Cotton root rot caused by the soil borne pathogen, Phymatotrichopsis omnivora is a very serious disease of cotton grown in the Southwest.

Field tests evaluating various chemical and cultural treatments for Phymatotrichopsis root rot control are being conducted at numerous offstation sites ranging from the Rio Grande Valley through the Coastal Bend region of southern Texas and the San Angelo area.

Experiments in Nueces County were established on a Clareville sandy clay loam soil in 1979, and data collected in 1980 and 1981. Early treatments in the 1980s compared various methods of anhydrous ammonia application at soil depths down to 16 inches. Additional treatments evaluated sodium chloride (table salt) and carbonate at 2000 pounds per acre.

Plant mortalities due to the disease were measured at various times during the growing season. Later studies combined ammonia with the fumigant Telone II at various rates to determine if lower rates of the costly Telone II could be used if combined with fertilizer grade ammonia.

Early counts of diseased plants showed no significant alleviation from salt or ammonia. Later diseased plant counts just prior to harvest indicated essentially no effect from all treatments with the exception of split depth application of ammonia and the sodium carbonate treatment.

Treatment averaged a mortality rate of 40 percent compared with 16 percent and 23 percent for split depth application of ammonia and sodium carbonate treatments, respectively. This was a non-significant suppression of the disease.

Other methods for disease control tried in previous years included soil fumigation using the chemical Telone II. It was used alone and also with anhydrous ammonia as a carrier for the chemical fumigant applied some four weeks prior to planting.

Soon after plant emergence phytotoxic effects were noted on the cotton plants in the form of stunting and chlorosis. These deleterious effects continued for five weeks when 50 gallons per acre or more of Telone II was used.

Ammonia at 120 pounds per acre did not appear to ameliorate this effect; however, the 240 pounds per acre rate did reduce the phytotoxic effect. Dry matter yields taken at the eight-week stage showed the phytotoxic effects from Telone II largely disappeared.

In this study early mortality counts from the disease showed little or no difference due to treatment. At midseason diseased plant counts showed significantly higher numbers when 240 pounds per acre of ammonia was used compared to the 120 pounds per acre rate. Deep chiseling alone appeared to reduce root rot damage slightly.

High rates of ammonia continued to show enhancement of the disease problem when final counts were made. Significant reduction in plant mortalities from the disease occurred only when 160 gallons per acre of Telone II was used. However, such a rate would not be economically feasible for commercial production of cotton.

Recent research has concentrated on developing formulations of chemical fungicides, which has shown toxicities to the cotton root rot pathogen.

One in particular, which has shown good suppression of the fungus, has been Propiconazole. Additionally, studies of the effects of depressing the alkalinity of the soils that are highly supportive of the disease have been compared with soil additions of chelated trace elements such as iron.

Most recently, in 2006 and 2007, for the first time biofumigation studies were initiated using winter cover crops of the genus, Brassica.

In cooperation with scientists Joe Bradford and Shoil Greenberg, the irrigated cover crop was grown during November, December and January at the USDA-ARS Center Farm at Weslaco, Texas. The crop was soil incorporated and cotton was seeded in early March.

This approach for possible biological suppression of the cotton root rot pathogen is based on other research studies in the Northwest United States where glucosinolate containing plants such as the Brassicas could provide toxins upon soil incorporation and hydrolysis. Ideally, these toxins (isothiocyanates) could kill the soil borne cotton root rot fungus or at least reduce the severity of the disease on the succeeding cotton crop.

Cotton grown previously at the Weslaco site suffered severe damage from root rot. In this experiment powdered elemental sulfur at 750 pounds per acre, band-applied preplant, suppressed the disease by 88 percent early, 58 percent at mid-season, and 62 percent at late-season below mortalities recorded for corresponding untreated controls.

Powdered sulfur maintained good disease suppression over the six-week critical period while disease pressure was increasing. Previous greenhouse research showed substantial disease suppression with soil pH reduction using elemental sulfur.

Other results from this new study were also encouraging. Chelated iron (2 pounds per acre) applied as stem drench was effective in reducing incidence of PRR with percentages of 41 percent, 21 percent, and 12 percent for the early-, mid- and late-season, respectively.

Stem drenching with Bayleton at 2 pounds per acre active ingredient reduced diseased plants by 70 percent below the corresponding untreated check at the 6/16 and 6/26 counting dates but decreased to 29 percent at the 7/31 date (Fig. 1). The reduced efficacy of Bayleton as time progressed indicates a need for controlled release formulation of this material,

The most effective treatment was a controlled release formulation of Propiconazole, a triazole fungicide applied in the seedrow at planting (Fig. 1). Disease suppression ranged from 75 percent early to 57 percent in late-season.

In our most recent investigation involving biofumigation with a winter Brassica cover crop, results were somewhat encouraging as presented in Figure 2. In the initial year, average reductions in plant mortalities were amazing and ranged from 50 percent early to 58 percent in late season. However, additional studies are needed on this new technology as the results were variable.

In summary, this serious Southwest cotton disease can be suppressed to varying degrees rather than completely eliminated. Our results indicate the following can be variably effective:

Further research should focus on combining soil amendments with biofumigation.