Abstract
Introduction
Cystic nematode is one of the limiting factors of sugar beet growth. Considering the importance and extent of this nematode in Iran and in order to prevent its damage, it is recommended to use resistant cultivars. The prerequisite for the development of resistant cultivars is the presence of resistant genotypes. Therefore, this study was aimed to evaluate the quantitative and qualitative yield of new sugar beet hybrids and determining their agronomic value in the field with natural infection under sugar beet cyst nematode in Razavi Khorasan province.
 Materials and Methods
In order to determine the agronomic value of advanced sugar beet hybrids, 15 new domestic hybrids along with two resistant (Aria) and sensitive (Sharif) control cultivars were evaluated during the years 2020 and 2021 in the field infected with cystic nematode in Torogh Agricultural Research Station (Mashhad) in the form of randomized complete block design with four repetitions. The investigation of resistance to sugar beet cyst nematode of 17 genotypes was also carried out under greenhouse conditions using a completely randomized design with 10 replications. Muller's method was used to evaluate cultivars under greenhouse conditions. After performing the homogeneity of variances test through Bartlett's test, composite analysis of two years' data was done. By using correlation coefficients between measured parameters, their relationships were compared to each other. In order to group the experimental materials, Ward's cluster analysis was used. LSD test was used at the probability level of five percent to compare the mean of the traits.
  Results and Discussion
The combined analysis of variance showed a significant difference between the hybrids in terms of all traits. Mean comparison showed that hybrid SBSI 126 had the highest root yield (57.91 tonha^-1). In addition, this hybrid had higher values of sugar content, sugar yield and white sugar yield than the resistant control cultivar (Aria) as well as the average of all genotypes. The SBSI 138 hybrid had the lowest values of root yield, sugar yield, white sugar yield, sugar content and white sugar content, and it was the most sensitive genotype to cyst nematode among the genotypes after the sensitive control variety (Sharif). The results of cluster analysis in the field experiment showed that hybrids SBSI 126, SBSI 129 and SBSI 128 had high yield potential and were placed in one cluster.Correlation coefficients of studied traits showed that the highest positive and significant correlation (0.97) was found between root yield and sugar yield and the highest negative and significant correlation (-0.93) was also obtained between extraction coefficient of sugar and sugar beet molasses. There was a positive and significant correlation at the level of 1%between white sugar yield and root yield (0.85), sugar content (0.40), sugar yield (0.95), potassium (0.57), nitrogen (0.51) and molasses sugar percentage (0.25). While a negative and significant correlation was observed between white sugar yield and alkalinity coefficient (-0.52) at the level of 1%. In this research, there was a significant difference at the level of 1% between the evaluated genotypes in greenhouse conditions in terms of female index of the cyst nematode (FI). The results of cultivar grouping according to Schmidt and Shannon showed that the SBSI 126 genotype with the lowest index (11.66%) has the highest resistance among genotypes.
 Conclusion
In general, the results showed that the SBSI 126 hybrid can be recommended as a cyst nematode-resistant hybrid for growing in infected fields due to its resistance and high white sugar yield.
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