The first issue of this newsletter usually coincides with questions about proper timing for control of apple scab on crabapples. This information may be most beneficial to growers in northern counties, where the disease is just beginning. Scab is caused by the fungus Venturia inaequalis. It is very common on crabapples and apples. Look for olive green leaf spots in the spring. These spots quickly become black, surrounded by leaf yellowing, and then extensive defoliation of susceptible varieties by late June. For details, 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 apple and crabapple scab.
Scab is present every year in Illinois. Its intensity depends on the weather. Primary infection is via a type of spores called ascospores that are present on last year’s leaves, currently on the ground in the garden, neighborhood, or nursery. These spores will be ready to infect foliage until the end of petal fall. If you have a susceptible 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 degrees F. If cooler, the wetting period must be longer. 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 are not able or willing to replace it, then spraying with fungicides might be your course of action. Fungicides are used as protectants, before infection occurs: You cannot wait until symptoms show. University recommendations say that the first spray should be applied when leaves just begin to emerge from buds, 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 2005 Commercial Landscape and Turfgrass Pest Management Handbook lists 30 possible products to use; on page 79, a table lists these products and their mobility. If possible, choose a systemic product. A systemic fungicide provides a bit of curative (kickback) action and will not be washed off by rains once absorbed by the leaves. Keep in mind that nearly all systemic fungicides will only move upward and outward toward new growth. Homeowners can refer to the 2004 Home, Yard, and Garden Pest Guide for fungicide options.
For commercial applicators needing more details on exactly when to spray for maximum benefit, this information from Extension plant pathology specialists Mohammad Babadoost and Bruce Paulsrud might be helpful. They suggest that for controlling apple scab on crabapples, one should make the first application between green tip and 1/2" green (target 1/4" green). Particularly for this application, they suggest the use of a systemic fungicide 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 Dr. Babadoost offers this approximation to help with your application choices:
- • Silver tip
- • +7 days to green tip
- • +5 days to 1/4" green
- • +5 days to 1/2" green
- • +10 days to tight flower bud cluster
- • +10 days to pink flower bud
- • +10 days to bloom
- • +10 days to petal fall
Paulsrud provides these references showing 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