Flyspeck and Spooty Blotch
Publication 310: Integrated Pest Management for
Ontario Apple Orchards,
Table of Contents
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.
Contrary to its name, flyspeck is not caused by insects but by the fungus Schizothyrium pomi (=Zygophiala jamaicensis).
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.
Figure 4-136. Flyspeck on fruit
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.
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.
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.
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
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.
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.
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.
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.
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