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White rot caused by Sclerotinia sclerotiorum (SS) is one of the most devastating plant diseases of sunflower. Controlling this pathogen by available tools hardly result in acceptable control. The aim of this study was to elucidate the effects of plant resistance inducers, BTH (benzothiadiazole in Bion 50 WG) and arbuscular mycorrhizal fungi (AMF) on disease development of white rot in three sunflower genotypes. Defence responses were characterized by measuring the disease severity and identifying cellular/ histological reactions (e.g. autofluorescence) of host plants upon infection. Depending on the host genotype, a single application of inducers reduced disease symptoms. Histological examination of host responses revealed that BTH and/or AMF pre-treatments significantly impeded the development of pathogenic hyphae in Iregi szürke csíkos and P63LE13 sunflower plants and it was associated with intensive autofluorescence of cells. Both localized and systemic induction of resistance was observed. Importantly, the frequency of mycorrhization of hybrid P63LE13 and PR64H41 was significantly increased upon BTH treatment, so it had a positive effect on the formation of plant-mycorrhiza interactions in sunflower. To our knowledge, this is the first report on the additive effect of BTH on mycorrhization and the positive effect of these inducers against SS in sunflower.

In the present study, the mechanism of resistance to Plasmopara halstedii, the downy mildew pathogen of sunflower, triggered by three resistance-inducing chemicals, benzo (1,2,3) thiadiazole-7-carbothioic acid S-methyl ester (BTH), DL-b-amino butyric acid (BABA) and 2,6-dichloroisonicotinic acid (INA), was investigated in susceptible, completely resistant and partially resistant sunflower genotypes. By applying P. halstediispecific primers, no detectable pathogen marker transcript accumulation was found in the infected but completely resistant sunflower hypocotyls; however, pretreatments with either of the three resistance inducers decreased the transcript accumulation in both the infected susceptible and the partially resistant lines. Benzothiadiazole pretreatment before inoculation considerably enhanced enzyme activities in the infected susceptible and the completely resistant genotypes but not in partially resistant plants. Pretreatment with resistance inducers before inoculation increased glutathione S-transferase, defensin and catalase transcript levels in the susceptible but decreased in the partially resistant plants. Our results indicate that the resistance-inducing chemicals can improve resistance in all of the sunflower genotypes to downy mildew and increase enzyme activities of peroxidase and polyphenol oxidase, as well as accumulation of mRNAs of glutathione S-transferase, defensin and catalase. However, it is important to emphasize that activation of these defence-related proteins did not correlate with the degree of resistance, but rather with the amount of necrotic tissues.