Apple Scab, Rust and Frog-eye Leaf Spot- Which is What?

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As Jim Chatfield and I were out in the “Crablandia” plot today, we noticed the onset of multiple foliar diseases.  Many tree lovers at this time of year start to panic as some of the crabapple leaves begin to turn yellow and drop.  They are often frustrated because they were told that the crabapple they chose was scab resistant, but it still gets spots, the leaves turn yellow and then they fall off.  That’s the Apple Scab fungus right… Well, maybe so and maybe not!

All of these fungal diseases cause spots as a result of their infections of leaf tissue and all of them result in the leaves ultimately turning yellow and dropping off!  And one leaf may have all three of these diseases present at the same time too.

The Apple Scab fungus causes the olive-gray to green lesions to form on the leaf.

Apple Scab fungus on 'Canary' crabapple
Apple scab fungus on crabapple but note the three spots (lesions) with purple borders which are the frog-eye leaf fungus

Frog-eye leaf spot causes circular lesions with purple borders to form initially and then the border often fades to a slightly darker brown than the rest of the lesion.

Frog-eye Leaf Spot on 'FoxFire' crabapple
Frog-Eye leaf spot fungal lesions on 'FoxFire' crabapple leaves

Rust lesions often appear as orange to reddish-orange spots on the crabapple leaves.

Rust lesions on 'Brandywine' crabapple leaves
Bright orange and reddish-orange Rust fungus lesions on 'Brandywine' crabapple leaves

Now that you can see the differences, go out and practice on your trees!