Sooty blotch and flyspeck (SBFS) or apple summer disease is a plant disease caused by a complex of saprophytic fungi which colonize the epicuticular wax layer of apple (Malus x domestica Borkh.). It is found worldwide in regions with moist growing seasons.
Description
Sooty blotch and flyspeck is a descriptive term for a condition of darkly pigmented blemishes and smudges caused by a number of different fungi affecting fruit including apples, pear, persimmon, banana, papaya, and several other cultivated tree and vine crops. The greenish black coating resembling soot or flyspeck-like dots grow into irregular stains and blotches during the summer or growing season. They can grow into each other and may cover the entire fruit surface. Frequently blotches run down in a track resembling tears (German: "Regenfleckenkrankheit"). The blotches can be removed by vigorous rubbing or be scratched off revealing a normal peel. Symptoms can be seen particularly well when apples are light or green colored. Late varieties are more susceptible, because the disease builds over a longer growing period.
Risk factors
Moist and cool weather favors the infection. Orchards with shade or regions near bodies of water with frequent morning dew are especially prone to disease. Infections can occur from June until autumn. Secondary infections are caused by conidia, which are transported by wind and rain onto other developing fruit. The first visible signs of SBFS colonies appear about 3 weeks after infection.
Biology
Microscopically, the flyspeck-like spots and sooty blemishes are fungal mycelium adhering to fruit. The fungi live as saprophytes on the wax layer surface of apples and do not invade the peel. The hyphae, fruiting bodies, and survival structures of these fungi become melanized over time. SBFS fungi also grow on the surfaces of stems, twigs, leaves, and fruit of a wide range of wild plants.
History of discovery
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.
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. 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". 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.
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