Managing apple scab

By BILL CRAIG, Horticulture Specialist
One of the disadvantages of growing apples in a Maritime climate is the amount of annual rainfall. Many may see this as an advantage, but with wet weather, disease pressures become an issue for apple growers.
The main disease that apple growers have to contend with is the fungus apple scab Venturia inaequalis. This disease overwinters on infected lives from the previous growing season. Ascospores are released from the pseudothecia during wetting events with the start of spore release coinciding with the start of apple bud growth that generally occurs the later part of April. Ascospores continue to mature and be released during wet weather into early June. The majority of spore release occurs during the pink to calyx stage of bud development when the tissue is the most susceptible to infections. Once the spores have been released and land on apple leaf or fruit tissue they need wetness to germinate and enter the tissue and the length of the wetting period is dependent on temperature. At a temperature of 1°C it may take 48+ hours for the infection to take place while at 16° C it will only take 9 hours.
Infections that occur from overwintering spore are called primary scab infection and these infections produce condia spores that wash down over the fruit and leaves during wet weather. Infections resulting from condia spores are referred to as secondary infections.
Secondary infection can take place from late May right up to the time of harvest. The majority of apple cultivars grown in North America are susceptible to apple scab and therefore fungicide applications are required to protect the foliage and fruit. The number of fungicide applications is dependent upon the length of the growing season and the number of infection periods. During the 2009 growing season, 16 infection periods were recorded at Kentville from late April to the middle of July. The number of fungicide applications to control the 16 infection periods would depend upon the timing of the application and could have been as few as eight. Failure to control apple scab infections can result in reduced yields while light infection may result in the down grading of the crop from fresh fruit sales to juice apples.
Apple growers have a number of fungicide options for the control of apple scab and it is critical that growers use these options wisely to prevent the development of apple scab resistance to a fungicide. In the late 1970s and early 1980s, resistance to the fungicide dodine started to appear and this was followed by resistance to benelate in the mid-to-late 1980s. The present concern for many apple growing regions is the development of resistance to the group 3 (DMI) fungicides Nova and Nustar and group 11 (Strobilurin) Sovran and Flint.
Resistance develops thought the repeated application of these product over time and occurs as the result of selecting for naturally occurring resistant spores or that organism gradually develops an immunity to the fungicide. To prolong the effective life span of these fungicides growers should restrict their use during a growing season. Never apply more than two back-to-back applications of a fungicide from group 3 or group 11 and never apply more than four applications of any of the fungicides from these two groups during a growing season. The other step to avoid is appling a fungicide from these groups in the hopes of weakening or burning out infections that have taken place.
The risk of resistance to the group M class of fungicides is low because they have a broad-spectrum means of control that has been compared to a sledgehammer virus; a fine needle for the groups 3 and 11. Many of the fungicide in this group have been used for over 30 years in Nova Scotia without resistance being documented. Losses of fungicide in this group will likely come about as the result of PRMA reviews of these older chemistries and concerns over environmental and health risks. AAFC and PRMA have already developed a reduced risk strategy for managing apple scab to minimize the use of captan. Some of the components of this strategy are to reduce over wintering spore by the means of leaf destruction through mowing and the application of urea to the leaves, the adoption of bio-fungicides and improved monitoring. The ability of producers to control primary apple scab though the reduction of overwintering spores, the application of fungicide at the proper timing and effective coverage will result in reduce pesticide usage. Controlling primary scab will reduce the amount of fungicide required to control secondary infections.
The key point of the message, when it comes to apple scab control, is to obtain good control of primary apple scab infection. This involves paying close attention to the weather; make use of disease forecasting models and the timely application of fungicide with a calibrated orchard sprayer. When selecting a fungicide, resistance management should be front and center. In an ideal world, everybody would be growing disease resistant cultivars that would meet the demands of the consumer 73thus eliminating the need to apply fungicides.
(For more information, please contact Bill at (902) 678-7722 or b.craig@agrapoint.ca)