Cotton root rot caused by the pathogen, Phymatotrichopsis omnivora is a very serious disease of cotton grown in the Southwest. Studies to control this disease have been conducted over many years and by numerous researchers. The purpose of these studies was to develop economically feasible and field applicable methods to suppress the disease to tolerable levels.

Field tests evaluating various chemical and cultural treatments for Phymatotrichopsis root rot control have been conducted at numerous off-station sites ranging from the Rio Grande Valley through the Coastal Bend region of southern Texas and the San Angelo area. USDA-ARS scientists, Dr. Shoil Greenberg and Dr. Joe Bradford at Weslaco also worked on the project.

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 to 16 inches. Additional treatments evaluated sodium chloride (table salt) and carbonate at 2,000 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 fumigant could be used if combined with fertilizer grade ammonia. These control measures failed to produce significant results.

Recent research concentrated on developing controlled release formulations of chemical fungicides, which have shown toxicities to the cotton root rot pathogen. One in particular, Propiconazole, has shown good suppression of the fungus. Additionally, studies of the effects of depressing the alkalinity (pH) of the soils that are highly supportive of the disease have been compared with soil additions of chelated trace elements such as iron and have shown some success on disease suppression.

Most recently, for the first time biofumigation field studies have been initiated using winter cover crops of Brassica juncea plants, a species of mustard (Not all mustard species have an effect on cotton root rot.). Under irrigation the winter cover crop was grown in studies at the USDA-ARS Center Farm at Weslaco, Texas, followed by soil incorporation of the grown plants and planting cotton in early March.

This approach to possible biological suppression of the cotton root rot pathogen is predicated on the theory that plants such as the Brassica juncea containing glucosinolate could provide toxins upon soil incorporation and hydrolysis. Ideally, these toxins (isothiocyanates) could inhibit germination of sclerotia and hyphal growth of the root rot fungus and reducing the severity of the disease on the succeeding cotton.

Results of these studies include are encouraging. Chemical treatments to soil show promise.

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 midseason and 62 percent late season below mortalities recorded for corresponding untreated controls (UTC).

Powdered sulfur maintained good disease suppression over the six week critical period while disease pressure was increasing. These field results substantiated previous greenhouse research that showed substantial disease suppression with soil pH reduction.

Other results from these recent studies were also encouraging. Chelated iron (2 pounds per acre) applied as a stem drench was effective early in reducing incidence of PRR with reduction of 41 percent, but became less effective with 21 percent and 12 percent during mid and late season, respectively.

The most effective treatment was a controlled release formulation of Propiconazole, a triazole fungicide applied in the seed row at planting (Fig. 1). Disease suppression ranged from 75 percent early to 57 percent in late season. This was followed closely by a preplant band application of ammonium sulfate in the seed row.

Biological treatments also are encouraging.

In field experiments at the Weslaco USDA-ARS Research Farm a winter cover crop, Brassica juncea (mustard) used as a biofumigant produced encouraging results and are presented in Figure 2. In the initial year, average reductions in plant mortalities ranged from 50 percent early to 58 percent in late season. In subsequent years this potentially new technology for suppression of the disease still showed an approximate 30 percent reduction in the cotton root rot disease.

Brassica juncea seed meal used for the first time as a sidedress treatment on dryland cotton in 2009 suppressed plant mortalities from 32 percent to 35 percent compared to untreated control (Fig.3). This computes to a 65 percent reduction in cotton root rot.

Laboratory studies (Table 1) on inhibition of Phymatotrichopsis omnivora sclerotial germination in soils by oilseed meals from Brassica plants conducted recently by Dr. Terry Gentry, microbiologist at Texas A&M University, and graduate students highly support the field test results presented earlier.

Brassica seed meal treatment at 1 percent and 5 percent application rates both completely inhibited fungal hyphal growth during the 4 weeks of incubation (Table 2).

Results of stem drenching with fungicides at the Tom Green County site are presented in Fig. 3. Stem drenching data for the October 2nd mortality counts showed the disease progressed very rapidly increasing from 25, 000 to 30,000 in late August to 50,000 to 63,000 dead plant per acre in early October. The untreated check figured approximately 95 percent plant loss from the original stand. The progression of the root rot disease was slightly faster for the new fungicide, Prothioconazole, when drenches were made earlier (42 days after planting) while higher mortalities were recorded for Propiconazole at the later drenches (56 days after planting).

These differences were non-significant between fungicides but both were significantly lower than both untreated checks. Seed cotton yields showed approximately 300 percent to 400 percent increase in yield from stem drenching (Fig.4). Prothioconazole appeared to be 85 percent to 90 percent as effective as the older Propiconazole in disease suppression and cotton yield increase, when applied at 42 days after planting.

So far, results from our studies indicate some success in suppression of this serious cotton disease. Use of controlled release dry formulations of Propiconazole at planting showed good results at most locations. Use of certain trace elements and soil amendments to reduce soil pH also resulted in disease suppression. Stem drenching with a new Prothioconazole or the older Propiconazole fungicide produced the most promising results.

Biofumigation via Brassica juncea winter cover crop and seed meal showed promising results in disease suppression. Use of certain trace elements and soil amendments to reduce soil pH also appeared very encouraging. Additional research should investigate combining the most promising treatments in suppressing this disease.