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Ministry of Agriculture, Food and Rural Affairs

Apple Scab

Young apple scab lesions appear velvety brown to olive green with indistinct marginsYoung apple scab lesions appear velvety brown to olive green with indistinct margins The olive green apple scab lesions eventually turn dark brown to black Lesions on older leaves are raised and appear dark green to gray brown with distinct margins A small scab lesion on a fruit Older scab lesion on fruit becomes brown, corky and scabby Scab lesions cause fruit to become deformed and cracked when infected at an immature stage Flail mowingFlail mowingMonitor weather conditions

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Beginner

Scientific Name
Venturia inaequalis

Identification
Leaves:

  • Lesion development occurs on both sides of leaves, usually developing first on the lower side
  • On leaves, young lesions are velvety brown to olive green with indistinct margins and are often not noticeable until after petal fall
  • With time, olive green lesions turn dark brown to black
  • Lesions on older leaves are typically raised, dark green to gray brown with distinct margins, and cause cupping on the underside of the leaf
  • Leaves that are heavily infected with scab will curl, shrivel and fall from the tree

Fruit:

  • Small black velvety spots develop on fruit
  • As spots grow and become older, the centre loses the velvety appearance and becomes brown, corky and scabby
  • Heavily infected fruit becomes deformed and cracked
  • Fruit infections occurring late in the summer may not be visible at harvest to the naked eye, but enlarge in storage to pinhead size (pinpoint scab) and tend to cluster around the calyx end of the fruit

Often Confused With

  • Blister spot - Blister spot on fruit appears as small raised blisters that affect only Mutsu and lesions penetrate the skin of the apple. Apple scab affects many different varieties and the small scab lesions do not penetrate the skin. However as the lesions grow the lesion will crack the skin.
  • Sooty blotch - Sooty blotch appears only on the fruit as brown to olive green blotches on the surface of the apple skin. Apple scab lesions can be on both fruit and leaves. The lesions on the fruit are green and velvety and as they mature they become black and corky.

Period of Activity
Apple scab activity occurs from the emergence of green tip until leaf drop in the fall.

Scouting Notes
From green tip to harvest leaves and fruit should be monitored for signs of apple scab and results should be recorded. Temperature and hours of leaf wetness should be recorded in order to determine ascospore maturity, primary and secondary infection periods.

Thresholds
Fungicides should be applied to prevent infection during periods when weather facilitates the spread of apple scab. See OMAFRA Publications 310 and 360 for a chart depicting the relationship of temperature and moisture to apple scab infections. Using the ascospore maturity degree day model (see publication 310) when 125 DDC is reached the risk of primary infection is high. When 418 DDC is reached, the primary infection period is over if enough precipitation has fallen. If no scab is observed following the end of primary infection period, the fungicide regime may be diminished accordingly. Secondary infections occur once lesions are present and produce conidia and weather conditions are conducive to infections. 

Advanced

Scientific Name
Venturia inaequalis

Identification
Infection and lesion development occur on both sides of leaves, usually developing first on the lower side. On leaves, young lesions are velvety brown to olive green with indistinct margins and are often not noticeable until after petal fall. With time, olive green lesions turn dark brown to black. Lesions on older leaves are typically raised, dark green to gray brown with distinct margins, and cause cupping on the underside of the leaf. Leaves that are heavily infected with scab will curl, shrivel and fall from the tree.

Small black spots develop on fruit, enlarging more slowly than on leaves. As these spots grow and become older, the centre loses the velvety appearance and becomes brown, corky and scabby. Heavily infected fruit becomes deformed and cracked when infected at an immature stage. Fruit infections occurring late in the summer may not be visible at harvest to the naked eye, but enlarge in storage to pinhead size. These lesions are “pin point scab” and tend to cluster around the calyx end of the fruit.

Often Confused With

  • Blister spot - Blister spot on fruit appears as small raised blisters that affect only Mutsu and lesions penetrate the skin of the apple. Apple scab affects many different varieties and the small scab lesions do not penetrate the skin. However as the lesions grow the lesion will crack the skin.
  • Sooty blotch - Sooty blotch appears only on the fruit as brown to olive green blotches on the surface of the apple skin. Apple scab lesions can be on both fruit and leaves. The lesions on the fruit are green and velvety and as they mature they become black and corky.

Biology
Apple scab overwinters in infected apple leaves on the orchard floor. During the winter and early spring, small black pseudothecia develop in the infected leaves on the orchard floor. By early spring, ascospores, which serve as the primary inoculum for early season infections, are formed inside the pseudothecia.
 
Maturation of the ascospores in the dead leaves on the orchard floor usually occurs at the same time the apple tree is emerging from dormancy. Mature ascospores are present and ready to infect the first green tissue in spring. The percentage of mature ascospores in the orchard generally peaks when apples are at the late pink to early bloom stages of bud development.

Mature ascospores are discharged from the pseudothecia by rain and carried up to emerging green tissue in the trees by wind currents. Moisture – dew or rain – is necessary for ascospore discharge and germination, as well as subsequent infection of apple tissue. Olive green, velvety lesions appear 10-28 days after infection by an ascospore. The lesions initiated by ascospores result in primary infections, and in turn, produce spores called conidia.

Conidia are spread from primary lesions by splashing raindrops and wind, and initiate further infections when the combination of temperature and leaf wetness enables them to germinate and become established. These are called secondary infections, and generally occur within a tree or between adjacent trees rather than at a long distance. 

The secondary cycle can be repeated many times during the growing season. With frequent rainfall, the control of apple scab becomes extremely difficult, particularly if the disease becomes established from primary infections in the spring.

Leaves are most susceptible to infection until they are fully expanded. Old leaves may again become susceptible to the fungus in late season, and previously inhibited mycelia inside the leaf tissues may resume growth, resulting in new visible lesions. This phase of epidemics in autumn has significant implications for disease management because it provides additional primary (ascospores) inoculum next spring.

Period of Activity
Apple scab activity occurs from the emergence of green tip until leaf drop in the fall.

Scouting Notes
From green tip to harvest leaves and fruit should be monitored for signs of apple scab and recorded. Temperature and hours of leaf wetness should be recorded in order to determine ascospore maturity, primary and secondary infection periods.

Thresholds
Fungicides should be applied to prevent infection during periods when weather facilitates the spread of apple scab. See OMAFRA Publication 310 Integrated Pest Management for Apples and OMAFRA Publication 360, Guide to Fruit Production, for a chart depicting the relationship of temperature and moisture to apple scab infections. Using the ascospore maturity degree-day model (see publication 310) when 125 DDC is reached the risk of primary infection is high. When 418 DDC is reached, the primary infection period is over if enough precipitation has fallen. If no scab is observed following the end of primary infection period, the fungicide regime may be diminished accordingly. Secondary infections occur once lesions are present and produce conidia and weather conditions are conducive to infections. 

Management Notes

  • Monitoring weather- Accurate weather data provides daily maximum and minimum temperatures, a necessary component in using the degree-day model for assessing ascospore maturity and primary infection (described below). Reliable weather monitoring equipment in the orchard to measure temperature, relative humidity, leaf wetness is needed to indicate ascospore maturity and when infection periods have occurred. For more information to determine primary and secondary infection periods and ascospore maturity refer to OMAFRA Publication 310 Integrated Pest Management for Apples and OMAFRA Publication 360, Guide to Fruit Production.
  • Determining the inoculum level- The incidence of fruit scab at harvest does not necessarily reflect the disease pressure in the orchard, since foliar infections continue to occur after harvest. To predict the amount of inoculum in the orchard in spring, assess the leaf scab present in the fall using Potential Ascospore Dose (PAD) method. See OMAFRA Publication 310 for more details.
  • Determining apple scab infection periods- Once the tree breaks dormancy and green tissue is present, a primary infection occurs if the following three conditions are met:
    • Mature ascospores are present in leaf litter in the orchard.
    • Weather conditions favour ascospore discharge and infection.
    • Fungicide protection is inadequate to prevent infections.
  • Secondary infections occur once lesions are present and produce conidia and weather conditions are conducive to infections. 
  • For more information to determine primary and secondary infection periods and ascospore maturity refer to OMAFRA Publication 310 Integrated Pest Management for Apples and OMAFRA Publication 360, Guide to Fruit Production, Notes on Apple Scab (PDF)
  • Management of apple scab with fungicides- Fungicides are used to control scab in most commercial orchards. See OMAFRA Publication 360, Guide to Fruit Production - Chapter 3 Apples (PDF) or Apple Calendar only (PDF) :
  • Scab-resistant cultivars- Several cultivars are now available with high resistance of field immunity to apple scab. Some of these cultivars also have resistance to other diseases. Refer to OMAFRA Publication 310, Integrated Pest Management for Apples, for a listing cultivars resistant to apple scab and other diseases.