New Developments in Combinations of Biological and Chemical Seed Treatments Designed to Manage Seed and Seedling Diseases of Cotton

J. E. DeVay, R. H. Garber, R. J. Wakeman, C. R. Howell, B. L. Weir, S. D. Wright, D. S. Munk, and R. N. Vargas


Combination seed treatments for management of seed and seedling diseases of cotton (Gossypium spp.)are designed mainly for control of three soilborne fungal pathogens: Pythium ultimum, Rhizoctonia solani, and Thielaviopsis basicola. Other fungi, such as Fusarium spp., including F. equisiti, may be seed borne and are known to cause seed and seedling diseases of cotton; however, registered seed treatments in current use have not been effective against these fungi. In contrast, combinations of chemical and/or biological seed treatments, which are selective for control of P. ultimum and R. solani, are commonly used. None of the currently registered seed treatments control T. basicola which causes black root rot of cotton. Thus, an urgent need exists for registration of Baytan (Gustafson, Inc., Dallas, TX) and Nu-Flow M (Wilbur-Ellis Co., Fresno, CA), both of which are effective seed treatments for control of T. basicola and also R. solani. Also needed, is the development of seed treatments for control of Fusarium spp. Performance trials of various combinations of both chemical and biological seed treatments were made in 1993 in five fields with histories of seedling disease problems. Isolations of pathogens from diseased cotton seedlings collected from these experimental plots and plated on several culture media, indicated which fungi were the predominant cause of the seed and seedling diseases. While R. solani was the main pathogen in experimental plots in Tulare and Kern Counties, T. basicola was isolated from most seedlings in plots from Merced and Fresno Counties. Commonly isolated were various mixtures of pathogens from a single diseased seedling. Although P. ultimum has been recognized as a major seedling pathogen of cotton in California, the development and release of Acala cultivars (Royale and Maxxa) which are resistant or highly tolerant of Pythium seed and root rot, have lessened the importance of this pathogen. A major unknown in these test plots is the role of Fusarium spp. which were isolated in most plots from 100% of the diseased seedlings. Greenhouse tests, using soil infested with single or combinations of fungal isolates have been useful for evaluating pathogenicity and the selective effects of various seed treatments; however, such tests have not been indicative of the disease potential of Fusarium spp. Results of field tests demonstrated the highly effective combination of Apron FL + Baytan 30F + Vitavax-PCNB (Gustafson, Inc.) for controlling seed and seedling diseases of cotton. When supplemented with Prevail or the biocontrol bacterium, Kodiak (Bacillus subtilis) as hopper box overtreatments (both from Gustafson, Inc.), no significant increases in seedling stands or lint yields were apparent. Also the combination of Nu-Flow AD + Nu-Flow M (Wilbur-Ellis Co.) gave excellent performance for controlling seedling pathogens in several field tests. Additionally, the combination of Apron TL + NU-Flow ND, which is registered for use on cotton, was excellent in its performance as a chemical seed treatment; it was used as a standard of comparison for biocontrol seed treatments involving Gliocladium virens. Two strains of G. virens were used: G-4, which selectively inhibits P. ultimum and G-6, which inhibits R. solani. Seed of three cultivars, Acala SJ-2, Acala Royale, and Acala Maxxa, were coated with Colorcon, a mixture of carbohydrate polymers, containing ground millet seed which had been colonized by one or the other of these biocontrol fungi. Approximately, 30,000 viable propagules were coated on each seed, assayed about two weeks before planting. Effective shelf-life of G. virens on seed (>1000 viable propagules/seed) was about four months. In four field tests in Kern, Tulare, and Merced Counties, none of the biocontrol treatments were significantly better than the nontreated control although in three of the tests some of the biocontrol treatments were as effective as the chemical seed treatment.

Reprinted from Proceedings of the 1994 Beltwide Cotton Conferences pg. 272
©National Cotton Council, Memphis TN

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Document last modified Sunday, Dec 6 1998