Reaction of Sunflower Genotypes against Physiological Races of Rust Disease Caused by Puccinia helianthi in Iran

Sunflower rust is one of the most important diseases of the host in main cropping areas of Iran and resistance sources among the plant germplasm are accessible. Sunflower rust disease caused by Puccinia helianthi was reported first time at 1822 on collected samples from Eastern south areas of United States. It is considered as an important disease for sunflower cropping areas of South and North America, Argentina, Africa, India, China, and Australia and has been reported on the host worldwide. Incidence and severity of the disease on commercial hybrids had been increased up to early 1990s in Canada. Between 1988-89, the incidence on crop was reported up to 60 percent; whilst highly infected fields were appeared at the end of crop season. Puccinia helianthi has a worldwide distribution on sunflower and all species of Helianthus. There are also four other species which cause rust on cultivated and wild sunflowers. P. enceliae and P. masalis have been reported on wild sunflower and composites from southwest of United States. P. xanthi with a worldwide distribution on Xanthium Spp from Australia and Coleosporium helianthi from eastern areas of U.S. have been reported. Investigations reported some areas of Mazandaran and Golestan provinces showing infection including Behshahr, Kalaleh, Golidagh, Kalpoosh plain and Gonbad. Sunflower rust in Iran is considered as one of the important diseases which cause economic losses every few years. For instance, it caused 10 percent yield loss and 9.8 percent loss for oil yield. Genetic studies revealed that resistance is controlled by two dominant genes R1 and R2. By these identified genes, four physiological races of the pathogen became determinable. The presence of several resistant sunflower lines to rust with different origins reveals genetic variation for resistance to the disease. In this study, the variation of physiological races of sunflower rust was investigated to identify possible new races of the pathogen, Puccinia helianthi, and reaction of the host genotypes was also evaluated to the identified races.
Rust-infected leaf samples were collected from provinces Golestan and West Azarbayjan in 2010, and 8 isolates were inoculated on susceptible cultivar Record for physiological race identification purpose. Employing single pustule technique, the isolates were purified. The isolates after mass-production in separate chambers were inoculated on 9 standard differential lines. Inoculation and incubation methods were the same as for race identification procedure. The reaction of sunflower genotypes was evaluated by measuring pustule coverage percentage (PCP) according to computer-generated leaf diagrams depicting various percentages of leaf area covered with rust pustules. The interpreting resistance method proposed by Gulya and Masirevic (1995) was used in which three reaction patterns are defined as follows: immune (PCP= 0%), highly resistant (0.5%>PCP>0%) and susceptible (PCP.1%).
All collected isolates from the two main areas (i.e. Khoy and Golidagh) infected the non-resistant Perodovic cultivar indicating the pathogenicity of them. In addition, lines HA-335 and 803-1 were susceptible to all isolates. Lines RHA-265 and QHP1 demonstrated resistance to isolates collected from Golidagh, whereas the isolates of Khoy infected both lines. The results showed physiological differences between the main collection areas and presence of two dominant races. On the basis of pathogenicity similarities on common differential lines, races 302 and 300 were determined as dominant for Golidagh (Golestan) and Khoy (West Azarbayjan), respectively. Resistance evaluation of sunflower germplasm against rust disease was done under controlled greenhouse condition. The genotypes were tested by powdering the plants with mixture of the race's spore and Talc powder. The results demonstrated that 10 and 6 sunflower genotypes including hybrids Ghasem and Barzegar, their restorer lines and cultivar Gabor individuals were resistant against races 302 and 300, respectively.
Occurrence and distribution of physiological races of sunflower rust has importance in plant breeding research; as the process of breeding for resistant genotypes would be broken without considering the variation and distribution of the disease physiological races. In addition, genetic resistance to rust is controlled by dominant genes. Thus, access to new resistant sources in sunflower breeding is more likely. Among 23 lines and individuals of sunflower derived at the breeding processes, 16 resistant ones could help breeders to improve and release new hybrids or varieties containing resistance sources to the rust.