| 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
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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 12°C and 25°C
- 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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