Field evaluation of some sugar beet cultivars for resistance to powdery mildew

Extended Abstract
Introduction
Sugar beet (Beta vulgaris) is an important industrial crop, ranking as the second-largest source of sugar production globally, accounting for approximately 20% of total sugar production. According to the latest reports, Iran, with around 116,000 hectares of area harvested and a production exceeding 6 million tons, is one of the significant sugar beet producers in the world. Powdery mildew is a major fungal disease affecting sugar beet worldwide. In Iran, the disease is prevalent in all regions where sugar beet is cultivated. Erysiphe betae (Vanha) Welzien, the causal agent of sugar beet powdery mildew has a limited host range and specifically targets species within the genus Beta. Powdery mildew causes reduction in both root yield and sugar content, potentially reducing crop yield up to 30%. Utilizing resistant cultivars is considered as the best management strategy for controlling this disease. So. This study aimed to evaluate the resistance of several new sugar beet cultivars to powdery mildew.
Materials and Methods
Seventeen sugar beet cultivars namely Ekbatan, Paya, Arya, Shokoufa, Motahar, Arta, Dena, Sina, Asia, Kimia, Nika, Tara, Homa, Hosna, Palma, Modex, and BTS335, alongside the susceptible genotype 191 were evaluated. Cultivars were cultivated in a randomized complete block design with three replications at the Research Field of the campus of Agriculture and Natural Resources, Razi University, Kermanshah. Each experimental plot consisted of four rows with 2 m long and 50 cm space apart. Two middle rows were dedicated to the genotype being evaluated, while the outer rows were planted with the susceptible control 191 to ensure uniform inoculum distribution.
The experimental field was monitored every two days, and after the first signs of infection, disease intensity was assessed weekly. Resistance evaluation was based on the Disease Intensity Index (DII) at the final assessment, the area under the disease progress curve (AUDPC), and the number of conidia produced per cm² leaf. Data analysis was conducted using SAS v9.3 software, employing the general linear model for variance analysis, and mean comparisons were made using Duncan's multiple range test at the 5% probability level. Additionally, Pearson correlation coefficients were calculated to determine the relationships between the indices used in the resistance assessment.
Results
The first signs of the disease were observed on July 15, 15 weeks after planting, appearing only in the cultivars Ekbatan, Arya, Motahhar, Kimia, Nika, Homa, and 191. After about one month, these signs completely diminished. Although the disease severity at this stage was not significant, the absence of the disease in several tested cultivars is epidemiologically important. The occurrence of the disease at this time creates disease foci in the field, accelerating the re-emergence and spread of the pathogen. This re-emergence occurred after nearly a 2-month pause in October, continuing with an increasing trend.
Variance analysis of the Disease Intensity Index data at the final assessment revealed significant differences among evaluated cultivars regarding resistance to powdery mildew (P < 0.0004). Among genotypes, Modex cultivar was identified as the most resistant, with a DII of 0.11 on a scale of zero to one, while Ekbatan was the most susceptible cultivar with a DII of 0.78, showing no significant difference from Paya, Arya, Sina, and Nika. The variance analysis of the AUDPC data also showed a significant difference among evaluated cultivars (P < 0.0053). In this context, Modex cultivar continued to show a significant difference with other evaluated cultivars.
The AUDPC provides several advantages over point assessments of disease for various reasons. By utilizing AUDPC, researchers can monitor changes in disease progression more accurately over time. This method allows for the integration of multiple parameters, such as disease occurrence, severity, and environmental effects, into a comprehensive analysis. Consequently, it enhances our understanding of disease dynamics and plant responses to the disease. Many researchers have employed AUDPC to assess the resistance of different cultivars. In this study, continuous monitoring of the disease throughout the growing season yielded valuable data on the onset, decline, and progression of the disease—information that cannot be obtained through point assessments at the end of the season.
Conclusion
In conclusion, Modex cultivar was identified as significantly more resistant to the disease compared with all other evaluated cultivars by all three assessment methods. Therefore, this cultivar is recommended for cultivation if exhibits favorable agronomic traits and yield.
Keywords
AUDPC, Kermanshah, Powdery mildew
References
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