Crabs, Scab and then So Sad... Drab!

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On the BYGL conference call, I shared that I am amazed at how the foliage of the greatest landscape small tree, the breathtaking crabapple, has remained relatively clean here in NE Ohio.  I was expecting with all of the rain this year, that we would quickly see what we Crabarians affectionately term “year of the scab dog”.  This “scab dog” effect is due to the fungal pathogen (Venturia inaequalis) causing extensive apple scab lesions on susceptible crabapple tree leaves.  Given our perfect environmental conditions for this fungal disease, I expected it to quickly overwhelm and wreak havoc on crabapples and the genus Malus, which includes eating apples.  Infected leaves turn yellow or other fall-type colors, then begin dropping to the ground like rain, resulting in unmistakable tree nudity, thereby rudely creating the “scab dog” tree.


Apple Scab fungus caused defoliation
Classic example of  a "scab dog" crabapple almost completely defoliated by apple scab


Our southern correspondent, Joe Boggs, again exclaimed his dismay “on how could this be possible when trees he was seeing, were literally covered in scab”.  I quickly retorted that it was due to the Northern part of Ohio is just more elite plant-wise!!  Apple scab lesions are a fuzzy, drab, olive gray-green in color and range from distinct lesions to covering the entire leaf surface.


Apple Scab on Crabapple


While I was elated there did not appear to be any apple scab lesions yet, I am confident that it is only a matter of time before symptoms begin to show up.  In fact, Jim Chatfield was quiet and when asked why so quiet, he said he was waiting for some crazy statement like “maybe there aren’t any infections!”  Jim reminded us about latent infections, which are infections that have already occurred in the leaf, but are as of yet, symptomless!


Apple Scab lesions on crabapple leaf
Leaf distortion and necrosis caused by Apple Scab fungus


Joe reminded us there are other diseases that can also cause crabapple leaves to turn yellow and fall off.  The disease that comes to mind is one that can also create some confusion because it can affect apple scab resistant crabapples!  Frogeye leaf spot, caused by the fungal pathogen, Botryosphaeria obtusa, is one that closely mimics apple scab symptoms.  The main difference is the appearance of the lesions on the leaf.  Frogeye leaf spots are small circular lesions with a distinct purple border and the interior of that lesion is beige in color, creating its namesake eye spot appearance.


Frogeye leaf spot on crabapple
Frogeye leaf spot (Botryosphaeria obtusa) on crabapple


Apple Scab and Frogeye Leaf Spot lesions



Apple Scab and Frogeye Leaf Spot lesions on Crabapple


Next time you notice leaves falling off your lovely crabapple, look closely at any lesions on the leaves will help you to determine which fungus might be causing the problem on your tree.