Flyspeck and Spooty Blotch

Publication 310: Integrated Pest Management for Ontario Apple Orchards,
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Table of Contents

  1. Summer diseases
  2. Flyspeck
  3. Sooty blotch

Summer diseases

Flyspeck and sooty blotch are two summer diseases that appear on apple fruit from mid summer to harvest. These diseases occur throughout the eastern apple-growing areas of North America, and are most severe in the central and southern regions. Flyspeck and sooty blotch are sometimes seen on commercially grown fruit in Ontario, and have become more of an issue recently - particularly in dry summers when growers reduce fungicide sprays for apple scab.

The symptoms of both diseases frequently appear together. The fungi causing sooty blotch and flyspeck overwinters on dead twigs of numerous woody plant species, including apple trees, wild blackberry and raspberry canes. Both diseases are favoured by moderate temperatures, high humidity and abundant rainfall, and require free water on the fruit surface for infections to occur. Both fungi grow only on the surface of the fruit, and do not damage the flesh.

Flyspeck

Contrary to its name, flyspeck is not caused by insects but by the fungus Schizothyrium pomi (=Zygophiala jamaicensis).

Symptoms

In commercial Ontario orchards, flyspeck is typically seen on fruit more often than sooty blotch, probably because a lower residue of fungicide is effective in managing sooty blotch. Flyspeck appears as groups of few to several small (approximately 0.5 mm in size), sharply defined, shiny black fungal bodies (thyriothecia) on the surface of the fruit (Figure 4-136). Although these fungal bodies appear to exist individually, they are connected by mycelium to form colonies. Colonies vary in size but are usually in round or irregular groups 1-3 cm in diameter.
Insect frass, particularly from the white apple leafhopper, can be mistaken for flyspeck. Frass tends to be less well-defined than flyspeck, and rubs off more easily, particularly if wiped with a damp cloth.

Figure 4-136. Flyspeck on fruit

Figure 4-136. Flyspeck on fruit

Biology

The flyspeck fungus overwinters as thyriothecia on apple twigs, culled apple fruit and on numerous wild hosts including brambles, oaks and maple. Mature ascospores are released during rainy periods for a one to two month period beginning at bloom. Warm and wet or humid (>96% relative humidity) conditions are conducive to the spread of the disease. In the lab, conidia germinate from 8-24°C and colony development occurs at 12-24°C. Symptoms are visible 10-12 days after infection under optimal conditions but may not appear in orchards for one month under less then optimal conditions.
Since the flyspeck fungus overwinters outside the orchard, there is not a huge risk of fruit infections until border row host plants begin producing second generation spores (conidia). This generally occurs at approximately 270 hr of accumulated wetting (hr AW) after petal fall, which usually occurs four to six weeks after petal fall. At this time, secondary spread of flyspeck occurs when conidia produced by non-orchard hosts are blown into apple orchards, causing the majority of infections that appear on fruit during the late summer. In orchards where fungicide protection is discontinued in early June, flyspeck infection on fruit becomes visible around 540 hr-accumulated wetting hours after petal fall (AWPF) when another generation of flyspeck is completed. Flyspeck also appears in September or October in orchards that have received summer sprays. In these cases, late summer rains often remove fungicide protection allowing fruit infection to occur prior to harvest. In these late season infections, flyspeck can suddenly appear on a high proportion of fruit within a few days.

Monitoring and management

Beginning at mid season through to harvest, monitor 25 fruit in the interior canopy of 10 randomly selected trees. Symptoms are more likely found in poorly-pruned trees in the wetter, foggy, slow-drying areas of the orchard. First symptoms usually appear by early to mid July. Apply fungicides if infections are observed. The presence of these diseases is an indication that fungicide surface residues are lacking or very low. There are no thresholds for either of these diseases.

The development of flyspeck is highly dependent on the microclimate surrounding the fruit, so be sure to undertake practices that facilitate drying of the trees and fruit. The most important practice for reducing damage by these diseases is proper pruning to maintain an open tree canopy. Researchers have shown that summer pruning reduces the incidence of flyspeck by almost 50% in unsprayed orchards. Thinning clustered fruit and summer pruning help promote better air circulation and improve coverage of fungicides. Removing alternate hosts, especially brambles from the orchard and surrounding hedgerows, helps reduce inoculum.

Both flyspeck and sooty blotch are controlled with most of the fungicides that are effective for apple scab. In Ontario and the northeastern United States, fungicides applied to apples from mid June through August provide subsequent control of flyspeck and sooty blotch. In wet seasons, four or five summer fungicide applications may be needed to control these diseases, while only two to three well-timed applications may be needed in dry years. Omitting summer fungicide applications may result in the appearance of flyspeck infections on fruit prior to harvest. Fruit can be vulnerable to these infections in orchards where fungicide programs are discontinued in early June (after the end of primary infection season for apple scab) and symptoms occur around 540 hr-AWPF. In orchards receiving summer fungicide sprays, flyspeck usually appears in September or October. These late season infections usually develop after late summer rains remove fungicide residues, leaving fruit vulnerable to infection prior to harvest. During these conditions, flyspeck can appear suddenly on a high proportion of fruit within a few days. As a result, re-apply fungicides in late August or September if more than 5 cm of rain have occurred since the last application. This is especially important in orchards adjacent to hedgerows for woodlots providing abundant inoculum. The safest approach to prevent flyspeck is protecting apple fruit with fungicides throughout the summer, recognizing that conidia infects fruit at any time after trees are exposed to 270 hr of accumulated wetting counting from petal fall. Coverage is essential for the efficacy of late summer fungicide sprays so travel speed, water volume and using a surfactant to enhance wetting are important. Research suggests fungicides applied in early summer do not eradicate pre-existing infections, and those infections can resume growing and become visible on the fruit in September when the fungicide residues are depleted.

Sooty blotch

Until recently, sooty blotch was believed to be caused by the fungus Gloeodes pomigena, but recent research suggests it is a complex of three different fungi including Peltaster fruiticola, Leptodontium elatuius and Geastrumia polystigmatis.

Symptoms

Sooty blotch appears as brown to olive green, cloudy blotches, with irregular margins on the surface of the apple skin. These are variable in size, and coalesce to cover large areas of the fruit (Figure 4-137). The blotches remove with vigorous rubbing. Do not confuse sooty blotch with sooty mold.

Figure 4-137. Sooty blotch on fruit

Figure 4-137. Sooty blotch on fruit

Sooty mold is caused by the visible growth of saprophytic species of fungi, which grow on the sticky, sugary honeydew excreted by aphids or other sucking insects. Sooty mold also grows on leaves and twigs, or wherever honeydew has fallen. Sooty blotch appears only on the fruit. Sooty mold can be removed much more easily from the surface of the apple than sooty blotch. If the sooty blotches on the skin are extensive, the market value of fruit is downgraded from fresh market to processing or juice quality. In storage, fruit with severe infections of sooty blotch shrivel more readily than uninfected fruit. Losses of 25% or more occur in wet seasons, even with fungicide use. Although the diseases occur separately, they are commonly found together on the same fruit. Typically fruit symptoms are observed by the first of July and become more obvious as the season progresses. Cultivars do not vary in their susceptibility to this disease.

Symptoms are more obvious on yellow or light coloured fruit. Fruit with thick cuticle seems to be more severely affected than other fruit.

Biology

Spores of the sooty blotch fungus overwinter as fruiting bodies on infected twigs of apple trees and other woody plants in hedgerows and woodlots including brambles (blackberry and raspberry), oaks, maple, ash, elm, grape, tulip tree and many others. Spores are dispersed from infected twigs by splashing rain in the spring and early summer, and begin causing fruit infections about two to three weeks after petal fall. First symptoms are usually apparent 20-25 days after infection, but can appear in 8-12 days under optimal conditions. Symptoms are usually more common and severe by late summer or early fall. While most primary spores are dispersed by early summer, the disease can spread extensively throughout the season by secondary infections caused by conidia. Optimum temperature for germination of conidia of P. fruiticola is 12-24°C and 12-32°C for L. elatius at a relative humidity of 95%. Production of conidia for both fungi is greatest when humidity exceeds 97%. Growth is very slow and limited at temperatures below 10°C and above 30°C. Sooty blotch appears more frequently during years with cool, wet springs, late summer rains and low temperatures in the early fall. Researchers in Pennsylvania have shown the development of sooty blotch is highly correlated with the amount of rainfall received in July, and to a lesser degree in August and September. In years with cool moist springs, followed by hot summers, sooty blotch may not appear on fruit until close to harvest. Disease outbreaks are often a problem when cool rainy weather in the spring is followed by late summer rains and cool fall temperatures prior to harvest.
Once fungal colonies appear, they spread by raindrops to other fruit, causing further disease development if environmental conditions remain favorable. This disease is most severe in years and orchards where conditions favour early disease development followed by extensive secondary spread. Sometimes infections not apparent at harvest will finish their development during long periods of cold storage when relative humidity is near 100%.

Flyspeck and sooty blotch are caused by very slow-growing fungi. Both have the ability to go dormant during unfavourable weather conditions such as hot, dry weather, and then continue development when favourable conditions return. This means symptoms of sooty blotch and flyspeck appear most often during the harvest season, even though infections may have taken place much earlier.

Monitoring and management

Beginning mid season, through to harvest, monitor 25 fruit in the interior canopy of 10 randomly selected trees. Symptoms are more likely found in poorly pruned trees in the wetter, foggy, slow-drying areas of the orchard. First symptoms usually appear early to mid July. Apply fungicides if infections are observed. The presence of these diseases is an indication that fungicide surface residues are lacking or very low.
Use cultural practices that facilitate drying of the trees and fruit to reduce sooty blotch.These practices are listed under the flyspeck section. Fungicide programs aimed at managing flyspeck provide subsequent control of sooty blotch.


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Author: OMAFRA Staff
Creation Date: 21 July 2011
Last Reviewed: 21 July 2011