Sooty blotch and flyspeck

In an 1832 paper written in Latin and published in Philadelphia, Ludwig Schweinitz described the first sooty blotch species as a fungus he named 'Dothidea pomigena'. It remained the sole species established as a cause until the beginning of the 1990s. In 1920, sooty blotch and flyspeck were mentioned together for the first time, blotch caused by Dothidea, renamed as Gloeodes pomigena and flyspeck caused by Schizothyrium pomi, respectively. Over the next 80 years various different looks of mycelia, that is morphologies were described.[4]

By the end of the 20th century three more fungal species had been identified as causes of sooty blotch on North Carolina apples, still based on their morphological type: Peltaster fructicola, Geastrumia polystigmatis and Leptodontium elatius.[5] The authors broke ground after 160 years of "confusion", stating that "sooty blotch fungi are difficult to isolate due to many contaminating microorganisms on the surface of plant parts".[5]^: 94 Also, fruiting structures -a major part of morphological identification- are rare on apple peels. The authors went back to the original historical deposits at the herbarium of the Academy of Natural Sciences in Philadelphia and found no conidia. They coined the term 'apple sooty blotch complex' for this group of fungi, which they suspected would include more.[5]^: 94

As soon as DNA sequence-based methods were used, many more fungi were found and new species and genera described. A sample from nine orchards in four Midwestern states near the cities of Indianola, Pella, Iowa Falls in Iowa, Rockford, Illinois, Simpson and Chester in Illinois, Mooresville and New Franklin in Missouri and New Munster, Wisconsin grew 422 isolates. Their 1 month old DNA was extracted and two regions, ITS1 and 28S ribosomal RNA sequenced. Parsimony analysis, bootstrapping and the minimum evolution principle led to groups of species, further described by conidial and colony morphology. The species were re-inoculated into apples grown at an Iowa State University research station in Gilbert, Iowa, re-isolated, sequenced, and morphology compared. Thirty isolates fulfilled Koch’s postulates as new species, all Dothideomycetes, 27 were within Dothideales, one was within Pleosporales and two with undetermined ordinal level. Only 2 species (Peltaster fructicola and Zygophiala jamaicensis) had previously been associated with SBFS [4]

A 2008 publication of the same sample plus a 2005 sample of 30 more orchards in 10 eastern U.S. states, (39 US apple orchards in 14 states) speciated by DNA- and phylogenetic analyses reported 58 putative species belonging to the Dothideomycetes, 52 of which were Capnodiales, and 36 were part of the Mycosphaerellaceae. Thus the SBFS species diversity had been underestimated by more than tenfold. The number of species per orchard varied from 2 to 15, with a higher diversity the longer an orchard had not been sprayed with fungicide.with blotch and flyspeck signs, reported four species of Zygophiala (Schizothyriaceae, Capnodiales), three of them newly identified. Schizothyrium pomi, Peltaster fructicola, and Pseudocercosporella sp. RH1, were found in nearly every geographic region. Species, such as Stomiopeltis sp. RS5.2, Phialophora sessilis, and Geastrumia polystigmatis, were found only in certain regions, leading to the conclusion that SBFS species differ geographically.[6]

Since then, slow-growing epiphytic fungi often belonging to the Capnodiales have been identified (Gleason et al. 2011), and a new species in 2014, Peltaster cerophilus from Europe.[7]

Most fungi are part of the Ascomycetes. The composition of the fungal complex varies depending on the region. Their complete life cycle is as of yet unknown. The fungi are hibernating on wild plants like willow, ash or brambles, which are the locus of primary infections.[5] Whether fungi stay within apple orchards is unknown. They probably thrive on apple juice, which exits through minute tears of the cuticula during growth.[2]

Flyspeck disease is caused by Schizothyrium pomi. It causes sharply demarcated grey blotches consisting of many small black flyspeck like dots. They can be rubbed off like sooty blotch, but no tear formation occurs. Fruit develop normally. Flyspeck frequently occurs in older trees with light colored peels (Golden Delicious, etc.). Because of their similarity the two diseases are often combined as sooty blotch and flyspeck ("Regenfleckenkrankheit" in German).[2]