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Ministry of Agriculture, Food and Rural Affairs

Sooty Blotch

Sooty blotch on fruit
Click to enlarge.

Beginner

Scientific Names
Peltaster fruiticola, Leptodontium elatuius and Geastrumia polystigmatis

Identification

  • Only on the fruit.
  • Brown to olive green, cloudy blotches, with irregular margins on the surface of the skin. 
  • Blotches are variable in size, and coalesce to cover large areas of the fruit and remove with vigorous rubbing.
  • In storage, fruit with severe infections of sooty blotch shrivel more readily than uninfected fruit.
  • Symptoms are more obvious on yellow or light coloured fruit.
  • Fruit with thick cuticle seems to be more severely affected than other fruit. 

Often Confused With

  • Sooty mold - Sooty blotch appears only on the fruit, sooty mold also grows on leaves and twigs. Sooty mold can be removed much more easily from the surface of the apple than sooty blotch.

Period of Activity
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.

Scouting Notes
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.

Thresholds
There are no established thresholds for sooty blotch.

Advanced

Scientific Names
Complex of three different fungi including Peltaster fruiticola, Leptodontium elatuius and Geastrumia polystigmatis.

Identification           
Sooty blotch appears only on the fruit. The disease appears as brown to olive green, cloudy blotches, with irregular margins on the surface of the apple skin.  Blotches are variable in size, and coalesce to cover large areas of the fruit and remove with vigorous rubbing. In storage, fruit with severe infections of sooty blotch shrivel more readily than uninfected fruit. Symptoms are more obvious on yellow or light coloured fruit. Fruit with thick cuticle seems to be more severely affected than other fruit. 

Often Confused With

  • Sooty mold- Sooty blotch appears only on the fruit, sooty mold also grows on leaves and twigs. Sooty mold can be removed much more easily from the surface of the apple than sooty blotch.

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. 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.

Flyspeck and sooty blotch are caused by very slow-growing fungi. Both have the ability to go dormant during unfavorable weather conditions such as hot, dry weather, and then continue development when favorable 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.

Sometimes infections not apparent at harvest will finish their development during long periods of cold storage when relative humidity is near 100%.

Period of Activity

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.

Scouting Notes
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.

Thresholds
None established.

Management Notes

  • Use cultural practices that facilitate drying of the trees and fruit to reduce sooty blotch.
  • The most important practice for reducing damage by these diseases is proper pruning to maintain an open tree canopy.
  • Thinning clustered fruit and summer pruning help promote better air circulation and improve coverage of fungicides.
  • Apply fungicides if infections are observed. The presence of these diseases is an indication that fungicide surface residues are lacking or very low.  
  • Fungicide programs aimed at managing flyspeck provide subsequent control of sooty blotch. For a list of fungicides and timings of sprays effective against flyspeck and sooty blotch see OMAFRA Publication 360, Guide to Fruit Production - Chapter 3 Apples (PDF) or Apple Calendar only (PDF) :