Kathy McLean and William Gazaway

Cotton, one of the most important agricultural commodities produced in Alabama, contributes significantly to Alabama’s economic and employment base. Seedling disease is a major obstacle to cotton production in Alabama, reducing cotton yields by approximately 6% annually, which translates to losses of approximately $10,282,200. In addition, if the seedling disease complex kills cotton seedlings, the producer must also pay for replanting, which includes additional seed, insecticides, herbicides, and application costs. Producers can only replant if enough time is left in the growing season to produce a crop. AAES studies indicate that using fungicides on cotton reduces losses to seedling diseases and generates sufficient additional revenues to cover the expense of fungicide application.

The value of the production of cotton lint and seed in Alabama in this decade has averaged $171,371,000 annually. Alabama produces cotton in the northern, central, and southern regions of the state in 64% of the counties. Cotton production influences employment in Alabama affecting not only the cotton producers, farm managers, consultants, and labors but also those employed by seed dealers, chemical companies, cotton gins, warehouses, and cotton brokers.

Seed borne and soil borne fungi, acting either singly or in combination, produce the seedling disease complex of cotton. Most fungal pathogens involved in the seedling disease complex are widespread fungi that are associated with many other plant hosts including cotton. The soil borne pathogens most commonly involved in the seedling disease complex in Alabama include four basic genera of fungi which include Rhizoctonia solani, Thielaviopsis basicola, Fusarium spp., and Pythium spp. (see Recognizing the Culprits, Figures 1-6).

The severity of the cotton seedling disease is greatest when the soil is moist for prolonged periods and spring temperatures are cool. 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.

Cotton seedling disease pathogens can attack and kill cotton seeds or seedlings or reduce the viability of cotton plants in several ways. Within hours after planting, the seeds are attacked by these fungi before the seed can germinate, which results in pregermination 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 that 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 vigor of the plant and thereby reduces yields.

Producers have several options to manage cotton seedling disease. Planting later in the season when soil temperatures are warmer reduces the chance of disease; however, this leaves a narrow window of planting time. Crop rotation also helps, but often is not 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. Fungicides are the least toxic of the pesticides to warm-blooded animals and they have specific toxicities to the fungi that they control.

Cotton seedling disease management research has been conducted at the AAES’s 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 this research was to examine the efficacy of selected fungicides for control of the seedling disease complex of Alabama cotton and subsequent effects on the growth and development of the cotton plants and yield responses.

In 1999, researchers examined more than 40 fungicide formulations, including seed treatments and in-furrow granular and spray formulations. Seed treatments are liquids or powders applied directly to the seed before planting. In-furrow granule applications are dry particles less than 0.1 inch in diameter that are placed in the seed furrow at planting. In-furrow spray applications may be liquid or dry formulations that are 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 conditions a producer faces in the field. Cotton was planted between April 15 and May 3 in a soil with a two-inch depth temperature of at least 68^oF. All production and management practices were based on real farm methods; however, plot size and fungal application were conducted on a smaller scale to make the test manageable for data collection. Each fungicide test plot consisted of two to four rows 25 feet long and 36 to 40 inches wide and was planted with five seeds per foot of row. Cotton varieties included in these tests were NuCotton 35B, DPL436RR, and Stoneville 474. 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 affect on cotton growth and development. All cotton plots were harvested between September 26 and 29 to determine the effects of the treatments on cotton yields.

The use of specific fungicides for the seedling disease complex had a beneficial effect on cotton stand development and subsequent yield production. 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 nontreated control. At 14 days after planting cotton seedling stands ranged from 0.4 to 1.76 plants per foot of row in the control and Terraclor Super X EC-treated plots, respectively. At 25 days after planting, the seedling stand ranged from 0.5 to 2.2 plants per foot of row in the control and Terraclor Super X EC treatments, respectively. The addition of a fungicide into the cotton production program increased seedling stands by an average of 27% in 1999.

Cotton seedlings were collected and cultured to identify the specific fungi present causing disease. Rhizoctonia solani and Fusarium spp. were isolated most frequently from the diseased cotton seedlings.

In these studies, seed cotton yields ranged from a high of 2,855.3 pounds per acre for the Quadris to a low of 1,449.7 pounds per acre in the 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 (Terraclor Super X and Ridomil Gold PC, Rovral, and Quadris), 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 direct cost of the fungicides. Therefore, sufficient additional revenues are generated to cover all extra cost.