Sooty blotch and flyspeck

Sooty blotch and flyspeck(SBFS) orapple summer diseaseis aplant diseasecaused by a complex ofsaprophyticfungiwhich colonize theepicuticular waxlayer ofapple(MalusxdomesticaBorkh.). It is found worldwide in regions with moistgrowing seasons.

Description

Haralsonwith its typical peel coloration from sooty blotch and flyspeck

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 andvinecrops. 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 byconidia,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 fungalmyceliumadhering to fruit. The fungi live assaprophyteson the wax layer surface of apples and do not invade the peel. Thehyphae,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 asGloeodes pomigenaand flyspeck caused bySchizothyrium pomi,respectively. Over the next 80 years various different looks ofmycelia,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 polystigmatisandLeptodontium elatius.The authors broke ground after 160 years of "confusion", stating that "sooty blotch fungi are difficult toisolatedue 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 theAcademy of Natural Sciences in Philadelphiaand 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 ofIndianola,Pella,Iowa Fallsin Iowa,Rockford, Illinois,SimpsonandChesterin Illinois,MooresvilleandNew Franklinin Missouri andNew Munster, Wisconsingrew 422 isolates. Their 1 month old DNA was extracted and two regions,ITS1and28S ribosomal RNAsequenced.Parsimony analysis,bootstrappingand theminimum evolutionprinciple led to groups of species, further described byconidialand colony morphology. The species were re-inoculated into apples grown at anIowa State Universityresearch station inGilbert, Iowa,re-isolated, sequenced, and morphology compared. Thirty isolates fulfilledKoch’s postulatesas new species, allDothideomycetes,27 were withinDothideales,one was withinPleosporalesand two with undetermined ordinal level. Only 2 species (Peltaster fructicolaandZygophiala 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 theDothideomycetes,52 of which wereCapnodiales,and 36 were part of theMycosphaerellaceae.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 ofZygophiala(Schizothyriaceae,Capnodiales), three of them newly identified.Schizothyrium pomi,Peltaster fructicola,andPseudocercosporellasp. RH1, were found in nearly every geographic region. Certain species, such asStomiopeltissp. RS5.2,Phialophora sessilis,andGeastrumia polystigmatis,were found only in certain regions, leading to the conclusion that SBFS species differ geographically.

Since then, slow-growingepiphytic fungioften belonging to the Capnodiales have been identified (Gleason et al. 2011), and a new species in 2014,Peltaster cerophilusfrom Europe.

Diversity

Most fungi are part of theAscomycetes.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 bySchizothyrium 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).

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.

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 withlime sulphuror coco soap during the main infectious periods is recommended.

See also

• Sooty mold
• Apple scab