Controlling Apple Scab

By Erica Pate

Growers need to ensure their apples are protected against scab as the primary infection period continues. Primary infection comes from the ascospores of the previous year’s infected leaves. As the season progresses the ascospores in these dead leaves on the orchard floor mature and become ready to be discharged.

However in order for an infection period to occur, sufficient moisture is needed to discharge these ascospores. Since many areas of the province have had little to no rain recently limited mature ascospores have been released. As the ascospores continue to mature without being discharged, there are many ascospores ready to be released during the next infection period. Because of this, the next infection event is anticipated to be a big one. With rain predicted in many areas across the province this weekend, this may be the infection period the ascospores have been waiting for.

For primary infection to occur, three critical conditions must be met. Infection will only occur if there are mature ascospores present in the leaves on the orchard floor, the weather conditions favour ascospore release, and there is inadequate fungicide protection. Mature ascospores are currently present, and this weekend may present favourable conditions for ascospore discharge. It is important that growers are vigilant in apple scab control over the next few days, and any following infection periods, to avoid any infection.

Recommendations for apple scab fungicides can be found in Publication 360, and Table 4-13. Provides more information on the characteristics of apple scab fungicides:

apple scab table

*Calculated from the start of the infection period
Data is adapted from New York (Geneva) from work done by Szkolnik et al, using conidia. These values are adapted from greenhouse tests on Golden Delicious. The after-infection activity of these fungicides may not be adequate to control primary scab in commercial orchards. Do not rely solely on after-infection activity for the control of apple scab.
These results are formulation specific. Newer products and formulations of some products have not been evaluated (e.g. Dithane DG, Polyram DF, Manzate 200DF and Pristine WG). Contact the manufacturer for more information.
1 Maximum after-infection activity is calculated from the start of the infection period
2 Data on Nova is adapted from 1993 Pest Management Recommendations for Commercial Tree Fruit Production, Cornell Cooperative Extension, New York.
3 After-infection activity of DMI (Nova and Nustar) and QoI (Flint and Sovran) fungicides may be reduced in orchards where scab populations have shifted toward resistance to DMI fungicides.
Blank cell means information is unavailable.
KEY: E = excellent; VG = very good; G = good; F = fair; P = poor; N = none. Values followed by ‘*’ are based on field observations.

As always it is important to continue to monitor for apple scab throughout the summer during the secondary infection period.

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