Biological Activity Of Five Novel Strobilurins Fungicides Against Pseudoperonospora Cubensis And Its Resistance Risk

It was studied five candidate strobilurin fungicide pyrametostrobin, SYP-3759, SYP-3375, pyraoxystrobin, SYP-3998 and so on in this paper that systemicity in potted plants of cucumber, mode of action against Pseudoperonospora cubensis, duration of activity, efficacy of controlling cucumber in the fields of five novel candidate strobilurin fungicides, and the risk of resistance to pyrametostrobin, the results were shown as followed:It was found in the test of fungicidal activity by application of fungicides at irrigation and by spray of fungicides onto detached leaves that the five novel candidate strobilurin fungicides have inhibitory activity of systemicity against Pseudoperonospora cubensis in the plants of cucumber, among acropetal translocation, symplastic translocation, lateral translocation and translaminar translocation inside cucumber plants, the fungicidal activity by translaminar translocation was highest. Among the five candidate fungicides, pyrametostrobin exibited the highest activity against Pseudoperonospora cubensis.Protective action, curative action and duration of fungicidal activity of the five candidate strobilurin fungicides were tested by inoculating sporangial suspension onto the leaf discs cut from potted plants sprayed with fungicide suspension, by inoculating sporangial suspention onto leaf discs sprayed with fungicide suspension, and by inoculating sporangial suspension onto the leaf discs cut from potted plants sprayed with fungicide suspension. The eradicative activity of the five candidate fungicides was tested by inoculating sporangial suspension onto the detached leaves and the leaf discs treated with fungicide suspension.The results showed that the five candidate fungicides have good action of protection, curation and eradication in controlling cucumber downy mildew. Pyrametostrobin 20EC stood out among the five candidate fungicides in action modes and duration of activity, its good efficacy of controlling could last 10 days. Besides, pyrametostrobin was found to inhibit the release of zoospores and elongation of germ tubes of germinated encysted zoospores by the test on concave slides.The controlling efficacy of the five candidate strobilurin fungicides under the condition of lab was tested by inoculating sporangial suspension onto the leaf discs cut from potted plants sprayed with fungicide suspension and by inoculating sporangial suspension onto the detached leaves sprayed with fungicide syspension, and the efficacy of the five candidate strobilurin fungicides and SYP-3399 20 SC in controlling downy mildew on cucumber grown in solar greenhouses was tested by random ranking of plots. The results showed that the candidate fungicides SYP-3399 20 SC, pyrametostrobin 20 EC, pyraoxystrobin 20 SC gave good efficacy of controlling cucumber downy mildew in the fields. In the field trial of efficacy, pyrametostrobin 20 EC and pyraoxystrobin 20 SC were applied 4 times at the rate of 50, 100, 150g a.i./hm~2 onto cucumber leaves, and SYP-3399 20 SC applied 4 times onto cucumber leaves at the rate of 900, 600, 300g a.i./hm~2 gave 70.23%～82.84%, 69.06%～80.26%, 70.01%～84.00%(7 days after the fourth spray), respectively. No significant difference between the control efficacy of the three candidate fungicides pyrametostrobin 20 EC (150g a.i./hm~2), pyraoxystrobin 20 SC (150g a.i./hm~2), SYP-3399 20 SC (900g a.i./hm~2) and the control efficacy of control fungicides azoxystrobin 25 SC(Amistar)100g a.i./hm~2, cymoxanil-mancozeb 72 WP(Curzate MZ)1440g a.i./hm~2, but the control efficacy of the three candidate fungicides pyrametostrobin 20 EC (150g a.i./hm~2), pyraoxystrobin 20 SC (150g a.i./hm~2), SYP-3399 20 SC (900g a.i./hm~2) was significantly higher than that of Golden metalaxyl-mancozeb 68 WG(Golden Ridomil MZ)675g/hm~2 and Pyraclostrobin 25 EC (carbio) 100g a.i./hm~2. Besides, it was found that the efficacy of the five candidate strobilurin fungicides and SYP-3399 20 SC considerably declined 14d after 4 times'application in controlling downy mildew on cucumber grown in solar greenhouses. Therefore, It was suggested that SYP-3399 20 SC (900g a.i./hm~2), pyrametostrobin 20 EC (150g a.i./hm~2) and didate fungicides pyrametostrobin 20 EC (150g a.i./hm~2) be sprayed onto the leaves of cucumber before occurrence of cucumber or at the initial stage of occurrence of cucumber with 900L hm-2, 2-4 times per growth season, at the interval of 7-10d. SYP-3399 20SC, pyrametostrobin 20 EC, pyraoxystrobin 20 SC should be applied in alternation with other fungicides of different modes of action, such as dimethomorph, cymoxanil and so on.Sensitivity to the five candidate strobilurin fungicides in the strains collected from Shandong, Hebei and Liaoning was tested by leaf disc floating test, the results showed that pyrametostrobin was the fungicide that P. cubensis appeared the lowest sensitivity to . Among the tested 101 strains, the pyrametostrobin-sensitive strains, strains tiny-resistant to pyrametostrobin, strains moderately-resistant to pyrametostrobin, strains highly-resistant to pyrametostrobin accounted for 13.86%, 28.71%, 48.51% and 8.91%.Mutants resistant to the five candidate strobilurin fungicides were accquired by subculturing sporangia of Pseudoperonospora cubensis on the detached leaves of cucumber treated with the target fungicides at sublethal dosages. It was supposed by comparision of possibility of mutants with accquired resistance to the target fungicides, fitness and competitive ability of mutants resistant to the target fungicides and their parent strains that moderate risk of resistance to the five candidate strobilurin fungicides pyrametostrobin, SYP-3759, pyraoxystrobin, SYP-3375 and SYP-3998 were spposed to exist, but the risk could be lower than the risk to azoxystrobin.