A review of powdery mildew in strawberries: the resistance of species, hybrids and cultivars to the pathogen is highly variable within and across studies with no standard method for assessing the disease

Powdery mildew incited by Podosphaera aphanis is an important disease affecting strawberry (Fragaria × ananassa Duch.). Cultivars that are resistant to the pathogen can reduce the dependence on chemicals for commercial production. There is little information on the variation in resistance across cultivar studies and the best way to measure the progress of the disease. This review highlights some of the important issues which need to be considered when conducting such experiments. The first objective of the review was to determine the variation in the severity of powdery mildew within and across experiments. The second objective was to determine the best way to record the development of the disease. In some of the experiments, the plants were rated on the severity of the disease based of the area of the leaf infected (0% = resistant cultivar and 100% = susceptible cultivar). In the others, these values were converted to scores between 0 and 6 or 0 and 10, or similar. There was a range in the behaviour of the cultivars. In some experiments, the cultivars varied across the whole spectrum from susceptible to resistant. In the others, the cultivars were mostly moderately susceptible to resistant, or mostly resistant genotypes with a few susceptible genotypes, or mostly resistant genotypes (N = 65 studies). There is information suggesting that the ranking of cultivars for resistance can vary under different conditions. In these studies, cultivars had the same classification (resistant and resistant, etc.) in about 60% of cases or 53 out of 89 comparisons across sites or years. Resistance to mildew is moderately to highly inheritable, with additive effects more important than non-additive effects. Only a few of the descriptions of new cultivars include data on the incidence or severity of the disease. There are numerous resistant cultivars, although they do not always have acceptable commercial production. Some of the historical resistant cultivars can be used as source of resistance in breeding programmes. I recommend that information be collected on the area of the leaf or fruit surface affected by the disease or these values converted to a scale (less preferred). The studies need to include at least one or two susceptible standard cultivars such as ‘Selva’ or ‘Torrey’. Initial experiments should be conducted under controlled conditions (growth chambers or glasshouses) using inoculated plants. Subsequent experiments should be conducted under protected cropping (tunnels) or in the open field in locations with a high rate of natural infection. These subsequent experiments across different sites and years could be used to determine if the relationship between yield and the severity of mildew differs between resistant and susceptible cultivars. Resistant cultivars possibly have lower slopes in the regression (higher yields per unit leaf area infected) than susceptible cultivars. It is hoped that adoption of these suggestions will improve collaboration across research groups. Genome-wide association studies are likely to accelerate the development of cultivars resistant to the disease.