Cotton Research Institute - Pathology Section

The pathology section carries out research work on major cotton diseases in the country which are Verticillium wilt, Bacterial blight and various ailments that affect young cotton plants (Seedling diseases). This section works closely with the breeding section mainly to screen all germplasm (strains, lines, varieties) for tolerance to the major cotton diseases. Only those cultivars that show good tolerance to these diseases are released and/or used in further screening/crossing programmes. An example of a variety with disease resistance is BC 853 which has good tolerance to Verticillium wilt. All released varieties in Zimbabwe have good resistance to Bacterial blight. Test fungicides against seedling diseases are also evaluated and rates determined before registration and recommendations for use by farmers.

• Screening of Germplasm for Bacterial Blight

This work assesses germplasm coming out of the breeding programme for its tolerance to Bacterial Blight caused by Xathomonas campestris pv malvacearum before the variety can be released. The germplasm that comprises strains, lines and varieties is assessed for tolerance using the stem inoculation method. Scores of each line, strain or variety are taken 21 days after inoculation on a scale of 1-5. A score of 1-1.4 indicates the material is resistant; 1.5-2.4 indicates intermediate resistance and scores above 2.5 indicates susceptibility. All lines with a BB score of more than 3 are discarded from the breeding program.

• Screening of germplasm for Verticillium Wilt

Verticillium wilt is on the increase in Zimbabwe. This disease is very difficult to control once it is established in the field. There can be significant yield losses of 20 – 100% if susceptible varieties like SZ9314 are grown in a field with Verticillium wilt. In wilt-infested fields at Cotton Research Institute, and Henderson infection percentages are calculated at the end of the season. Most Zimbabwean cotton varieties have good tolerance to Verticillium wilt under natural infection.

Achievements

The pathology section developed inoculation methodologies, screening techniques and also perfected storage of the bacterial blight pathogen. This has led to better screening of breeding materials for blight like FQ 902 and BC 853.   The pathology section also developed and perfected inoculum preparation, screening techniques and the timing of stem inoculation for Verticillium wilt screening. This has helped in the selections for breeding materials used in crosses and in the release of cultivars with Verticillum wilt tolerance like BC 853 and CY 889. The section also established that Alternaria leaf spot is a problem if the crop is predisposed to stress conditions e.g. potassium deficiencies. Amelioration of potassium deficiencies reduced disease incidence.

This section developed cost effective inoculum preparation and storage techniques for Rhizoctonia solani which helps in effective screening of fungicides against the pathogen. Effective fungicides for seed dressing and in-furrow treatments were recommended to farmers. The section recommended Maxim XL fungicide for seed treatment to prevent seedling diseases. Fungicides reduce seedling diseases and extended cotton production to areas not possible before because of stand losses caused by seedling diseases.

The varieties CRI-MS-1 and CRI-MS-2 were released in 2006. These varieties are significantly superior to SZ 9314 in both yield and environmental resilience and are now a new benchmark for CRI’s variety development program. DR&SS hopes that these varieties will be available to growers soonest as further delays will prejudice the whole cotton industry.

CRI is committed to continuous improvement in cotton variety development, production technologies development, evaluation of agro-chemicals used in cotton, and information dissemination.