Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
10/4/99
Contacts:
Mark Gleason, Extension Plant Pathology, (515) 294-0579,
mgleason@iastate.edu
Elaine Edwards, Extension Communication Systems, (515)
294-5168, eedwards@iastate.edu
Yard and Garden Column for the Week Beginning Oct. 8
Why Is My Crab So Scabby?
By Mark Gleason
Extension Plant Pathologist
Iowa State University
"Why is my crab so scabby?" has become a common refrain of disgruntled garden-center customers. The source of their anger is their prized crabapple trees, which lost their leaves in midsummer -- defoliated, in other words -- and wound up looking hideous instead of decorative. The nursery center sold these crabs with guarantees that they were genetically resistant to a defoliating disease called apple scab. Assurances notwithstanding, the crabs defoliated.
So who's responsible for these bare branches? The customer feels deceived and the garden-center owner feels baffled and defensive. Buyer and seller eye each other suspiciously, and the situation threatens to deteriorate into "yard rage."
At this volatile moment, a caped plant pathologist leaps between into the fray and calls for a time-out. "Wait," he says. "If you'll put down that hoe and those pruning shears, I can explain."
The customer and the garden-center owner are skeptical; "OK," they say, "but this better be good!" Down go the hoe and the pruning shears.
It turns out that several factors may contribute to the crabs' poor performance. One piece of the puzzle is what might be called the "snowball effect." Like most fungi that attack leaves, scab burrows into the leaf tissue in the summer and hides inside fallen leaves during the winter. Next spring, during rainy weather, scab spores squirt out of the decaying leaves like toothpaste out of a tube, then drift up into the air, onto the new leaves. Infections by these spores produce the smudgy-looking, olive-green leaf spots typical of scab, then the leaves turn yellow and fall to the ground. Because scab was bad last year, a scab epidemic is more likely next year. More scab spores beget more scab infections, which beget more spores, and the scab population snowballs.
Scab's snowball effect buries crabapples when spring and summer weather is wetter than normal. Spores of this rain-loving fungus become so numerous, and attack leaves so often, that they overwhelm the resistance of moderately scab-resistant crabapple varieties. When you realize that much of Iowa saw above-normal rainfall in every growing season of the 1990s - that's 10 straight years - it's easy to understand how scab's snowball effect became an avalanche. Moderate resistance simply can't hold out against wave after wave of hungry scab spores.
Scab can break down crabapple resistance in another way, too -- by changing its own genetic makeup. Natural selection, the process driving this change, is the same process that made dinosaurs into birds and apes into us.
Here's how it happens: Scene 1: A grove of a single variety of crabapple trees, moderately resistant to scab. As the rain pelts down, scab spores struggle desperately to infect crabapple leaves. Only a handful of lucky spores, genetically more aggressive than the others, succeed. All the other spores die. The crabapples look great, because just a few infections occurred. Scene 2. It's some time later, in the same grove, during another rainstorm. Again, the scab spores attack the crab leaves. But now more spores cause infections, because many spores are the offspring of spores that were successful infectors in scene 1. Once again, the wimpy spores die. Soon, the crabs don't look all that great, and some leaves are falling off. Scene 3. More time has passed. It's still the same grove, in yet another rainstorm. The action resembles that of scene 2, only worse. Now, almost all the scab spores, which descended from successful infections, cause infections in turn. Almost no wimpy scab spores are left. Soon, defoliation is rampant, and the crabs look like the dog's breakfast.
This process, resistant crabs selecting for scab strains that are able to overcome the resistance, has been documented for scab on apple trees, so it probably happens on crabs, too. The net effect is that repeated "scenes" (cycle of selection) can break down the crabs' ability to withstand scab attack. A tree's genetic makeup doesn't change; it's the scab population that changes, thanks to natural selection.
Luckily, there are ways to solve the crab-scab puzzle. One is to plant only crab varieties with a high level of resistance to scab. These varieties are likely to be less vulnerable than the moderately resistant ones to succumbing to scab. How can you find the highly scab-resistant crabs? Almost all good garden centers and nurseries will have tags or catalogs that specify the level of each crab variety's resistance. If it doesn't say highly resistant, pass it by.
Good luck, in the form of drier weather, can break the snowball effect, but you can help out, too. Annual pruning of crabs, to thin out the inner branches, remove the water-sprouts and open the canopy to better air movement, speeds up drying, thereby lowering scab risk. And proper pruning results in a better-looking crab, too.
Raking up the fallen leaves in late October also helps melt the scab snowball. Because most of next year's scab spores come from this year's old leaves, raking helps minimize spore survival into the new millennium. Be sure to burn, bury or dispose of the scabby leaves, because they will probably survive the winter very well in your yard-waste pile.
Is there any way to spike the natural selection process that can wear down scab resistance? One approach would be to scramble your crab-planting plans. In other words, rather than planting 20 crabs of the same resistant variety together in a grove or colonnade, intermix them with other crab cultivars or, even better, with other tree species. Because they avoid concentrating one type of crab in the same area, these strategies can put the brakes on selection for resistance-tolerant scab.
Fungicides can be part of scab management too. Three sprays in the spring, starting a few days before the flowers open and repeated at 10-day intervals thereafter, can provide fairly good control of scab in most years. But almost nobody is willing to wage annual chemical warfare against scab, because it's expensive, tedious and liable to be unpopular with the neighbors. The cultural approach -- selecting highly resistant varieties, scattering rather than massing trees of the same variety, annual pruning and raking fallen leaves -- turns out to be cheaper, easier and probably more effective. With a little planning and maintenance, you can keep your crabs looking great, and avoid the perils of yard rage. And caped plant pathologists can give it a rest.
ml: isugarden
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