Effects of the dinitroaniline fungicide fluazinam on Fusarium fujikuroi and rice.

Fusarium fujikuroi is the primary causal agent of rice bakanae disease. Fluazinam is a protective dinitroaniline fungicide which could interrupt the fungal cell's energy production. Little is known about the effects of fluazinam on F. fujikuroi. In this study, baseline sensitivity of F. fujikuroi to fluazinam was determined using 103 isolates collected from diseased young rice of different fields in Shaoxing of Zhejiang Province and Huaian of Jiangsu Province of China in 2016. The EC50 values of fluazinam on inhibiting mycelial growth against 103 isolates of F. fujikuroi ranged from 0.0621 to 0.5446 µg/mL with the average value of 0.2038 ±â€¯0.0099 µg/mL (mean ±â€¯standard error). The EC50 values of fluazinam on suppressing conidium germination against 103 isolates of F. fujikuroi ranged from 0.1006 to 0.9763 µg/mL with the mean value of 0.3552 ±â€¯0.0181 µg/mL. Treated with fluazinam, hyphae of F. fujikuroi were contorted, offshoot of top mycelia increased, conidial production descreased significantly and exopolysaccharide (EPS) content did not change significantly while peroxidase (POD) activity significantly decreased. Meanwhile, cell membrane permeability increased after treated with fluazinam. The analysis of cell ultrastructure indicated that fluazinam could damage the membrane structure of F. fujikuroi and cause a large number of vacuoles formed. In addition, fluazinam did not affect germination rate, plant height and fresh weight of rice, which indicated that fluazinam was safe to rice. All the results indicated that fluazinam had strong antifungal activity against F. fujikuroi and a potential application in controlling rice bakanae disease. These results will provide useful information for management of rice bakanae disease caused by F. fujikuroi and further increase our understanding about the mode of action of fluazinam against F. fujikuroi and other phytopathogens.

Effects of a novel SDHI fungicide pyraziflumid on the biology of the plant pathogenic fungi Bipolaris maydis.

Pyraziflumid is a novel member of succinate dehydrogenase inhibitor (SDHI) fungicide. Southern corn leaf blight (SCLB) caused by Bipolaris maydis is an important foliar disease of maize crop. In this study, baseline sensitivity of B. maydis to pyraziflumid was determined using 100 strains of B. maydis collected from different geographical regions in Jiangsu Province of China during 2015 and 2016, and EC50 values ranged from 0.0309 to 0.8856 µg/ml with the average value of 0.2780 ±â€¯0.2012 µg/ml for mycelial growth, and 0.032 to 0.9592 µg/ml with the average value of 0.3492 ±â€¯0.2450 µg/ml for conidium germination. After treatment with pyraziflumid, the distribution of cell nucleus and septum of mycelium was not changed, but hyphae of offshoot and conidia production decreased, cell secretion decreased, the cell membrane was damaged, mycelium electrolyte leakage increased, and organelles in mycelial cell dissolved and vacuolated. The protective and curative activity test of pyraziflumid suggested that pyraziflumid had great control efficiency against B. maydis on detached corn leaves. In protective activity assay with application of pyraziflumid at 5 µg/ml and 10 µg/ml, the control efficacy reached to 87.32% and 100% respectively. In curative activity assay with application of pyraziflumid at 20 µg/ml and 50 µg/ml, the control efficacy reached to 82.10% and 100% respectively.

Molecular and biological characterization of Sclerotinia sclerotiorum resistant to the anilinopyrimidine fungicide cyprodinil.

Cyprodinil belongs to the chemical class of anilinopyrimidines fungicides. In this study, baseline sensitivity of Sclerotinia sclerotiorum (Lib.) de Bary to cyprodinil was determined using 100 strains collected from the fields in Jiangsu Province of China. The EC50 (50% effective concentration) values ranged from 0.0636-0.8163 µg/ml with a mean value of 0.1869 (±0.1118) ug/ml for mycelial growth. Nine cyprodinil-resistant mutants (Range of resistance factor: 20.22-271.59) were obtained from sensitive strains exposed on PDA medium amended with cyprodinil and the resistance was stable after their ten transfers on PDA without the fungicide or stored at 4 °C for two months. There was positive cross-resistance between cyprodinil and pyrimethanil but not to fludioxonil, dimetachlone, procymidone, carbendazim and boscalid in S. sclerotiorum. Compared with the parental strains, all of the nine cyprodinil-resistant mutants decreased in sclerotial production. The dry weight of mycelia, pathogenicity and cell membrane permeability of most resistant mutants decreased. The mycelial growth, oxalic acid content, and the response to various stress for resistant mutants were almost the same as the sensitive parental strains. Sequencing alignment results showed that there was no alteration of amino acid in cystathionine γ-synthase (MetB) and cystathionine ß-lyase (MetC) between cyprodinil-resistant mutants and their sensitive parental strains, which indicated that MetB or MetC was not the molecular target of cyprodinil in S. sclerotiorum. The addition of amino acids L-methionine, L-cystine or L-cysteine decreased the inhibition of cyprodinil against mycelial growth of S. sclerotiorum, which indicated that cyprodinil could not only inhibited methionine biosynthesis but also suppressed cystine and cysteine biosynthesis. These results will contribute to evaluating the resistance risk of cyprodinil for management of the plant diseases of Sclerotinia stem rot caused by S. sclerotiorum and further increase our understanding about the mode of action of cyprodinil.

Resistance mechanism of Fusarium fujikuroi to phenamacril in the field.

BACKGROUND: Rice bakanae disease, mainly caused by Fusarium fujikuroi, is an important disease of rice. Phenamacril has been used to control the disease for a few years in China. In 2016, nine phenamacril-resistant strains were found in the field in Zhejiang Province. The aim of the study was to clarify the mechanism of resistance of F. fujikuroi to phenamacril and the fitness of resistant strains. RESULTS: The nine F. fujikuroi strains examined were highly resistant to phenamacril. Eight of them had the point mutation TCA (Ser) → CCA (Pro) at codon 219 in the Myosin-5 protein, while the other had the point mutation TCA (Ser) → TTA (Leu) at codon 219. Myosin-5 replacement between resistant and sensitive strains confirmed that the point mutation in Myosin-5 caused the resistance of F. fujikuroi to phenamacril. Docking of phenamacril into the modeled binding pocket of Myosin-5 showed that the affinity between phenamacril and Myosin-5 decreased and a hydrogen bond could not be formed between phenamacril and the amino acid at codon 219 after it changed to Pro or Leu. There was no cross-resistance between phenamacril and other fungicides. The eight resistant strains containing the point mutation S219P had almost the same fitness as the sensitive strains, while the one resistant strain containing the point mutation S219 L showed decreased mycelial growth, sporulation and pathogenicity. CONCLUSION: In the field, the point mutation S219P or S219 L in Myosin-5 conferred high resistance to phenamacril in F. fujikuroi. The point mutation S219P did not affect the fitness of F. fujikuroi, while the point mutation S219 L decreased its fitness. © 2017 Society of Chemical Industry.