Resistance risk assessment of Fusarium pseudograminearum from wheat to prothioconazole

Fusarium crown rot (FCR), primarily caused by Fusarium pseudograminearum, poses significant threats to cereal crops worldwide. Prothioconazole is a demethylation inhibitor (DMI) fungicide used to control FCR. However, the risk of resistance in F. pseudograminearum to prothioconazole has not yet been evaluated. In this study, the sensitivity of a total of 255 F. pseudograminearum strains obtained from Henan Province, China to prothioconazole were determined by the mycelial growth inhibition. The results showed that the effective concentration to 50% growth inhibition (EC₅₀) of these strains ranged from 0.4228 μg/mL to 2.5284 μg/mL, with a mean EC₅₀ value of 1.0692 ± 0.4527 μg/mL (mean ± SD). Thirty prothioconazole-resistant mutants were obtained out of six selected sensitive parental strains by means of fungicide taming. The resistant mutants exhibited defects in vegetative growth, conidia production, and pathogenicity on wheat seedlings compared to their parental strains. Under ion, cell wall, and temperature stress conditions but not osmotic stress, all the mutants exhibited decreased growth rates compared with their parental strains, which was consistent with the control treatment. Cross-resistance test showed that there was a cross-resistance relationship between prothioconazole and four DMI fungicides, including prochloraz, metconazole, tebuconazole and hexaconazole, but no cross-resistance was observed between prothioconazole and carbendazim, phenamacril, fludioxonil, or azoxystrobin. Although no site mutation occurred on Cyp51a and Cyp51b genes, the constitutive expression level of the Cyp51a gene was significantly increased in all mutants. After being treated with prothioconazole, the Cyp51a and Cyp51b genes were significantly increased in both the resistant mutants and their parents. These results suggested that the resistance to prothioconazole of the mutants may be attributed to the changes of the relative expression level of Cyp51a and Cyp51b genes. Taken together, these results could provide a theoretical basis for the scientific use of prothioconazole in the field and fungicide resistance management strategies.

melonis against phenamacril, a myosin inhibitor

Fusarium wilt caused by Fusarium oxysporum f. sp. melonis (FOM) is one of the most notorious seed-borne diseases worldwide. Phenamacril is a cyanoacrylate fungicide with novel chemical structure and strong inhibitive activity against FOM. To evaluate the risk of FOM developing phenamacril resistance, five phenamacril-resistant mutants with >800μgml^-1 minimum inhibitory concentration were obtained by repeated exposure to the fungicide in the laboratory. Compared with the parental isolate, four of the five phenamacril-resistant mutants showed enhanced biological fitness in sporulation and virulence, but not in sensitivity to various stresses (oxidative and osmotic pressure, cell membrane and wall inhibitor). No positive cross-resistance was observed among phenamacril and the other five fungicides, including azoxystrobin, carbendazim, boscalid, fluazinam and tebuconazole. Sequencing alignment results of the myosin 5 from the five resistant mutants and the parental strain indicated that the three resistant mutants fo-2, fo-3 and fo-4 had a single point mutation (S175L), which may confer the resistance of FOM against phenamacril. Interestingly, the resistant mutant fo-4 harbored not only one mutation (S175L) at myosin 5, but also the other mutation (A52G) at β₂-tublin. Our data supported that resistance risk of Fusarium oxysporum f. sp. melonis against phenamacril was between the moderate to high level.

Baseline sensitivity and resistance risk assessmemt of Rhizoctonia cerealis to thifluzamide, a succinate dehydrogenase inhibitor

During 2010-2012, a total of 120 isolates of Rhizoctonia cerealis were collected from wheat with symptoms of sharp eyespot in four provinces (Henan, Shandong, Anhui and Jiangsu) in China. All the isolates were determined for baseline sensitivity to thifluzamide, a succinate dehydrogenase inhibitor (SDHI) with strong antifungal activity. The sampled pathogenic populations, never exposed to SDHIs, had similar sensitivity to trifluzamide (0.025-0.359 µg/ml) in the four regions and over the two years. The baseline sensitivity was distributed as a skewed unimodal curve with a mean EC50 value (effective concentrations for 50% inhibiting mycelial growth) of 0.064 ± 0.013 µg/ml. The resistance risk of R. cerealis to thifluzamide was further evaluated in vitro. Two thifluzamide-resistant mutants of R. cerealis were obtained by culturing on thifluzamide-amended plates. The resistance factors (RF = EC50 value of a mutant/EC50 value of the wild type progenitor of the mutant) were 120 and 40 for two R. cerealis mutants, respectively. All the mutants exhibited similar fitness after 10 successive transfers when compared to their wild-type parents in mycelial growth, sclerotia production, and virulence. However, the two thifluzamide-resistant mutants differed significantly in sensitivity to boscalid and flutolanil. Therefore, a low-to-moderate risk of resistance development was recommended for thifluzamide.