William McPhee
Apple Scab Cycle
Apple scab, caused by the fungus Venturia inaequalis, can be a very serious problem in the Okanagan. The most difficult characteristic, relative to control strategy, is that the disease is not a major annual problem here like it is in other areas in the country. Ontario, Quebec and Nova Scotia have weather much more conducive to scab development and as a result they are more concerned about monitoring and applying controls. Here, the intermittent condition for scab development leads to lax vigilance and sets the area up for “surprise outbreaks.”
The many different microclimates and weather patterns in the Okanagan mean certain areas are more likely to have a scab infection than others. These areas should be identified as needing close monitoring each spring. There will be years when the scab pressure is wide spread and a calendar spray strategy necessary to “play it safe,” but most years, if the high risk areas are under low, or no pressure, it should be possible to avoid unnecessary sprays in the majority of the valley.
The potential damage caused to fruit can be horrendous in some areas but in the Okanagan the fruit infections are much less dramatic; however the demand for perfect fruit at markets is so high that even slight damage from scab results in fruit being rejected. A single missed spray can divert an entire block of fruit to juice.
LIFE CYCLE OF THE FUNGUS
The life cycles of fungi are important to develop the best control strategies.
There are two distinct segments to the life cycle of fungus V. inaequalis. The pseudothecia segment involves the development of a specialized structure that contains ascospores. These ascospores develop over the winter and early spring in the leaf litter under the tree. They are released in the spring and spread onto the newly developing leaves and fruit causing primary infections.
Once primary infections have occurred then the next distinct segment, secondary infections, are initiated. Secondary infections that come from an initial primary infection can cycle throughout the summer if weather conditions are right and control is slackened.
Stage 1 is the over-wintering stage. If there was apple scab last summer pseudothecia (a specialized fungal structure) will be imbedded in infected leaves in the leaf litter and capable of finishing development in the spring.
Stage 2 illustrates the most important stage of the fungus as far as the control program is concerned. Control depends on preventing the ascospores (orange in the illustration) from infecting the newly developing tissue in the spring. If this is accomplished the summer will be scab free.
These ascospores are contained within an ascus and must be mature to infect. They require heat units and moisture to reach maturity at which time they are ejected from the leaf litter by splashing rain drops and are carried up into the tree by air movement. Generally this release of the ascospores coincides with late pink.
Since ascospores are absolutely necessary for primary infection to occur, the first sprays do not need to go on until the ascospores are released. Monitoring for ascospores in the orchard is a technique that avoids spraying too early.
Most mature ascospores are released within two hours after the start of rain. Once released there are still specific environmental conditions necessary for a primary infection. This is where the leaf wetness/temperature charts in the Tree Fruit Production Guide are used.
Released ascospores require water and time to penetrate the leaf cuticle. Once established these infections begin to mature and lesions become visible on the leaves. Early leaf infections can be very faint but, as they mature and produce spores, they will develop into more severe lesions.
Stage 3 is the secondary cycle, which can be repeated many times during the growing season if wetting periods occur. When fruit is infected during the summer infections may not be visible to the naked eye at harvest but they will enlarge in storage and cause “pin point’ scab, which results in a cull. Scab infections can occur anytime up to harvest if environmental conditions are met, but as fruit matures it becomes increasingly resistant to scab.
Note: Apple scab is easy to control in our area. Poor management is the usual cause of problems.
ADDING UREA TO REDUCE ASCOSPORES
Research has shown the application of urea to the leaf litter in the fall aids in breaking down leaf litter and prevents pseudothecia development, which reduces ascospore production in spring. A fall application of Dolmitic lime will similarly reduce spring ascospores. However, even a reduction of 90 – 95% of the ascospores still leaves the block vulnerable to primary infection in the spring. In heavily infected orchards the fall applications of nitrogen and lime can be useful, but are not a cure.
OVERHEAD SPRINKLERS
Overhead sprinklers can extend a wetness period so any extended use must be included in the infection period calculation whether at the beginning of the wetness period or at the end.
DRYING PERIOD
The infection period is interrupted once the leaves dry off for 10 hours. Often a leaf wetness will be interrupted by a drying period. If that dry period is less than 10 hours then continue to record the length of the next leaf wetness period and add it to the first. If there are several short dry periods continue to add the wetness period and if accumulated time indicates an infection period, then spray.
RESISTANCE:
There has been resistance reported in the valley to certain chemicals, but resistance is avoided by rotating fungicides from different action categories. Check with your field person or the BC Ministry extension specialist to be sure there is no resistance in your area.
If there is resistance to a chemical avoid it for the season – the resistance should revert and the fungus become sensitive to the fungicide again. ■