Effect of cultivar mixture in control of four foliar diseases of wheat

Introduction
One of the effective strategies to control wheat diseases is to use a mixture of varieties. Basically, varieties with good and similar agricultural traits, but different in diseases resistance should be used. It is common to use a mixture of cultivars, when the high-yielding cultivar is sensitive and the low-yielding cultivar is resistant, so that their mixture has an acceptable efficiency. The mixture of cultivars is defined as a mixture of different cultivars that are similar enough to grow together, but differ in many traits such as disease resistance. The mixture of cultivars does not completely stop the disease, instead it reduces the disease progression rate by eliminating a large number of pathogen spores in each reproductive cycle. The aim of this research was to investigate the effect of mixed cultivation of three cultivars, Kohdasht, Moghan and Morvarid, in controlling four diseases, powdery mildew, tan spot, stripe rust and leaf rust.
Materials and methods
To investigate the effect of the mixture of cultivars in control of wheat leaf diseases, a research was performed in a randomized complete block design with 12 treatments and four replications in the Gorgan Agricultural Research Station, Gorgan, Iran. The studied cultivars included three cultivars, Kohdasht, Moghan, and Maorvarid, and the evaluated diseases were four leaf diseases including powdery mildew, tan spot, stripe (yallow) rust, and leaf (brown) rust. Powdery mildew and tan spot were measured based on contamination rate using the Sari and Prescott scale, and atripe and leaf rusts were measured based on the disease type and severity. Final disease severity and standardized area under disease progression curve (SAUDPC) were evaluated for all four diseases.
Research findings
The results of the statistical analysis of data revealed that there was a very significant statistical difference (P<0.01) between experimental treatments for the final disease severity and SAUDPC of all four diseases. The results of comparison of means for powdery mildew showed that all treatments had a final disease severity higher than 30% and lower than 45%. Therefore, none of the treatments could reduce the severity of this disease. However, in term of stripe rust, the resistance of Kohdasht and Morvarid cultivars and their mixtures was clearly observed. For tan spot, the lowest final severity of disease was recorded in Morvarid and 1:1 mixture of Kohdasht-Moghan, and the other mixtures of Kohdasht-Moghan could not significantly reduce the severity of this disease. In term of leaf rust, the lowest final severity of the disease was observed in Morvarid and Moghan cultivars with a density of 150 seeds per m², and the highest final severity in Kohdast cultivar.
 
Conclusion
In this research, the effect of the mixture of cultivars in the control of wheat leaf diseases was investigated. In the case of powdery mildew, all treatments had a final severity of the disease higher than 30% and lower than 45%, and therefore, there is no proper efficiency in any of the treatments to reduce the severity of this disease. For yellow rust disease, the resistance of Kohdasht (treatment No. 1) and Marwarid (treatment No. 3) cultivars and their seed mixture (treatment No. 10) was clearly observed. In term of tan spot disease, the lowest final severity of the disease was observed in two treatments, Morvarid cultivar (treatment No. 3) and 1:1 mixture of Kohdasht and Mughan cultivars treatment No. 4), and the other proportions of the seed mixture of Kohdasht and Moghan varieties could not properly control the disease. In the case of brown rust disease, the lowest final intensity of the disease was observed in two treatments, Morvarid (treatment No. 3) and Moghan (treatment No. 12) with a density of 150 seeds per m², and the highest severity of this disease was observed in the treatments belonging to Kohdasht variety.