IPDM in focus: black spot (apple scab and pear scab)
Research & ExtensionThe inoculum for primary infections of black spot in spring is infected fallen leaves from the previous season. Now, at harvest and before leaf fall in autumn, is the time to monitor and determine carry-over levels.
Both apples and pears across Australian growing regions are affected by black spot. Venturia inaequalis (apple scab) and Venturia pirina (pear scab) are separate diseases, sharing similar symptoms, development and methods of management. Although caused by similar pathogenic fungi, they do not cross-infect. The diseases affect flowers, fruit and foliage, causing yield and quality loss. As a consequence of a run of particularly wet years, reports of black spot have spiked. To ensure reduced severity next spring, there are management decisions that can be taken from now, as part of your IPDM plan.
Figure 1: Apple scab infection on apple fruitlet. Photo: Kevin Dodds, NSW DPI
Establishment
Infections primarily occur from fallen leaves that were infected the previous season. Young leaves and fruit are the most susceptible and are infected when temperature and leaf wetness conditions are conducive to spore germination, such as during spring rains. Primary infections (ascospores) mature from shortly before bud break to late November, with most maturing during flowering. Leaves are most susceptible when expanding and fruit when young, although older fruit can also be infected by secondary infections (conidia) during prolonged moist conditions.
If the diseases become well established, fungicide applications may not have a great effect.
Identification
Foliage: Look for lesion development on both sides of the leaves. Young lesions are very small and are olive-green, with diffuse edges, noticeable until petal fall. As the lesions get older, they become larger and velvety, turning a darker olive-green. In time, as spores are dispersed from the lesions, they become brown and areas of the leaf die.
Fruit: Lesions – small black velvety spots – can be seen on flower stalks or young fruit . They are noticeable after petal fall. When spots grow, they become brown, corky and scabby. Fruit infections will occur in summer and may be visible before harvest as ‘pinpoint’ summer spots. However, they may not become evident until storage. Lesions are superficial and the fungus does not extend to a great degree into the flesh of the fruit.
Monitoring
- At harvest and before leaf fall, monitor to determine the levels of carry-over and, hence, potential for spring primary inoculum.
- Monitor temperature from before bud break (late August) to December to determine primary infection maturity, and rainfall events to determine potential spore germination.
- From green tip to harvest, monitor to determine disease pressure and new unprotected plant tissue.
- At harvest, monitor fruit in bins to determine effectiveness of the IPDM program.
For additional monitoring tips, timing and thresholds, see pages 166–176 of the IPDM Manual.
Figure 2: Apple scab infection on apple foliage. Photo: Kevin Dodds, NSW DPI
Management
To control and prevent primary infections, post-harvest sanitisation is the key to reducing infection of susceptible plant tissues during the infection periods.
The aim is to make conditions as least favourable to the disease as possible, then to use sanitation practices to reduce inoculum and thereby reduce the number of fungicide applications required. The need and timing of fungicide applications can be guided by predictive models that account for weather conditions, ascospore maturity and leaf canopy development.
Across eastern growing regions in 2022, frequent rainfall interrupted spraying schedules and efficacy to combat black spot outbreaks. A major focus now is to manage proactively by removing leaf material from orchard floors as soon as practicable for the next season.
Figure 3: Apple scab infection on lower side of leaves Photo: Kevin Dodds NSW DPI
More information
The IPDM Manual provides guidance on developing an IPDM (integrated pest and disease management) plan for your orchard to identify, monitor and manage key pests, diseases and weeds through an integrated approach that aims to disrupt life cycles and prevent susceptibility while encouraging and nurturing beneficial species that can act as natural control agents in your orchard.
For detailed information on black spot (apple and pear scab) in Australia’s apple and pear growing regions, including detail on the life cycle of the disease, see page 158 of the IPDM Manual, found on the home page of extensionaus.com.au/ozapplepearipdm.
Watch some short videos with David Williams, where he discusses ground cover management and early season management techniques to minimise and control scab. Visit extensionaus.com.au/ozapplepearipdm/scab-management.
The PIPS3 Program’s Strengthening cultural and biological management of pests and diseases (AP19002) project has been funded by Hort Innovation, using the apple and pear research and development levy, contributions from the Australian Government and co-investment from Agriculture Victoria. Hort Innovation is the grower-owned, not-for-profit research and development corporation for Australian horticulture.
This article was first published in the Autumn 2023 edition of AFG.
