Effect Of Fungicides On DON Production By Fusarium Graminearum And Wheat Senescence Physiology

Fusarium graminearum (teleomorph, Gibberella zeae) causes head blight of cereals and contaminates grains with deoxynivalenol (DON) toxin that are harmful to humans and domesticated animals. To safeguard human health, the People's Republic of China Ministry of Health advises a limit of less than 1 mg kg-1 for finished flour products. Various strategies have been developed to control Fusarium head blight (FHB) and to reduce DON contamination of cereals, with chemical control having an important role in an integrated FHB control program. Benzimidazole fungicides, particularly carbendazim (MBC), have been used to control FHB in China for over 40 years. However, F. graminearum has seriously developed resistance to MBC with a site-specific mode of action, leading to inefficacy of this fungicide. Therefore, there is a challenge now to find alternative fungicides for MBC in China. Besides, there are no data on the effect of MBC-resistance on DON production by F. graminearum strains. These data are needed to assess the risk of resistance of F. graminearum to MBC. Moreover less attention has been addressed towards the effect of fungicide on the senescence process of wheat treated with the fungicides. So the major results of the present study are shown as below.We used real-time quantitative PCR (RQ-PCR) and gas chromatography with electron capture detector (GC-ECD) to evaluate the efficacies of JS399-19, tebuconazole, a mixture of tebuconazole and thiram, azoxystrobin, MBC, and thiram on the development of FHB and DON contamination of winter wheat after artificial inoculation under field conditions with Fusarium graminearum. The incidence of spikelets infected (ⅡS), amount of F. graminearum Tri5 DNA (Tri5 DNA), and total DON [containing DON,3-acetyl-(3-ADON) and 15-acetyl-deoxynivalenol (15-ADON)] concentration were quantified in 2006 and 2007. A strong positive correlation was found betweenⅡS or Log10Tri5 DNA and total DON concentration in the harvested grain. The JS399-19, tebuconazole, and the mixture of tebuconazole and thiram significantly reducedⅡS of FHB, amount of Tri5 DNA, and total DON within the grain. Azoxystrobin, MBC, and thiram had no effect on the occurrence of F. graminearum compared with those of the untreated controls. Surprisingly, azoxystrobin and MBC significantly increased the total DON content in the harvested grain because they might have stimulated the amount of total DON production per Tri5 DNA. The fungicides JS399-19, tebuconazole, and the mixture of tebuconazole and thiram were the most effective in controlling FHB and reducing DON contamination of the wheat.Sixty-seven strains were evaluated for DON production in shake culture or in the field. The strains included 60 field strains (30 MBC-resistant and 30 MBC-sensitive), three MBC-resistant site-directed mutants at codon 167 inβ2-tubulin, three MBC-sensitive site-directed mutants at codon 240 inβ2-tubulin, and their MBC-sensitive progenitor strain ZF21. The incidence of infected spikelets and the amount of F. graminearum DNA in field grain (AFgDNA) also were evaluated for all strains. MBC resistance increased DON production ability in shake culture or in the field. Although MBC resistance did not change the incidence of infected spikelets, it did increase AFgDNA. Tri5 gene expression increased in MBC-resistant strains grown in shake culture. We found a significant exponential relationship between DON production ability and Tri5 gene expression in shake culture. However, no association existed between the the incidence of infected spikelets and the ratio of total DON content to the amout of F. graminearum DNA in field grain. These indicated that total DON content was determined by the amount of F. graminearum mycelia in field grain and not influenced by the DON production ability of F. graminearum.The impact of four fungicides JS399-19 (2-cyano-3-amino-3-phenylancryic acetate) (a novel fungicide), azoxystrobin (a strobilurin), tebuconazole (a triazole) and MBC (a benzimidazole), applied as foliar spray at the recommended field rate, on the physiology and biochemistry of the senescence process and grain yield was studied in winter wheat (Triticum aestivum L. cv.'Nannong No.9918') under natural environmental conditions. Fungicide treatments to wheat plants at growth stage [ZGS] 57 (3/4 of head emerged) significantly increased the CHL and SP content and decreased the MDA content and electrolyte leakage. Additionally, activities of the antioxidative enzymes SOD, CAT and POD in flag leaves of the fungicide-treated plants were also higher than that in untreated plants. These coincided with elevated levels of H2O2 and reduced level of O2- in the fungicide-treated plants. The results suggested that the fungicide-induced delay of senescence was due to an enhanced antioxidant enzyme activity protecting the plants from harmful active oxygen species (AOS). Because all fungicides can induce the delay of wheat senescence, fungicide-treated wheat shown higher grain yield than untreated wheat. Of all tested fungicides, JS399-19, azoxystrobin and tebuconazole showed similar effects on delaying senescence of wheat and enhancing the grain yield of wheat, but JS399-19 was more efficient in general.