Survey of take-all (Gaeumannomyces tritici) on cereals in Tunisia and impact of crop sequences. (September 2020) - Gargouri, S.
Abstract: Cereal cultivation is a major component of Tunisian agriculture. Take-all of cereals caused by the soil-borne ascomycete Gaeumannomyces tritici (syn. Gaeumannomyces graminis var. tritici ) and G. avenae (syn. G. graminis. var. avenae ) is a common disease worldwide. The objectives of this study were to evaluate the incidence and distribution of the disease across the cereal growing areas in different climatic regions of Tunisia, to assess the relative importance of the two species, and to investigate the impact of crop sequences on inoculum density in soil and disease incidence. During the cropping seasons of 2009/10, 2010/11, 2012/13, and 2013/14, the percentage of fields infested varied from 8.1% in 2011 to 39.6% in 2013 and the incidence of take-all varied from 0 to 60% in the 298 fields surveyed. Disease incidence was significantly higher in wetter climatic regions than in semi-arid regions. The disease was not detected in the driest area. The highest incidence of take-all was recorded in durum wheat fields. However, overall, no significant differences were observed in the incidence between durum wheat, bread wheat, and barley crops. Take-all was not detected in any oat crop. All findings in this study including surveys, molecular identification of isolated fungi, and determination of DNA concentration in soil, indicate that G. tritici is the dominant species and that G. avenae is rarely found in Tunisian cereal cropping soils. Rotation trials conducted over four cropping seasons on the experimental station showed that a one year break crop could significantly reduce the incidence and severity of take-all in durum wheat. In addition, the inoculum density of Gt/Ga in soil estimated by quantitative PCR was significantly reduced after a legume crop compared to monoculture of durum wheat, and consequently there was a significant decrease in the incidence and severity of the disease in the following year. This study highlights the influence of climatic conditions on the distribution of take-all in the cereal growing areas of Tunisia and the importance of rotation in reducing incidence and severity of take-all. Highlights: The distribution of take-all varied according to the climatic regions and the cropping seasons in Tunisia. It was restricted to wetter regions. The highest incidences of take-all were recorded in durum wheat compared to bread wheat and barley. Oat was not infected. Break crops, such as faba bean, fenugreek, vetch and oat, reduced inoculum of Gt in the soil and thus severity of take-all.