QoI fungicides, inhibitors of mitochondrial respiration at the Qo site of cytochrome b in the mitochondrial bc(1) enzyme complex, are commonly applied in vineyards against Plasmopara viticola (Berk. & MA Curtis) Berl. & De Toni. Numerous treatments per year with QoI fungicides can lead to the selection of resistant strains in the pathogen population owing to the very specific and efficient mode of action. In order to evaluate the resistance risk and its development, two different methods, biological and molecular, were applied to measure the sensitivity of oospores differentiated in vineyards, both treated and untreated with azoxystrobin, from 2000 to 2004. Assays using oospores have the advantage of analysing the sensitivity of bulked samples randomly collected in vineyards, describing accurately the status of resistance at the end of the grapevine growing season. Both methods correlated well in describing the resistance situation in vineyards. QoI resistance was not observed in one vineyard never treated with QoI fungicides. In the vineyard where azoxystrobin had been used in mixture with folpet, the selection of QoI-resistant strains was lower, compared with using solely QoI. In vineyards where QoI treatments have been stopped, a decrease in resistance was generally observed.

QoI resistance in P. viticola was first detected in France and Italy in 1999. Molecular and biological assays have been carried out since 2000 in order to provide reliable methods of detecting and quantifying resistance. Oospores were collected in vineyards located in northern and southern Italy. QoI resistance was evaluated by the germination rate of oospores on azoxystrobin amended medium and the frequency of mutant alleles in the DNA extracted from oospores. Both methods correlated to each other and were used side by side to test QoI resistance. Due to the spontaneous occurrence of the G143A mutation in wild type populations and the immigration from surrounding vineyards, resistance frequencies up to 10% were found in samples collected from vineyards never treated with QoIs. Particularly high values, about 90%, were associated with the application of five to six QoI treatments within the same season, while lower percentages, about 30%, were detected in vineyards treated with QoI used in mixture with fungicides belonging to a different resistance group. A progressive decrease of resistance frequency was observed when QoI applications were reduced in number or completely suspended for at least one season. Therefore, a full recovery of sensitivity may be achieved even in vineyards characterized by high levels of resistance, if particular care is taken during disease control by using QoIs only in mixtures and reducing the number of QoI treatments.

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The mandelic acid amide, mandipropamid, which belongs to the carboxylic acid amide (CAA) fungicides, is active against Plasmopara viticola, the causal agent of grapevine downy mildew. The fungicide primarily inhibits the germination of encysted zoospores, thus preventing the pathogenÕs penetration into the host tissues, but it also shows curative effects. In this study, the infection structures of P. viticola in both leaves and berries were investigated to detect the histological and ultrastructural alterations induced by mandipropamid when applied after inoculation. Compared to the untreated samples characterized by a diffuse colonization of the tissues and by a normal ultrastructure of the pathogen, the application of mandipropamid 24 h after inoculation with P. viticola reduced pathogen colonization in leaves and berries. In addition, detachment of the plasmalemma from the hyphal and haustorial walls was observed 72 h after inoculation. In the berries, an abnormal proliferation of the pathogen plasma membrane was observed. Collapsed hyphae and haustoria in treated leaves were surrounded by callose or encapsulated in an amorphous material inside the host cell 72 h after inoculation, while a similar effect was observed in later stages (7 days) in berries. The results confirm that mandipropamid, which acts at the interface between the pathogen plasmalemma and cell wall, has curative activity against P. viticola, appearing more rapidly in leaves than in berries.