Disease of the Month - Apple Scab

 
  Hort Snacks - June 2018
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 Causal Organism: Venturia inaequalis

Crops Affected: apples, crab apples, mountain ash

Disease Cycle:

  • Fungal pathogen
  • Overwinters in infected leaves on the ground underneath orchard plants
  • Fungal spore-producing bodies (pseudothecia) are produced over winter and in early spring, with spores (ascospores) being released as the new growth appears in spring.
    • Initial spores are released during rainy periods and are carried on wind currents to infect leaves, blossoms and fruit
    • Moisture is a critical requirement for infection to take place
  • Lesions develop on the plant, with the disease increasing rapidly if conditions are suitable
    • Conidia are produced and may spread without the need for precipitation (as is the case with spring ascospore release)
    • Short distance spread and pathogen transfer results in secondary infections when lesions develop and rain splashing and wind transfers conidia within the plant canopy and between adjacent trees
  • Lesions may “reactivate” at different times, increasing inoculum levels
Symptoms:
  • Leaves
    • Lesions are circular or irregularly-shaped, brown to olive-green spots, with feathery/indistinct margins
      • Lesions on older leaves are often raised and have more distinct margins
    • Lesions may develop on both upper and lower leaf surfaces, however lesions usually develop on the lower surface first
    • Severe infections may results in leaves that curl, shrivel and fall prematurely
  • Fruit
    • Lesions are circular and brown to black in colour
    • The skin around lesions may rupture to reveal dark layers of spores
    • Lesions look more corky as they age
  • Earlier infections result in large fruit lesions and fruit that is severely cracked and deformed
Lesions on both fruit and leaves, with cracks forming on fruit
Photo by OMAFRA

Conditions Favouring Disease Development
  • Moisture (precipitation or dew), and the duration of wetness, is a critical factor determining spore germination and infection levels
  • Temperature is a factor in determining the required wetness period for infection to take place
    • Generally, cooler temperatures result in an increase in the required wetness period, for both ascospores and conidia
      • Peak infection occurs at temperatures between 12C and 25C
    • Conidia normally need a slightly longer wetness period than ascospores, at all temperatures
  • Leaves are susceptible to infection from emergence to full expansion, then again as they approach the end of the season
    • Fruit are less susceptible to infection as they mature, requiring longer wetness periods for infection to occur
Management:
  • Most commercial apple varieties are susceptible to scab to some extent
  • If possible, isolate orchards from sources of inoculum
    • If planting resistant varieties, avoid planting near susceptible varieties, to avoid loss of resistance
  • Water trees at the soil surface, to avoid triggering ascospore release and to reduce wetness periods
    • Avoid irrigating when precipitation is expected
  • Ensure orchards have good air flow, both inside the orchard, and within the plant canopy, as this will speed up drying of the foliage
  • Reduce inoculum by reducing the amount of leaf litter in the orchard
    • This may be done through raking, vacuuming or blowing
    • Some methods have been tried to accelerate the decomposition of leaf litter, including a small urea application just prior to leaf drop in the fall, or an application of dolomitic lime to leaf litter
    • Chopping or shredding leaves in spring or fall can be effective
  • Protective applications of registered fungicides to new growth during or in advance of rainy periods can help to reduce infection and disease
    • Fungicides may also be applied on a schedule, starting as leaf buds break and emerge, repeating regularly, as required
      • This would be more important in high pressure areas (high moisture, lots of disease inoculum)
OMAFRA “Apple Scab” factsheet
BC Ministry of Ag “Apple Scab Management in BC” factsheet
 
 
 
 
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For more information about the content of this document, contact Robert Spencer.
This information published to the web on May 29, 2018.