Oct. 30th, 2012: Zimbabwe rust survey summary – Stem and Leaf rust at low levels, but indications of possible new leaf rust races

Credit: Bruce Mutari

Following the widespread distribution of rusts diseases observed in 2011 main season, a survey team co-led by Mr Bruce Mutari (Crop Breeding Institute) and Mrs Mutisi (Plant Protection Research Institute) carried out surveys throughout the key wheat growing areas of Zimbabwe during the period 10th – 16th September 2012. The primary focus of the surveys was to assess the status of wheat stem rust in Zimbabwe compared to the situations in 2009, 2010, 2011 and to monitor the potential spread of virulent races of wheat stem rust (Ug99 race lineage) in Zimbabwe and Southern Africa as a whole. The majority of the crops observed were at dough to maturity, with some crops already harvested. In the survey area, wheat was grown at elevations ranging from 422m to 1605m. A total of 22 wheat fields, distributed throughout the main agro-ecological zones, were surveyed using standard BGRI survey methodology. The highveld region contained the highest density of wheat fields. Surprisingly, wheat stem rust was not widespread and only positively recorded at 1 (4.5%) of the 22 survey sites; Rattray Arnold Research Station, in the middleveld region. Whilst not at the same level of disease pressure as 2010 and 2011, leaf rust was the most widespread disease as it was recorded at 9 (40.9%) of the 22 survey sites (Nyanga 1, Nyanga 2, Sisal farm, Save Valley, Birchnough Bridge, Panmure Research Station, Gwebi Variety Testing Center, Rattray Arnold Research Station and ARTfarm. For the first time in four years, stem rust was not recorded in the Lowveld. At Rattray Arnold Research Station, high levels of stem rust infections were observed in a single experimental line which exhibited highly susceptible reactions. At Birchnough Bridge, high incidence and severity of leaf rust was observed in an irrigation scheme with SC Nduna, Kana and SC Sekuru (improved varieties) showing highly susceptible reactions. Low incidence and very high severity of leaf rust was recorded at GVTC in a Kana breeder’s seed field. Trace amounts of leaf rust were recorded at Agricultural Research Trust farm in contrast to last year where very high incidence and severity was recorded at the same station. In addition to one stem rust sample, nine leaf rust samples were collected and sent to Prof.Z.Pretorius, University of Free State, RSA for race analysis. One stem rust sample was sent to Dr.L. Szabo, Cereal Disease Laboratory (CDL), U.S.A for DNA – molecular diagnostic analysis. As compared to the previous seasons, powdery mildew was much more widespread as it was regularly recorded at survey sites. No stripe rust was observed at any of the survey sites visited.

Results from the DNA analysis indicated that the stem rust race present was TTKSF, although it must be noted that the CDL molecular diagnostic assay used is currently unable to differentiate between races TTKSF and TTKSF+. Conventional race analysis at the University of the Free State confirmed that the stem rust race was TTKSF.

The difference in the incidence and severity of stem and leaf rust between 2010, 2011 and 2012 may be the result of the unfavorable environmental conditions that prevailed during the 2012 season. The 2012 wheat cropping season was characterized by long periods of dry conditions and cool temperatures which are not favorable for rust development. The decline in wheat hecterage in Zimbabwe (from 10 000ha to 5 000ha) might also have resulted in this decrease in rust incidence because of the reduction in suitable hosts. In addition, for the past two seasons, most farmers in Zimbabwe have been adhering to the recommended planting dates which is very important for rust escape. By planting early, the crop escapes stem rust infection since it is common/develops late in the season when temperatures begin to rise.

The breakdown of leaf rust resistance on improved cultivars, SC Sekuru, SC Nduna, Kana and Insiza (highly susceptible reactions) may indicate the presence of new biotypes of leaf rust. However further laboratory analysis needs to be undertaken to confirm if this is the case.

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