Peronophythora litchii is the causal agent of litchi downy blight. Enestroburin, SYP-1620, SYP-2815 and ZJ0712 are four novel QoI fungicides developed by China. Eight mutants of P. litchii resistant to these QoI fungicides and azoxystrobin (as a known QoI fungicide) were obtained in our preliminary work. In this study, the full length of the cytochrome b gene in P. litchii, which has a full length of 382 amino acids, was cloned from both sensitive isolates and resistant mutants, and single-site mutations G142A, G142S, Y131C, or F128S were found in resistant mutants. Molecular docking was used to predict how the mutations alter the binding of the five QoI fungicides to the Qo-binding pockets. The results have increased our understanding of QoI fungicide-resistance mechanisms and may help in the development of more potent inhibitors against plant diseases in the fields.

In in vitro tests with 18 plant pathogens, the fungicide 3-[5-(4-chlorophenyl)-2,3-dimethyl-3-isoxazolidinyl] pyridine (SYP-Z048) was highly effective on inhibiting mycelial growth of various ascomycota and basidiomycota, with EC50 values ranging from 0.008 to 1.140 μg/ml. SYP-Z048 had much weaker activity against growth of oomycota with EC50 values > 100 μg/ml. In a second in vitro test with Monilinia fructicola isolates, SYP-Z048 inhibited mycelial growth (EC50 = 0.013 μg/ml), germ tube elongation (EC50 = 0.007 μg/ml), and sporulation but did not affect spore germination. In a detached pear fruit assay inoculated with M. fructicola isolates, SYP-Z048 showed protective and curative activity. Field tests indicated that SYP-Z048 was an efficacious fungicide for control of brown rot disease in two peach orchards. When applied to a single spot on a tomato leaflet in a compound leaf, SYP-Z048 suppressed the growth of Botrytis cinerea isolates on the rest 4 leaflets, indicating that the fungicide has systemic movement in plant tissues. These results indicate that SYP-Z048 has potential for management of brown rot causing by M. fructicola and other diseases caused by ascomycota and basidiomycota.

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The above results suggest that the resistance risk of flumorph may be similar to that of dimethomorph but lower than that of azoxystrobin and can be classified as moderate. Thus, it can be managed by appropriate product use strategies.

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