Effect of Sulfur Nutrition and Thiobacillus Inoculation on Suppression of Take-all Disease Caused by Gaeumannomyces tritici

Wheat take-all caused by Gaeumannomyces tritici fungus is one of the most destructive diseases in the world. The disease is common in alkaline to natural, non-fertile and low-drainage soils. Thereby, wheat take-all can be suppressed by reducing soil pH. In recent years, sulfate-containing fertilizers had been substituted with new sulfur-free fertilizers which decrease soil content of sulfur and sulfur-oxidizing bacteria, significantly. So, soil amendment with mineral sulfur or Thiobacillus spp. could be reconsidered to reduce soil pH. However, soil amendment with mineral sulfur seems to be more effective when the rhizosphere is colonized by sulfur-oxidizing bacteria. This study aimed was to investigate the effect of mineral sulfur and Thiobacillus sp. on take-all disease and wheat growth.

Physicochemical properties of soil were analyzed based on standard methods. Sulfur was mixed with soil in 0, 2, 4 and 8 g/kg of soil with three methods. In the first method, sulfur amended soil was inoculated with the pathogen and sown immediately. In the second method, amended soil was incubated for two months then inoculated and seeded with wheat. In the third method, sulfur amended soil was inoculated with Thiobacillus sp. bacteria and sown immediately. G. tritici inoculum was prepared on oat seeds and was placed in a layer under plant seeds. Disease severity was checked once plants seedling in control pots exhibited the disease symptoms. The experiments were carried out in a completely randomized design with four replicates. Analysis of variance was done by SAS 9.1.

Analysis of soil showed the examined soil was calcareous and moderately alkaline. It has appropriate nitrogen and zinc content but slightly Fe, Cu and P are deficient. Results exhibited that sulfur amended has no significant role on take-all disease neither with incubation nor without incubation. However, mixed treatment with sulfur and Thiobacillus sp. decreased disease severity in concentration depended on manner. The highest disease suppression achieved in 8 g /kg sulfur of soil which decreased disease severity up to 45%. Soil amendment with 2 g/kg sulfur of soil either incubated or not increased plant dry weight 20 and 25%, respectively. However, a further increase in sulfur concentration showed no more increase in plant growth. In a combination with sulfur and Thiobacillus sp., wheat aerial parts dry weight was increased paralleled with an increase in sulfur concentration and reach to 25% higher than control in 8 g /kg sulfur of soil. Calcareous soils are highly buffered and it is very difficult to change their pH. In our experiment, the Soil amendment with mineral sulfur was unable to reduce pH as much as necessary for Take-all suppression. However, sulfur had synergistic interaction with Thiobacillus sp.. Indeed, these bacteria can colonize the plant rhizosphere efficiently. Thereby, they can reduce the pH of the rhizosphere zone instead of whole balk soil. In this situation, G. tritici could not survive in an acidic condition of the rhizosphere. On the other hand, mineral sulfur improved plant growth in all concentrations. It should be considered that sulfur could be uptake by the plant as a nutrient element. However, sulfur-oxidizing bacteria will help the plant to uptake sulfur more efficiently. It should be considered that Thiobacillus is compatible with other biocontrol bacteria such as Bacillus and Pseudomonas. These bacteria are highly effective in Take-all disease but cannot survive well in calcareous soils. Co-inoculation of these bacteria with Thiobacillus sp. in the presence of sulfur seems to be a promising approach in biological control of Take-all disease.

Our results showed that a combination of sulfur with sulfur-oxidizing bacteria could be a promising approach in the suppression of Take-all in wheat. In recent years, integrated disease management and integrated crop management receive high attention in susceptible agriculture. In these strategies, agricultural practices such as plant nutrition are considered as multifaceted practice. This means that soil amended with specific fertilizer not only increases plant growth but it can increase or decrease specific plant disease. In the case of sulfur, this element not only decreases Take-all disease but also improves the availability of other nutrients such as Fe, Zn and Cu. Furthermore, it should be considered that soil amendment with a low amount of sulfur is not effective to reduce soil pH in buffered calcareous soils and we need to add sulfur-oxidizing bacteria to improve fertilizer efficiency. This combination is effective against Take-all disease.