Crabapple scab is one of the most common targets of fungicide applications to ornamentals in Illinois. Most sprays are done by commercial operators. Fungicide sprays protect against current-season scab infection. You cannot wait until symptoms show to make the first fungicide application. The first spray needs to be applied as soon as new leaves begin to emerge. Chemicals are used when a problem has been established in the past and other methods of control have not worked. Ideally, one would plant crabapples with resistance to scab, fire blight, rust, and powdery mildew. Still, many established trees are susceptible to this disease. Scab may not kill the tree but will cause unsightly foliage, leaf drop, and general tree decline over the years.
Crabapple scab is caused by the fungus Venturia inaequalis. Undoubtedly, most of you have seen or heard of this disease. It is very common on crabapples and apples. We are addressing only crabapples in this article. Look for olive green leaf spots in the spring. These quickly become black spots surrounded by leaf yellowing and then extensive defoliation of susceptible varieties by late June. For details about apple and crabapple scab, refer to Report on Plant Disease, no. 803, available in your Extension office or on the Web at http://www.ag.uiuc.edu/~vista/horticul.htm. Scroll down to the apple and crabapple scab entries.
Scab is present every year in Illinois. Its intensity depends on the weather. Primary infection is by ascospores present on last year’s leaves, currently on the ground in the garden, neighborhood, or nursery. Sprays usually target this primary infection. These spores will be present and ready to infect foliage until the end of petal fall. If you have a susceptible crabapple cultivar, when might you expect fungal infection to occur? The apple scab fungus infects under a wide range of temperatures but requires a wetting period to become established on a tree. The minimum wetting period on the leaves is only about 6 hours if temperatures stay near the optimal 60°F. If temperatures are cooler, the wetting period must be longer for infection to occur. In a normal spring, scab symptoms might start to show on the leaves from 8 to 18 days after infection. Under cool, dry conditions, this incubation period might be longer.
If you have a susceptible variety and you are not able or willing to replace it, then spraying with fungicides might be your course of action against this disease. University of Illinois recommendations state that the first spray should be applied when leaves just begin to emerge from buds (about 1/4 inch green). This is to protect new leaves. Sprays must be continued according to label intervals until 2 weeks after petal fall to give maximum protection against ascospore infection. The current Commercial Landscape and Turfgrass Pest Management Handbook lists 30 possible products you could use. Additional, broad-spectrum products are listed on page 84 of that publication. At the end of the chapter, on pages 85 and 86, a table lists products and their mobility. Some products are protective-contacts, and some are systemics. A systemic fungicide provides a bit of curative (kickback) action and is not washed off by rains once absorbed by the leaves. Keep in mind that nearly all systemic fungicides move only upward and outward toward new growth. Homeowners can refer to the current Home, Yard, and Garden Pest Guide for fungicide options. These manuals are available for a fee in Illinois Extension offices or online at https://pubsplus.uiuc.edu/.
For commercial applicators needing more details on exactly when to spray for maximum benefit, one should make the first application between green tip and 1/2 inch green (target 1/4 inch green). Particularly for this application, a systemic fungicide is recommended to provide curative (kickback) activity. The second application should be made between tight flower cluster stage and pink flower bud stage. Additional, later applications (at labeled intervals) may be needed, depending on the year and client expectations. We cannot be any more specific because each product has slightly different recommendations. You really do need to read the label.
The question we often receive is how long it takes for the tree to move from one growth stage to the next. That depends on the weather, as well as the location and variety; but fruit pathology specialist Dr. Babadoost offers this approximation to help with your application choices:
Approximate time between apple growth stages
- Silver tip
- + 7 days to green tip
- + 5 days to 1/4 inch green
- + 5 days to 1/2 inch green
- + 10 days to tight flower bud cluster
- + 10 days to pink flower bud
- + 10 days to bloom
- + 10 days to petal fall
Try these Web site references showing apple growth stages.
Many crabapple cultivars have resistance to scab, and resistance is definitely the long-term solution to infection. If you are planting new crabapples this year, look for varieties with resistance to scab, rust, fire blight, and powdery mildew. A publication that may help is this reference by U of I professors Dave Williams and Gary Kling: Recommended Crabapples for Illinois Landscapes. Look for it on the Web at http://www.extension.uiuc.edu/IPLANT/plant_select/trees/Selecting_Crabapples.pdf.