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 season A season is a division of the year marked by changes in weather, ecology, and the amount of daylight. The growing season is that portion of the year in which local conditions (i.e. rainfall, temperature, daylight) permit normal plant growth. Whil ...

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 A banana is an elongated, edible fruit – botanically a berry – produced by several kinds of large herbaceous flowering plants in the genus ''Musa''. In some countries, bananas used for cooking may be called "plantains", distinguis ...

papaya The papaya (, ), papaw, () or pawpaw () is the plant species ''Carica papaya'', one of the 21 accepted species in the genus ''Carica'' of the family Caricaceae. It was first domesticated in Mesoamerica, within modern-day southern Mexico and ...

, 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.Rußflecken
Kompetenzzentrum Obstbau-Bodensee in Bavendorf (KOB), not dated, retrieved 25 October 2015

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 A conidium ( ; ), sometimes termed an asexual chlamydospore or chlamydoconidium (), is an asexual, non-motile spore of a fungus. The word ''conidium'' comes from the Ancient Greek word for dust, ('). They are also called mitospores due to the ...

, 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

hypha A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one or ...

e, 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 ''Schizothyrium pomi'' is a plant pathogen of the sooty blotch and flyspeck 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 ( ...

, 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 Geastrumia polystigmatis is an ascomycete fungus that is a plant pathogen Plant pathology (also phytopathology) is the scientific study of diseases in plants caused by pathogens (infectious organisms) and environmental conditions (physio ...

and

Leptodontium elatius Leptodontidium elatius var. elatius is a slow growing pathanogenic ascomycete fungus A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the ...

. The authors broke ground after 160 years of "confusion", stating that "sooty blotch fungi are difficult to

isolate Isolate may refer to: Arts, entertainment, and media * ''Isolate'' (film), a 2013 Australian film * ''Isolate'' (Circus Maximus album), 2007 * ''Isolate'' (Gary Numan album), 1992 Language * Isolating language, with near-unity morpheme/word ...

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 Indianola may refer to: Places in the United States * Indianola, California (disambiguation) ** Indianola (Eureka), California * Indianola, Florida * Indianola, Georgia * Indianola, Illinois * Indianola, Iowa * Indianola, Kansas, a former settleme ...

, Pella,

Iowa Falls Iowa Falls is a city in Hardin County, Iowa, United States. Iowa Falls is the home of Ellsworth Community College. It is also a regional transportation center, located along U.S. Routes 20 and 65 and the Canadian National and Union Pacific Rai ...

in Iowa,

Rockford, Illinois Rockford is a city in Winnebago County, Illinois, located in the far northern part of the state. Situated on the banks of the Rock River, Rockford is the county seat of Winnebago County (a small portion of the city is located in Ogle County). ...

, Simpson and

Chester Chester is a cathedral city and the county town of Cheshire, England. It is located on the River Dee, close to the English–Welsh border. With a population of 79,645 in 2011,"2011 Census results: People and Population Profile: Chester Loca ...

in Illinois, Mooresville and New Franklin in Missouri and

New Munster, Wisconsin New Munster is an unincorporated community and census-designated place in the town of Wheatland, in Kenosha County, Wisconsin, United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) o ...

grew 422 isolates. Their 1 month old DNA was extracted and two regions, ITS1 and

28S ribosomal RNA 28S ribosomal RNA is the structural ribosomal RNA (rRNA) for the large subunit (LSU) of eukaryotic cytoplasmic ribosomes, and thus one of the basic components of all eukaryotic cells. It has a size of 25S in plants and 28S in mammals, hence th ...

sequenced. Parsimony analysis, bootstrapping and the minimum evolution principle led to groups of species, further described by

conidial A conidium ( ; ), sometimes termed an asexual chlamydospore or chlamydoconidium (), is an asexual, non- motile spore of a fungus. The word ''conidium'' comes from the Ancient Greek word for dust, ('). They are also called mitospores due to t ...

and colony morphology. The species were re-inoculated into apples grown at an Iowa State University research station in

Gilbert, Iowa Gilbert is a city in Story County, Iowa, Story County, Iowa, United States. The population was 1,211 at the time of the 2020 United States Census, 2020 census. It is part of the Ames, Iowa Metropolitan Statistical Area, which is a part of the large ...

, re-isolated, sequenced, and morphology compared. Thirty isolates fulfilled

Koch’s postulates Koch's postulates ( )"Koch"
''

Dothideomycetes Dothideomycetes is the largest and most diverse class of ascomycete fungi. It comprises 11 orders 90 families, 1300 genera and over 19,000 known species. Traditionally, most of its members were included in the loculoascomycetes, which is not par ...

, 27 were within Dothideales, one was within

Pleosporales The Pleosporales is the largest order in the fungal class Dothideomycetes. By a 2008 estimate it contains 23 families, 332 genera and more than 4700 species. The majority of species are saprobes on decaying plant material in fresh water, marin ...

and two with undetermined ordinal level. Only 2 species (Peltaster fructicola and Zygophiala jamaicensis) had previously been associated with SBFS 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

, 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 The Schizothyriaceae are a family of fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms ...

, Capnodiales), three of them newly identified.

, Peltaster fructicola, and Pseudocercosporella sp. RH1, were found in nearly every geographic region. Species, such as Stomiopeltis sp. RS5.2,

Phialophora sessilis ''Phialophora'' is a form genus of fungus with short conidiophores, sometimes reduced to phialides; their conidia are unicellular. They may be parasitism, parasites (including on humans), or saprophytic (including on apples). Genetic analysis o ...

, and

, were found only in certain regions, leading to the conclusion that SBFS species differ geographically. 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.

Diversity

Most fungi are part of the

Ascomycetes Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The defi ...

. 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. Whether fungi stay within apple orchards is unknown. They probably thrive on apple juice, which exits through minute tears of the cuticula during growth. Flyspeck disease is caused by

. 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).

Effect

The blotches are cosmetic damage "unacceptable to consumers" and downgrade fruit from premium fresh-market grade to processing use, i.e. reduce its market value, but leaf and fruit development are not affected.

Control

Preventive measures are pruning which allows light and air to enter the tree, to achieve fast drying. Strong growth within the root area dams up moisture and facilitates infection. A prognostic model called Sooty Blotch RIMpro has been developed, which still awaits validation. Similar to the apple scab model it numerically grades risk and degree of infection and can serve as a warning system. It allows conventional growers to spray more targeted. The parameters for calculation are wetness of leaves, amount of rain fall and temperature.RIMpro forecast model "Sooty Blotch"
Fruitwebinfo, not dated, retrieved 25 October 2015 Conventional orchards that spray fungicides against apple scab, treat soot blotch and flyspeck at the same time. Therefore, the problem is not seen in conventional non-resistant varieties. However, scab-resistant varieties, which are not sprayed frequently show the infection. In organic orchards, spraying 4–5 with

lime sulphur In horticulture, lime sulfur (British spelling lime sulphur) is mainly a mixture of calcium polysulfides and thiosulfate (plus other reaction by-products as sulfite and sulfate) formed by reacting calcium hydroxide with elemental sulfur, used in p ...

or coco soap during the main infectious periods is recommended.

References

{{DEFAULTSORT:Sooty Blotch And Flyspeck Ascomycota Fungal plant pathogens and diseases