Controlling apple scab on crabapples is essentially the same as controlling it on apples, with the exception that most folks don’t use the fruit on crabapple, and don’t continue a spray program into summer to protect the fruit. Once leaves have matured they are less susceptible to apple scab.
Spray Timing and frequency will vary depending on temperature, rains and severity of the problem, and the interval specified on the spray product’s label. Most labels spray interval will be 7-14 days. Ask your service to show you the label of the product they use.( Suggestion- they could take a picture of it on a smart phone if the packaging is too large to bring to you.)
Here are details on when to start control, and spray intervals-
“Fungicides: Materials available to home growers for scab control include captan, lime-sulfur, and powdered or wettable sulfur. Applications of lime-sulfur closely following captan sprays can damage leaves and flower buds, so use caution when rotating these two materials.(Professional services have access to other chemicals)
All-purpose sprays, containing combinations of fungicides and insecticides, are also available. For scab sprays just after petal fall, when insecticide sprays may also be necessary, these chemical mixtures may be appropriate. If the goal of a spray is only to control apple scab, however, the insecticide portion of the spray is wasted. In addition, you may need to spray for scab while your trees are in bloom. Pollinators, including honeybees, bumblebees, solitary bees, and many other beneficial insects, will be killed by a spray that includes an insecticide, so never use an all-purpose fruit spray during bloom.
When should you start spraying?
To protect leaves and fruit from scab, most home apple growers need to spray fungicides in spring. Sprays should start at the growth stage known as "half-inch green tip," when the leaf buds have swollen and begun to open so that about 1/4 -1/2 inch of leaf tissue is visible.
How often should you spray?
Scheduling fungicide sprays for scab after the first application at half-inch green can involve a little guesswork. In warmer weather, leaves grow quickly, and newly-exposed tissue will be unprotected. In cooler weather, growth slows or stops
Check fungicide labels for the recommended spray interval. Most labels offer a range of days to wait before spraying again. (E.g. seven to ten days after spraying, you will need to spray again). In plantings where there was a severe scab infection the previous year, use the shortest interval. In plantings where scab has not been a problem, a longer interval will probably give adequate protection. In addition if the weather is dry the longer interval is acceptable.
When can you stop spraying for scab?
In mid-June, examine the leaves on your trees for scab lesions. Be very thorough, checking upper and lower leaf surfaces, leaves on the interior and exterior of the canopy, leaves close to the ground and those higher in the tree. If you find no or very few apple scab leaf spots, you need not spray fungicide again. If you find scab lesions, or if there are unsprayed trees in your neighborhood with scab lesions, you should continue to spray, because the lesions on the leaves will release more scab spores all summer long.
If scab has been a problem in your apple planting, it may take a year or two to get it under control. If you continue with appropriately timed sprays that cover all leaf and fruit tissue, and practice excellent sanitation of fallen leaves, and if outside sources of fungal spores are few or distant, you should find in the second or third year that you only need to spray from half-inch green tip to mid-June.” (Source- Univ. of Minnesota Extension service)
The above is for apple trees grown for fruit, but it applies to crabapples, too. Here is a link to a crabapple specific guide, with a chart your professional service can follow:
I hope this is helpful. Thanks for using our service.