Linda Benedict | 5/3/2005 12:03:15 AM
Boyd Padgett, Patrick Colyer, Eugene Burris and Ken Whitam
Cotton seedling diseases, caused by fungi, can reduce seedling emergence and plant establishment. The fungi commonly found attacking cotton in Louisiana are Rhizoctonia, Fusarium and Pythium. The detrimental effects from these pathogens are typically non-uniform plant populations and reduced plant vigor. In severely damaged fields, producers may be forced to replant, costing time, money and yield potential. In 2000, statewide losses to seedling diseases were estimated at 3 percent, or $7,051,832. Producers manage seedling diseases using cultural practices and fungicides. Cultural practices that help avoid or reduce the incidence of seedling disease include planting when conditions are conducive to rapid seed germination and seedling emergence. Conditions are considered favorable when the 4-inch soil temperature is 65 degrees F or higher for three consecutive days, and there is no approaching cold front or excessive rain. The weather during the planting period of April and May, however, is unpredictable and extremely variable.
Fungicides help protect cotton from the effects of seedling disease pathogens. Fungicides are present on the seed at purchase, and supplemental fungicides can be applied to the seed in the hopper, known as hopper box treatments, or in the furrow at planting. Though limited by the amount of material that adheres to the seed, hopper box applications are relatively easy to perform and cost from $1.68 to $4.50 per acre. In-furrow fungicides treat the seed as well as the surrounding soil and cost from $4.35 to $22.00 per acre.
While hopper box and in-furrow applications of fungicides are effective, they may not always be economical. Therefore, to determine the efficacy of hopper box applications as options for seedling disease management, experiments were conducted at the LSU AgCenter’s Macon Ridge, Northeast and Red River research stations. Fungicides Delta Coat AD (Wilbur-Ellis), Prevail (Trace Chemical Inc.) and System 3 (Setre Chemical Co.) were evaluated in 28 field tests conducted from 1991 through 2001.
Delta Coat AD was applied at 11.75 fluid ounces per 100 pounds (cwt) seed; Prevail at 12 ounces per cwt seed; and System 3 at 12 ounces per cwt seed. In three tests at the Macon Ridge location, disease pressure was enhanced with an at-planting, in-furrow application of inoculum. During this 11-year period, planting dates ranged from April 2 to May 13. Hopper box treatments were compared with commercially treated seed and in-furrow applications of Ridomil PC or Terraclor Super X at recommended rates.
Fungicide treatments were evaluated in field plots and template experiments. The field plots were four 100-foot row or 45-foot rows spaced 40 inches. Cotton was grown using conventional tillage practices. Plant densities (plants per foot of row) were recorded two, three, four and six weeks after planting, and seed cotton was harvested when possible.
Four template studies were conducted at the Macon Ridge Research Station. These plots consisted of 50 seed arranged in 10 rows, five seed per row spaced 1.5 inches apart, of a selected seed treatment. Plant emergence was recorded from six to 32 days after planting.
Eleven field plot tests evaluating Delta Coat AD were conducted at the Macon Ridge and Northeast research stations. Five tests were harvested. When compared to commercially treated seed, greater plant densities were observed in cotton treated with Delta Coat AD in five tests. Yields from cotton treated with Delta Coat AD were increased over those with commercially treated seed in one test (enhanced disease pressure). Plant densities and yields from tests evaluating Prevail or System 3 were not improved over commercially treated seed or in-furrow fungicides. Plant densities for Delta Coat AD, Prevail and System 3 averaged across the harvested tests were not different from commercially treated seed and cotton treated with an in-furrow fungicide (Table 1).
In template studies, plant emergence for hopper box treated seed did not differ from commercially treated seed. When averaged across tests, percentage of emergence six days after planting for Delta Coat AD and commercially treated seed averaged 58 percent and 61 percent, respectively. Emergence six and 32 days after planting for cottonseed treated with System 3 and Prevail ranged from 50 percent to 76 percent and 56 percent to 77 percent, respectively, compared to 58 percent and 81 percent for commercially treated seed.
In six tests conducted at the Red River Research Station evaluating Delta Coat AD, plant densities were increased over commercially treated seed in two tests; however, yield was not affected. Plant densities and yields from cotton treated with Prevail were not improved over those observed in commercially treated seed. Average plant densities and seedcotton yields across tests for Delta Coat AD and Prevail were similar to those observed in commercially treated seed and in-furrow treatments (Table 2).
Hopper box applications increased plant density over commercially treated seed in 25 percent of the tests and resulted in an increase in yield in 6 percent of the tests. The performance of hopper box fungicides was similar to in-furrow applications of TSX or Ridomil-PC. However, in-furrow fungicides treat the seed and surrounding soil, covering a more extensive area than hopper box applications. This increased efficacy of hopper box fungicides over commercially treated seed is most likely related to environmental conditions at planting. Consequently, planting during favorable conditions for rapid seed germination and seedling emergence should be the first line of defense against seedling disease. If cool, wet conditions prevail, additional fungicides can help preserve plant populations. These increases in plant density rarely translate to increased yield because many other factors affect plant development and yield after planting.
Boyd Padgett, Associate Professor, Macon Ridge Research Station; Patrick Colyer, Professor, Red River Research Station, Bossier City, La.; Eugene Burris, Professor, Macon Ridge Research Station, Winnsboro, La.; and Ken Whitam, Plant Science Specialist, Louisiana Cooperative Extension Service, LSU AgCenter, Baton Rouge, La.