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Apple Trees Looking a Little Rusty

WD40 won't help you fight the dread disease they call Rust

Question. Every spring our ornamental crab apple tree is infested by a rust that ruins the leaves and causes them to dry and shrivel in the summer. I am aware that these trees need to be sprayed with an oil-based solution immediately prior to budding,but this has not worked in the past.  I have also been told that suppository-like things can be drilled into the tree or put into the ground during the fall to prevent the problem. Any other suggestions would be appreciated.  The tree looks terrible most of the spring and summer months.

    ----Nancy in Media, PA

Answer. Yes, rust really does like to go after apple trees—both fruit bearing and ornamental types like your flowering crab apple—big time. It generally shows up late in the Spring, in the form of pale yellow spots that gradually get bigger and take on that familiar orangey color.

The answer you always hear is to remove any nearby cedar trees—especially Eastern red cedar—and, less frequently, other members of the Juniper family (yes, cedars are junipers—plants have families every bit as confused as yours). There are a number of different 'species' of spore-borne fungal diseases grouped under the general name 'rust'—another well-known one loves to attack roses—but this type needs to also grow on cedars (or other junipers) before it can infect apple trees (that's why its common name is 'cedar-apple rust'). And it's far from the most complicated kind—one type of rust has to 'jump' to five different types of plants in a season to complete its life cycle. Kind of like that bug the kids brought home from school for your Christmas present.

Anyway, the spores can overwinter in debris on the ground (which makes a good clean-up underneath the tree in the Fall essential), and/or they can spend the off-season on cedars and junipers, which I'm now just going to pretend are two different things so I don't have to insert family tree explanations into the dialogue every time I mention the dang things. On cedar and junipers, the disease takes the shape of large, soft galls on the branches that, like teenagers, develop long colorful horn-like structures when the weather warms up. Those horns then spit out disease spores that go looking for your flowering crabapple—their hands-down favorite victim. (If you ever see these kinds of structures on your cedars or junipers, prune them off and trash them—even if you don't have apple trees; you might be ending the rusty misery of an apple grower miles away!)  

If you've got an apple tree planted close to a cedar or juniper, it'll never be happy till one or the other goes bye-bye. But rust may still come a callin' even if you do get rid of any cedars and junipers close by. Those rusty spores can travel up to four miles on a windy day, and you may never even see the playmate that's making your plant sick.

That's why you should make your apple trees as resistant to the problem as possible:


You Bet Your Garden   Question of the Week ©2005 Mike McGrath

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