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One of the objectives of sunflower plant breeders is to develop hybrids with a moderate resistance level to head rot caused by Sclerotinia sclerotiorum, one of the most important diseases affecting this crop. In this work we evaluated the impact that genetic improvement had on the resistance to this disease in hybrids grown in Argentina.


MATERIALS AND METHODS

Twenty-four hybrids, released in Argentina between the years 1973 and 2000 (YR), were evaluated in two environments, following a randomized complete block design with two replicates. Artificial S. sclerotiorum infections were made according to a current protocol.

Disease level was estimated at maturity for the following resistance components: incidence, incubation period, and severity.


Responses of sunflower hybrids, released in Argentina between 1973 and 2000, to S. sclerotiorum infections in two environments.

(&): across environments



RESULTS AND DISCUSSION

Combined analysis of variance detected highly significant effects of hybrids for all resistance components, and of environments and hybrid-environment interaction for incidence and severity. The correlation between YR and incubation period was significant and low (r = 0.24), as so as the regression of incubation period on YR (b = 0.005).





Hybrids did not show any variation of their disease level across YR for incidence and severity, but the first symptoms of the disease on modern hybrids appeared later than on the older ones, thus indicating there was a genetic gain for the incubation period for the hybrids released between 1973 and 2000. If this trend continues in the future, next hybrids could also show a lower incidence, which is the only component of head rot resistance being selected by most Argentinean sunflower breeders.

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