Mike: Apple scab damage is dependent on two things: how susceptible or resistant a particular apple variety is and how wet a mid-April through early June we have. If you have several varieties, you probably have noticed that some get scab worse than others. Red Delicious is notoriously susceptible. The susceptible ones need more attention than the ones that hardly ever show any damage. Lime sulfur (calcium polysulfide) has been used for a long time and is quite effective. There is a winter mixing rate and the summer mixing rate. Use the summer rate! Otherwise you can tale the leaves/small fruit right off the tree.  But it got pulled from the regular consumer market over some eye concerns. It still is available from farm stores like Wilco.

A more available alternative, though a bit less effective, are sulfur powder sprays. This finely ground sulfur can be found at BiMart and Linnton Feed and Seed, at least the last time I looked. It can take some time to mix into a good solution. Read instructions carefully. Can be applied at "pre-pink" flower stage through the rainy periods in May and early June on sunny days between rains. If it is a dry spring, you may not need to use it much at all. But a moist, drizzly week can really ignite the scab on susceptible varieties.  Golden Delicious will sometimes get "russeted" by this spray. I hope this helps. Here is a link to our PNW Handbook section on apple scab. It is focused toward commercial growers but does have some cultural information useful to homeowners. Call me<personal data hidden> cell if you have questions. Chip  https://pnwhandbooks.org/plantdisease/host-disease/apple-malus-spp-scab

On 01/28/2022 10:27 AM Ask Extension <<personal data hidden>> wrote:

Mike: A delayed dormant, like early to mid-March, the rate is 6-12 gallons of concentrate in 100 gallons of water. That would equal ~2. 5 pints/ 5 gallons (at the 6 gallons/100 rate) or ~5 pints/5 gallons (at the 12 gallons/100 rate.) The lower rate should be fine and would only be used once.

After buds start to swell up to when the pink flower tip shows, the spray rate is much lower at 1.5 – 3 gallons of Polysul per 100 gallons of water. For 5 gallons, that would equal .63 to 1.25 pints gallons per five gallon sprayer. Then stop spraying during bloom and two weeks past bloom.

Once you are 2 weeks past bloom and petal fall, and you can spray again at the rate is ½ gallon per 100 gallons which translates into 3.2 ounces of Polysul per 5 gallons of water which is about ¼ of a cup of Polysul per 5 gallons. Apples can be sensitive to higher concentrations once they are formed. The spray can be repeated fairly often but if it has been dry, you can skip weeks. Usually by late-June, we don’t need to spray any more. Watch the week long weather forecasts and try to spray a few days before a rainy pattern starts.

Hope this helps. Chip

On 02/11/2022 10:19 AM Ask Extension <<personal data hidden>> wrote: