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How to fight cotton seedling diseases

If not properly managed, cotton seedling disease can reduce stand and vigor, forcing producers to replant. When managing seedling disease, the major objective is to minimize the risk associated with these pathogens.

While it is impossible to eliminate this risk, a better understanding of seedling disease development will allow producers to make a better-informed decision on what options to implement in their program. Seedling disease development is different from traditional plant disease development.

Unlike most diseases that thrive when conditions favor the development of the pathogen, seedling disease development depends on conditions affecting development of the host. This disease is most severe when conditions are adverse for seed germination and seedling development. Therefore, the first and most important step for managing seedling disease is to plant when conditions favor rapid seed germination and plant establishment.

A successful program begins with a uniform density of two to three plants per foot. This, in part, is accomplished by planting high-quality seed when soil temperatures (4 inches deep) have been at least 65 degrees F for three consecutive days with no threat of an approaching cold front. Soil moisture should be adequate to insure rapid germination and seedling establishment.

Insist on high-quality seed. High-quality seed will have a percent cool germination ranging between 60 percent and 85 percent. The cool germination test result is not listed on the seed tag; therefore, the producer will need to contact the company of origin to obtain this result.

If a producer must plant when conditions favor seedling disease, it will be necessary to determine if an in-furrow application of a fungicide is needed. Depending on the fungicide used and rate, an in-furrow application can cost from $6 to $22. In-furrow applications can be economical; however, several factors should be considered before adding this to your program. In-furrow applications are likely to be most beneficial during the following conditions:

  1. Planting early (before May 1). Temperatures fluctuate drastically during April compared to temperatures in May. Cool soil temperatures will slow germination and increase the risk of seedling disease. In-furrow fungicides can provide additional protection during brief periods of inclement weather. In-furrow applications will not provide the protection necessary when conditions are severe or persist for an extended period of time. Avoid planting during these conditions.
  2. Fields with a history of seedling disease. In-furrow applications will protect against seedling disease pathogens in problem fields, but these fields should be planted after the first week of May to help minimize the risk of seedling disease.
  3. Poor-quality seed. Avoid using poor-quality seed. If poor quality seed must be used, it should be planted during mid-May. An in-furrow application can be used to provide additional protection against seedling disease pathogens.
  4. Planting during adverse conditions for seed germination and seedling development. It is not recommended to plant when bad weather is imminent (i.e. forecasted cool and/or wet weather); however, an in-furrow application can protect against seedling disease. The protection from in-furrow applications is superior to seed treatments alone, but this protection will not last forever, and will deteriorate over time during severe conditions.
  5. Reduced seeding rates (three seeds per foot or less). Fewer seeds mean fewer plants; therefore, it is important to protect those seed. Consider using an in-furrow fungicide when reducing the seeding rate to three seeds per foot or less.

After deciding to make an in-furrow application, the next question is: What product to use? It is important to determine what pathogens to target with your application. In Louisiana, seedling disease is usually caused by Rhizoctonia solani, Fusarium spp., and/or Pythium spp.

Rhizoctonia is pathogenic over a wide temperature range, while Pythium is most active during cool, wet conditions. In problem fields make a note of what conditions are occurring when disease epidemics are most severe. This observation will help implicate the pathogens involved. Plant samples from infected plants can be used to identify the pathogens responsible. For additional information about identifying these pathogens, contact you local county agent or your consultant.

The most predominant pathogen affecting Louisiana's cotton is Rhizoctonia solani; therefore, a product providing protection against this pathogen is always necessary. When cool, wet weather exists, a product providing protection against Pythium spp. should be added. Eliminating the need for a product effective against Pythium will save the producer around $4 per acre. There are a vast array of products and formulations of fungicides available. Prices also vary considerably; therefore, be sure you are using the product you need, and not spending money unnecessarily.

Another consideration is the formulation. In most cases, the protection provided by a liquid or granule formulation of the same active ingredient would not differ. However, liquid formulations offer several advantages. Liquids provide better coverage and give producers more tank-mix options. For example, the active ingredients in granule products can be applied only at either a high or low rate. However, liquid tank mixes can deliver the desired rate of each product (i.e. high rate for Rhizoctonia and a low rate for Pythium). In addition, there are more liquid products on the market, which increases competition and potentially reduces costs.

When can you reduce the rate? Fungicide rates can be reduced according to the situation at planting. For example, lower rates should be considered when planting in late April. In general, as conditions favor seed germination and the weather stabilizes, the benefits of an in-furrow application diminish.

The major objective is to minimize risk. Consider field history, the pathogens involved, the conditions at planting, and what conditions are likely to occur several weeks after planting.

Boyd Padgett is a LSU AgCenter plant pathologist.

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