A network meta-analysis of the susceptibility of apple genotypes to powdery mildew under organic management

The susceptibility of apple tree genotypes to powdery mildew has been quantitatively evaluated in the field by various researchers. Our aim was to attempt to overcome difficulties such as sampling error, and differences in the designs, time periods and geographical locations of studies, to form a consensus about the variation in this susceptibility over different apple genotypes under organic management by applying the methodology of meta-analysis. A systematic search of appropriate databases was conducted for relevant articles published from January 1998 to December 2017. It was required that susceptibility to powdery mildew was measured by incidence in terms of the percentage of affected leaves. Where possible, the incidence levels of the studies that satisfied given criteria were standardized using an association scheme and adjustment factors to the mean environmental conditions in what was chosen to be the base study. The literature search found 22 studies that satisfied all the inclusion criteria of the meta-analysis, and for 20 of these, incidence levels could be standardized. Since the overall virulence of the fungus varied greatly across studies, standardizing incidences was crucial. Of the 12 genotypes having the lowest mean standardized incidences in the main analysis of 94 genotypes, the least well-known are 'Earlidel', 'Florina-Quérina', 'Lourdes', 'Piros', 'Rajka', 'Topaz', '436' and '468' of which the former 6 genotypes are arguably the more appealing to producers in terms of fruit yield and the appearance, taste and texture of their fruits. Many genotypes appear to have susceptibilities relative to other groups of genotypes that are not that sensitive to changes in environmental conditions, including the native strains of the pathogen that are present, however for other genotypes this sensitivity appears to be reasonably high.