex. Once again most products combine fungicides to control both Rhizoctonia solani and Pythium spp. Deltacoat AD combines Apron (metalaxyl for Pythium spp.) and Demosan (chloroneb for Rhizoctonia solani). Prevail combines Apron + Terraclor (for Rhizoctonia solani) + Vitavax (for Rhizoctonia solani). Kodiak is a biological control agent which contains the bacterium Bacillus subtilis which can be effective against Rhizoctonia solani.