Sooty blotch and flyspeck disease of apple: expansion of the fungal complex in Turkey and evaluation of a warning system for the Upper Midwest
Sooty blotch and flyspeck (SBFS) is a disease complex caused by nearly 80 fungal species that blemish the surface of apple fruit, reducing crop value in humid regions worldwide. The severity of SBFS is a concern for apple producers in northeastern Turkey. However, the composition of the SBFS complex in Turkey is unknown. Therefore, the first objective of my research was to isolate and characterize the species assemblage from SBFS infected apples collected in 2008 from the Rize Province, in Turkey. In this study, pressed SBFS colonies with subtending apple cuticle were shipped to Iowa State University for isolation. The internal transcriber spacer (ITS) ribosomal DNA and a portion of the 28S large subunit region (LSU) were compared to previously identified fungi using parsimony analysis. Twelve species were delineated based on parsimony analysis of ITS sequences and morphology. A newly discovered and described species from the survey was Scleroramularia abundans; newly discovered putative species included Zygophiala sp. FS3.3, Stomiopeltis spp. RS7.1 and RS7.2, and Chaetothyriales sp. F1. Previously recovered putative species included Zygophiala sp. FS6 and Stomiopeltis sp. RS4.1. Previously discovered and named SBFS species included Schizothyrium pomi, Zygophiala wisconsinensis, Microcyclosporella mali, Microcyclospora tardicrescens, and Peltaster fructicola. Based on parsimony analysis of the LSU, one species was placed in the subclass Chaetothyriomycetidae and 11 species were placed in subclass Dothideomycetidae. Ten species were placed in the order Capnodiales. This study provided the first documented description of the composition of the SBFS complex from Turkey and expanded the documented extent of genetic diversity of the SBFS complex worldwide.
The second objective was to evaluate a SBFS disease-warning system for the Upper Midwest in replicated field trials conducted in Iowa in 2010 and 2011. The Gleason-Duttweiler warning system uses cumulative hours of relative humidity (RH) to predict the timing of first appearance of SBFS signs. The trial compared on-site RH measurements with site-specific RH estimates as system inputs, and also compared strobilurin-containing fungicides with a tank mix of thiophanate-methyl plus captan when used in conjunction with the warning system. Using on-site RH measurements, the warning system saved three and two fungicide applications in 2010 and 2011, respectively, compared to calendar-based spray timing, while providing statistically equivalent control of SBFS. Using site-specific RH estimates substantially underestimated hours of RH, resulting in a control failure in 2010. In 2011, after improving accuracy of the site-specific estimation system, 3 fungicide applications were saved compared to the calendar-based treatment while providing equivalent control of SBFS. Strobilurin-containing fungicides resulted in equivalent or better control of SBFS than thiophanate-methyl plus captan in the warning-system treatments. This study was the first field evaluation of a relative humidity-based SBFS disease-warning system while providing the first efficacy data for multiple fungicides and suggestions of alternative RH thresholds to obtain more consistent and reliable warning system performance in the future.