Ministry of Agriculture & Lands
Tree Fruit Leader, Vol. 4(1) Mar. 1995
Sanitation for Apple Scab Control
Gayle Jesperson, Plant Pathologist, Kelowna
It is possible to reduce the amount of overwintering apple scab in your orchard using two practical and inexpensive methods.
1. Spraying trees with a urea fertilizer solution at leaf-fall or spraying fallen leaves on the ground.
2. Shredding fallen leaves with a flail mower
How it is done:
Urea application: A 5% solution of urea (46-0-0) in water may be applied to apple trees as leaves begin to fall in the autumn. This should be done as late as possible to prevent the urea from being translocated into the tree. However, it should be early enough to have most of the leaves still on the tree. Note that trees sprayed with urea may defoliate more quickly than unsprayed trees. Urea may also be sprayed on the leaves on the ground, after all the leaves have dropped. The ground spray can also be done in the spring.
Leaf shredding: Mulch leaves with a flail mower in the fall. Mowing in the spring before budbreak will also be helpful. The amount of leaf litter that you can shred up will depend on the type of mower, as well as the tree canopy and spacing. If you can get at the leaves in the weed-free strip under the trees, it will work better than just mowing between the rows.
How it works:
Urea inhibits the development of apple scab fruiting bodies on the fallen leaves. The high nitrogen content also helps the leaves to decompose much faster than normal. Leaves sprayed within a week of leaf fall have been shown to produce few or no apple scab spores in the spring. If the urea-sprayed leaves hang on the tree longer than a week, too much nitrogen will be translocated out of the leaves, and scab suppression will not work as well. A follow-up spray of leaf litter on the ground would help to improve control of the fungus. Leaf shredding also hastens the decomposition of the leaves.
How well does it work?
R.G. Ross (Kentville, Nova Scotia) published an article in 1969 titled "Can Apple Scab be controlled by breaking its life cycle?" He reported on research from England that showed a 97% reduction in ascospore productivity in leaves sprayed with 5% urea at leaf-fall. He reported that similar results had been obtained at Kentville.
R.T. Burchill, (East Malling Research Station, England) published a paper in 1968 describing experiments with urea. This may have been the research referred to by Ross. Burchill found that when apple plants were sprayed in autumn with 5% urea, followed by a second pre-bud-burst application of 2% urea to leaf litter, ascospore production in the spring was significantly suppressed. Spraying in the fall was effective when leaf abscission occurred within a week of treatment. However, a single autumn application still allowed some ascospore production. In 2 orchard trials conducted by Burchill, urea treatments resulted in a reduction in the number of scab lesions on blossom spur leaves by 46 and 59%.
William MacHardy (University of New Hampshire), has done some recent work on evaluation of sanitation practices. In a 1994 article, he reported on effectiveness of spring and autumn leaf shredding in orchards using a flail mower, as well as effectiveness of urea application in both spring and fall:
- Autumn leaf shredding with a flail mower reduced leaf litter density an average of 55% and reduced severity of primary scab on leaves by an average of 62%. Ascospore dose was reduced by 55%.
- Spring leaf shredding reduced ascospore dose by 89% and foliar scab by 80%.
- Spring urea treatment reduced ascospore dose by 74% and foliar scab by 80%.
- In small plot studies, pre-leaf-fall urea, spring urea, autumn leaf-shredding, spring leaf-shredding, and a combined autumn and spring leaf-shredding treatment reduced ascospore productivity by 97, 82, 50, 65 and 83% respectively.
How does this fit into a control program?
Sanitation measures will help to reduce the amount of initial inoculum. This will make scab easier to control, but fungicide sprays will still be necessary during primary scab season.
An orchard that had a severe scab problem last year will benefit from reduced diseased pressure, making effective control with fungicides during primary scab season more likely. And remember - good control during primary scab season means you won't need to continue spraying for the rest of the summer. Another benefit - lowering the disease pressure will make it harder for the fungus to develop resistance to fungicides such as Nova.
An orchard where scab levels were low last year could also benefit. By reducing the potential ascospore dose even more, it may be possible to delay the first fungicide spray. There has been considerable research done in other areas on "PAD" or "potential ascospore dose". MacHardy has shown that orchards with a very low PAD can delay the first fungicide spray until after the first 3 infection periods, or the pink bud stage, whichever comes first. This has not yet been investigated in the Okanagan.
Kelowna Urea Trial
A small plot urea trial was established in a south Kelowna orchard in 1994. A 5% urea solution was sprayed on several trees at approximately 50% leaf fall. After the spray had dried, leaves with scab lesions were removed and placed into plastic mesh cages on the orchard floor. An equal number of unsprayed leaves were also placed into separate cages. In the spring the leaves will be collected at intervals for ascospore discharge and maturity tests. This is a component of a larger 2 year project to look at the pattern of ascospore maturity and release in the north Okanagan, funded by the BCMAFF IPM program.
