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Apple and Pear Scab

Ravageurs et maladies
English
Pear Scab.
Photo: Jardin botanique de Montréal (Lise Servant)

Onglets

Description

Summary

Cool, rainy weather promotes the development of the fungi responsible for apple and pear scab. Apple, crabapple and pear trees are particularly affected. The disease first appears as numerous circular lesions (scabs) on leaves and fruit. A heavy infestation may cause many of a tree's leaves to drop, and seriously affect fruit quality.

Signs and symptoms

  • Young leaves and fruit are especially sensitive to apple and pear scab during the growing period. Mature tissues are more resistant.
  • On foliage: The first sign of the disease is small, circular, olive-green, velvety-looking patches. As the infection progresses, the lesions become black and slightly raised. Seriously infected leaves may turn yellow and drop prematurely.
  • On flowers: A single scab on a flower pedicle or sepal can cause a flower to dry out and drop.
  • On fruit: An early infection can cause young growing fruit to drop or be deformed (the tissue will not grow around the scabs). A late infection will mark mature fruit with corky scabs, similar to those on foliage. These fruit can be eaten, because the scabs remain on the surface and are easy to scrape off.
  • On new shoots: Apple and pear scab can cause spots and cankers to form on young branches, killing them, particularly on pear trees.
  • A serious infection can cause many leaves, flowers and fruit to drop prematurely. The tree may come into leaf a second time, but this will drain its reserves.
  • Apple and pear scab does not usually kill trees, but weakens them and makes them more vulnerable to other insect pests and diseases.

Latin name (genus)

Venturia inequalis, Venturia pirina

Host plants

A few fruit and ornamental trees, including apple, crabapple, hawthorn, pear and mountain ash.

Especially certain cultivars of the following species:

Apple: 'McIntosh', 'Cortland', 'Lobo', 'JerseyMac' and 'Empire'.

Crabapple: 'Columnaris', 'Echtermeyer', 'Hopa', 'Kelsey', 'Profusion', 'Radiant', 'Red Jade', 'Red Splendor', 'Royalty' and 'Selkirk'.

Pear: 'Flemish Beauty' and 'Williams'.

Development cycle

Description and life cycle

Fungi in the genus Venturia are microscopic members of the class Ascomycetes, spore-sac fungi.

They produce thread-like tubular mycelia that form between the leaf cuticle and epidermis. They form two types of spores: conidia (asexual) and ascospores (sexual). The latter are usually arranged in groups of eight in an ascus, a small sac inside which meiosis occurs.

They overwinter in the form of immature ascospores deep inside fallen leaves and fruit or hidden between the scales on buds.

In spring, the mature ascospores are released and dispersed by wind and rain. Given favourable weather conditions, they quickly form fructifications and the mycelia proliferate, producing scattered spots on the surface of wet leaves and fruit. This is the primary infection period, which lasts for six to ten weeks, from the time when the buds burst until late June.

Later, the scabs become downy looking, indicating that the mature mycelia have produced fructifications and summer spores (conidia). As these are dispersed, they spread the disease to other tissues. These secondary infections continue as long as weather conditions remain favourable.

In fall, the Venturia fungi invade the tissues more deeply, where they form overwintering reproductive structures that release ascospores the following spring.

Prevention and control

Favourable conditions

Cool, rainy weather in spring is conducive to primary infections. Winter spores must have water in order to germinate. The risk of infection diminishes with warmer weather, but may increase again in fall.

Drastic pruning and heavy feeding with high-nitrogen fertilizer promote the rapid growth of tender shoots that are more susceptible to infection.

Identification

In spring, during the period when the ascospores are being released and new leaves are forming, regularly inspect susceptible trees and those that have already been infected. In summer, watch for the disease to spread, particularly if the weather stays cool and rainy.

Prevention

  • Choose resistant cultivars:
    • Apple: 'Britegold', 'Crimson Spire', 'Dayton', 'Emerald Spire', 'Freedom', 'Greening', 'Golden Russet', 'Jonafree', 'Liberty', 'Macfree', 'McIntosh Savio', 'Moira', 'Murray', 'Nova Easygrow', 'Novamac', 'Priam', 'Prima', 'Priscilla', 'Red Baron', 'Redfree', 'Richelieu', 'Rouville', 'Sir Prize', 'Trent', 'William's Pride', 'Wolfe River'
    • Crabapple : 'Adirondack', 'Anne E', 'Autumn Glory', M. baccata 'Jackii', M. baccata 'Roshtern', 'Baskatong', 'Beverly', 'Bob Blanc', 'Callaway', 'Camelot', 'Canterbury', 'Cardinal', 'Christmas Holly', 'Coralburst', 'David', 'Dolgo', 'Excalibur', 'Flame', 'Florida Rosea', 'Foxfire', 'Geneva', 'Gibb's Golden Gage', 'Golden Raindrops', 'Guinevere', M. halliana var. parkmanii, 'Hamlet', 'Henningi', 'Holiday Gold', M. hupensis 'Tea', 'Jewelberry', 'King Arthur', 'Lancelot', 'Lisette', 'Lollipop', 'Louisa', 'Mary Potter', 'Maybride', 'Molten Lava', 'Morning Sun', 'Ormiston Roy', 'Pink Princess', 'Pink Satin', 'Pink Spires', 'Prairie Fire', 'Prairie Maid', 'Professor Sprenger', M. prunifolia var. rinkii, 'Purple Prince', 'Ralph Shay', 'Rawhide', 'Red Jewel', 'Royal Beauty', 'Royal Raindrops', 'Royal Ruby', 'Rudolf', M. sargentii, M. sargentii 'Candymint', M. sargentii 'Firebird', M. sargentii 'Tina', 'Serenade', M. sieboldii 'Calocarpa', M. sikkimensis 'Rockii', 'Silver Moon', 'Sinai Fire', 'Sir Galahad', 'Sir Lancelot', 'Snow Magic', 'Spring Snow', 'Strawberry Parfait', 'Sugartyme', M. toringo 'Fuji', 'Tschonoskii', 'Turesi', 'Van Eseltine', 'White Angel', 'White Candle', 'Winter Gold', 'Shaugnessy Cohen'
    • Pear: 'Ure', 'Nova'
  • Keep trees healthy by fertilizing them properly and watering them during dry spells.
  • Prune overly dense trees to allow air and light to reach the centre of the plant.
  • Avoid drastic pruning and heavy feeding with high-nitrogen fertilizers, both of which promote the rapid growth of vulnerable, tender shoots.

Physical control

  • Rake up and dispose of infected leaves and fruit as they drop.
  • Never compost infected plant material.

Biological control

None available.

Chemical control

In case of a serious infection in the previous year or years, as a preventive measure, apply a low-impact pesticide with sulphur, calcium sulphide or calcium polysulphide (lime sulphur) as the active ingredient. Read the product label carefully and follow the manufacturer's recommendations.

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