Cotton seed treatments are designed to protect seed and seedlings from a complex of soil fungi that cause a rotting of seed, roots, and hypocotyl tissues. The extent of disease is mainly dependent on the concentrations of propagules of individual fungal pathogens, soil temperature, and the degree of susceptibility of disease resistance of cotton cultivars. Seldom are cotton fields alike in the kinds and concentrations of pathogens present in soil. Ideally, seed treatments should be customized for individual fields. However, since the major fungal pathogens are Pythium ultimum (seed and root rot, damping-off), Rhizoctonia solani (seed rot, sore-shin, damping-off), and Thielaviopsis basicola (black root rot, stunting), seed treatments and the search for disease resistant cotton germplasm have mainly dealt with these fungi.

Among registered cotton seed treatments, Apron [N-(2,6-dimethylphenyl)-N-(methoxyacetyl) alanine methyl ester], is most widely used to control Pythium species, while Chloroneb (1,4-dichloro-2,4-dimethoxy-benzene), Vitavax (5,6-dihydro-2-methyl-1,4-oxathiin-3-carboxanilide), and Terraclor or PCNB (pentachloronitrobenzene) are effective against R. solani. At present, none of the registered chemical seed treatments are effective in the field against T. basicola, although Imazalil [l-(2-(2,4-dichlorophenyl)-2-(2-propenlyoxy) ethyl)-l-H. imidazole] has been useful in greenhouse tests. Baytan (1-(4-chlorophenoxy)-3,3-dimethyl-l-(lH-1,2,4-triazol-1-y1)-2-butanone], which is nearing registration, has shown exceptional protective action against T. basicola in field trials. Protective action against complexes of Pythium species and R. solani has usually resulted from the use of combination seed treatments, such as Apron 2.66F (1 oz/cwt) plus a mixture of Vitavax (17%)--PCNB (17%) at 6 oz/cwt (Gustafson, Inc., Dallas, TX) or a combination of Apron 2.66F (1 oz/cwt) and Nu-Flow ND (14.5 oz/cwt) (Wilbur-Ellis Co., Fresno, CA). Nu-Flow ND is a mixture of Chloroneb (23 %) and TCMTB [2-(thiocyanomethyl) (thio)benzothiazole] (9%). TCMTB and Vitavax are more broadspecturm, in their fungicidal activity but less effective against R. solani than PCNB and Chloroneb. A hopper box treatment using a combination of Apron Terraclor (17% Vitavax plus 17% PCNB) (Gustafson, Inc.) also has been effective against seedling diseases. In the present research, comparisons were made of seedling survival from seed with these combination treatments a well as with several unregistered materials such a Baytan and Rovral [3-(3,5-dichlorophenyl)-N-(l-methylethyl)-2,4-dioxo-l-imidazolidine] (Rhone-Poulenc, Inc., Fresno, CA) at six different field locations. These mixtures comprise the most effective chemical seed treatments for cotton and have increased seedling stand counts from an average of 58% for untreated see to 74%. for treated seed in the field tests. Chemical seed treatments vary in their effectiveness from field to field because of differences in the concentrations of various pathogens as well as from the effects of other microorganisms and soil characteristics.

With increasing constraints involving the use of synthetic agricultural chemicals, there has been an increased interest in the use of naturally occurring biological controls of soilborne pathogens. Although very few biocontrol organisms have been registered fo this purpose, Quantum 4000 (Bacillus subtilis) (Gustafson, Inc.) is an example of a spore preparation registered for use on several crops including cotton. Considerable biocontrol research on cotton has involved soil fungi such as Trichoderma species and Gliocladium virens. Strains of these fungi have been selected for activity against Pyrhium species or R. solani. A strain of T. harzianum used in this research has been registered for use by Eastman Kodak Co., (Syracuse, NY) but it is commercially unavailable at present.

Field tests comparing the protection afforded by biocontrol fungi and bacteria as cotton seed treatments were begun in California in 1974. Several kinds of bacteria and fungi developed by different companies and research laboratories as well as microorganisms isolated in our laboratories have been tested. A major problem has been the lack of a satisfactory material for coating seeds with spores of biocontrol organisms. A breakthrough came in 1990 when it was discovered that a mixture of carbohydrate polymers used for coating pharmaceuticals, Opadry Blue Type F (Colorcon, West Point, PA), was ideal and provided an excellent carrier and seed coating for biocontrol fungi and bacteria (1). It is now possible to obtain seed and seedling protection with strains of biocontrol fungi, such as G. virens (3), equal or better than that obtained with widely used chemical seed treatments. For example, in the 1991 field trials, the average seedling survival in two fields for chemical seed treatments was 68%, while for G. virens (Pythium antagonist) and G. virens (Rhizoctonia antagonist), seedling survival reached 84% and 81%, respectively. Additionally, seedling survival from seed treated with T. harzianum (Eastman Kodak Co.) was 66% and with Kodak (B. subtilis, Gustafson, Inc.), it was 59 %. The treatments with G. virens were significantly better (P=0.05) than the other seed treatments including the untreated control seed.

A third area of research emphasis for controlling cotton seedling diseases is the development of disease resistant cotton cultivars. Except for the TAMCOT MAR cultivars, most cotton cultivars are highly susceptible to seedling diseases. Only recently have cultivars been developed which are highly resistant to Pythium spp. (2). Most outstanding in field performance are Acala Royale, Acala Maxxa, and a breeding line Acala C-396. As yet, no resistance has been found toward T. basicola (black root rot), while a low level of resistance to R. solani is present in Acala C-396.