Seedling disease continues to be a major obstacle to cotton production in Alabama, reducing yields by approximately six percent annually and causing losses of more than $10 million.
And if the seedling disease complex kills cotton seedlings, growers also must pay for replanting, which includes additional seed, insecticide, herbicide and application costs. Producers can replant only if there's enough time left in the growing season to produce a crop.
Recent studies conducted by the Alabama Agricultural Experiment Station indicate that using fungicides on cotton reduces losses to seedling diseases and generates sufficient additional revenues to cover the expense of fungicide applications.
Lead researchers in the studies were Bill Gazaway, professor emeritus of entomology and plant pathology and Kathy McLean, assistant professor.
Seed-borne and soil-borne fungi, acting either singly or in combination, produce the seedling disease complex of cotton, according to the study. Most fungal pathogens involved in the seedling disease complex are widespread fungi that are associated with many other plant hosts, including cotton.
The severity of cotton seedling disease is greatest when the soil is moist for prolonged periods and temperatures are cool, says Gazaway. “These conditions are optimal for fungal growth and stressful for cotton seed germination, emergence and growth.
“Producing cotton in the same field for several consecutive years also increases the severity of cotton seedling disease. These fungi increase in population and pathogenicity in the presence of a susceptible host plant,” says the pathologist.
Cotton seedling disease pathogens can attack and kill cotton seeds or seedlings or reduce the viability of cotton plants in a variety of ways, he adds. Within hours after planting, seeds are attacked by these fungi before the seed can germinate, resulting in pre-germination decay of the seed. In addition, pre-emergence “damping off” can occur, which is defined as death of the cotton seed after it germinates, but before it emerges above the soil surface.
Postemergence damping also may occur, which means the cotton seedling is killed after the seed has germinated and emerged above the soil surface.
“Seedling root rot also may occur up to several weeks after planting and often kills the tap root, causing the cotton plant to respond by producing secondary roots. While seedling root rot may not kill the plant outright, the cotton plant will have a shallow root system, which leads to a loss of plant vigor and a resulting reduction in yields,” explains Gazaway.
Producers have several options for managing cotton seedling disease. Planting later in the season when soil temperatures are warmer reduces the chance of disease. However, this leaves a narrow planting window.
Crop rotation also helps but often isn't a viable management option due to economic differences in crop values. Also, the wide host range, and the ability of these seedling disease organisms to survive as saprophytes reduces the effectiveness of crop rotation.
The most reliable and economic method for seedling disease control is the use of fungicides, notes Gazaway. Fungicides are the least toxic of the pesticides to warm-blooded animals and they have specific toxicity to the fungi they control.
Cotton seedling disease management research has been conducted at the Tennessee Valley Research and Extension Center in Belle Mina, the Wiregrass Research and Extension Center in Headland and the Prattville Experiment Field.
The objective of the research was to examine the efficacy of selected fungicides for control of the seedling disease complex seen in Alabama cotton and subsequent effects on the growth and development of the cotton plants and yield responses.
Researchers examined more than 40 fungicide formulations, including seed treatments and in-furrow granular and spray formulations. Seed treatments were liquids or powders applied directly to the seed before planting. In-furrow granule applications were dry particles less than 0.1 inch in diameter that were placed in the seed furrow at planting.
In-furrow spray applications were liquid or dry formulations that were dissolved in water and then sprayed in the seed furrow at planting. Each fungicide was applied at the manufacturers' recommended rates and application methods at planting. A control was included in each test to provide an example of disease severity and yield losses that can be expected if no fungicides are included at cotton planting.
The tests were conducted to simulate actual field conditions faced by Alabama producers. Cotton was planted between April 15 and May 3 in a soil with a two-inch depth temperature of at least 68 degrees F. All production and management practices were based on real farm methods.
All treatments were rated biweekly for five weeks after planting to determine the percent of seedlings killed by cotton seedling disease. The cotton was monitored throughout the summer to determine if the fungicides had any effect on growth and development. All cotton plots were harvested between Sept. 26 and 29 to determine the effects of the treatments on cotton yields.
The use of specific fungicides for seedling disease complex had a beneficial effect on cotton stand development and subsequent yield production, says Gazaway. Cotton seedlings generally emerged within seven to 14 days after planting. At 14 and 35 days after planting, more cotton seedlings survived in the Terraclor Super X 18.8 G, Terraclor Super X EC, Ridomil Gold PC, Rovral and Quadris treatments than in the non-treated control.
The addition of a fungicide into the cotton production program increased seedling stands by an average of 27 percent during this test, conducted in 1999.
In these studies, seed cotton yields ranged from a high of 2,855 pounds per acre for the Quadris to a low of 1,449 pounds in the untreated control. Lint yield data indicated an average lint yield across all fungicide treatments of 1,005 pounds, representing a 108-pound increase over the control.
A market price of 60 cents per pound of cotton would indicate the additional 108 pounds of lint per acre is worth $64.80 per acre to the producer. Averaging the cost of the three commercial materials, the average cost per acre for a fungicide is $16.70 per acre.
If the additional cost of $16.70 per acre for the fungicides is compared with the additional revenue of $64.80 generated from the added lint production, the producer realizes a $48.10-per-acre return for the fungicide investment.
“The economic analysis indicates that the fungicide treatments produced a positive return above the direct cost of the fungicides. Therefore, sufficient additional revenues are generated to cover all extra costs,” according to the study.
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