Evaluation of genetic variability for Sclerotinia sclerotiorum Lib. de Bary resistance in a F2 population from a cross between susceptible and resistant sunflower

The inbred line R28, coming from Helianthus argophyllus, displays low susceptibility to both artificial and natural infection by basal stem and head rot and to artificial infection by fungus filtrate (oxalic acid). A cross was made between this line and 9304 (a susceptible inbred line). F2 plants, parental plants and F1 plants, were artificially infected by: i. pathogen mycelium over the basal stem (basal stem attack), ii. ascospores on the head (head rot attack). As result of basal stem infection, 72.4% of the F2 plants showed symptoms and died. Incubation period (IP), as the number of days from infection to the appearance of first symptoms, varied from 5 to 25 days, with a population mean of 12.5±6.1 days. The IPs in 9304, F1 and R 28 were 7, 10 and 16 days, respectively. The same IP character and the percentages of lesions on the capitulum, monitored at the end of flowering time (EF) and at physiological maturity (PM), were also measured after the infection with ascospores. After the ascospore test, 60.7% of the infected F2 plants showed disease symptoms on the capitulum. In this case the IP ranged between 16 and 45 days, with a population mean of 27.6 ± 6.4 days. The IPs for 9304, F1 and R 28 were 18, 25 and 35 days, respectively. Among all the plants with lesions at PM, 24% was completely safe at EF, thus showing different reactions of F2 genotypes against Sclerotinia. Relationship among EF, PM and IP were investigated and the obtained results suggest two different Sclerotinia attack mechanisms. This paper discusses the possibility of obtaining some results from classical selection programs utilizing these parameters. We are also considering a molecular markers assisted approach. Recently, several hundred microsatel-lite markers were developed for sunflower. For this purpose we have selected a first set of primer combinations on the basis of amplicon length to facilitate multiplexing. SSR markers were screened for polymorphism using three-color multiplexes.