October is National Apple Month and also host to my favorite holiday of the year, Halloween.  (Does the latter really surprise you given my my moniker, Dr. Death?)  So, in honor of both events, I decided that for this month’s web article, I’d talk a bit about diseases that can adversely affect apple fruits and make them less than optimal for use in making pies and seasonal treats like caramel apples.  The pathogens involved in these diseases do not produce compounds toxic to humans, so technically the infected apples are not poison per se.  However, the damage these pathogens cause often makes apple fruits unusable.

Apple Scab

This disease is the most common that I see causing issues on apple fruits.  The causal fungus (Venturia inaequalis) commonly infects apples leaves, often leading to defoliation in susceptible apple varieties.  Once leaf infections occur, the fungus can eventually infect fruits leading to symptoms that range from blackish surface blemishes to fruit growth distortions (if infections occur early in fruit development).  Of the apple fruit diseases, apple scab is the most cosmetic in my mind.  Fruits are often usable if peeled.  The downside to the disease is that consumers expect perfect, perfect fruit and are not accepting of fruits with apple scab lesions.  Also, scab-blemished fruits typically do not store as well as unblemished fruits.  For this reason, commercial apple growers spend a lot of time, effort, and money spraying apple trees with fungicides to prevent this disease.

Gymnosporangium Rusts

The fungi involved in these diseases (which include cedar-apple rust, cedar-hawthorn rust, and cedar-quince rust) must infect both woody rosaceous plants (most commonly, apples and hawthorns), and junipers to complete their life cycles.  Red cedar (Juniperus virginiana) is a particularly susceptible juniper host.  I most commonly see Gymnosporangium rust symptoms on leaves, where bright yellow to orange spots (about the size of a dime or nickel) form.  But, fruit infections on hawthorn (where you will see spiny, salmon-colored fruit) are also very common.  I have seen fewer apple fruit infections, but they do occur, and the lesions tend to have a spiny appearance.  Infected fruits are often frightening and fascinating at the same time.

Fire Blight

2023 was a banner year for fire blight in my estimation.  I diagnosed more fire blight this year than any other year in my 25 years at the PDDC.  The bacterium that causes the disease (Erwinia amylovora) commonly gains entry into trees through flowers.  Honeybees pick up the bacterium as they feed on oozing sap from infected branches.  The bees then drop the pathogen off as they pollinate.  Erwinia amylovora can rapidly colonize branches leading to significant dieback, and even tree death.  Interestingly this year, I received samples of apple fruits (not full developed, but well on their way) with blotchy, rotted areas that were quite sticky.  These areas tested positive for Erwinia amylovora.  This was a first for me.  I had never seen fireblight on apple fruits before.

Black Rot

I don’t see this fungal fruit disease often, but I commonly see the pathogen involved (a Sphaeropsis species) cause cankers and dieback on apple branches.  Often, when clients think they have a fire blight problem, the real problem is the canker phase of black rot.  The black rot pathogen also causes frogeye leaf spot on apple leaves.  Interestingly, the pathogen does not sporulate well on leaves, but it sporulates profusely on branches and fruits.  Fruit symptoms are often large, roughly circular, dark areas of softened tissue.  If you look closely within the decayed tissue, you may notice a series of concentric rings showing how the fungus has colonized the fruit.  You may also notice tiny, pimple-like structures embedded in the rotten tissue.  These are the fruiting bodies (i.e., reproductive structures) of the pathogen, filled with relatively large, oblong, brown spores.

Bitter Rot

Bitter rot is caused by fungi in the genus Colletotrichum (typically Colletotrichum gloeosporioides or Colletotrichum acutatum).  This genus contains many fungi that cause anthracnose leaf diseases.  On apple, the bitter rot pathogens are primarily fruit pathogens (leaf symptoms are only rarely observed), leading to symptoms that look somewhat similar to those observed with black rot.  Bitter rot lesions are often large, roughly circular, and somewhat tannish to brown in color.  Concentric rings are typically quite obvious with the diseased tissue, and pimple-like fruiting bodies abound.  Individual spores of the bitter rot pathogens are smaller than those of the black rot pathogen, oblong and colorless.  En masse on the surfaces of fruits however, these spores can have a pink, salmon, or orange color.

Management of apple fruit diseases varies from disease to disease, but in general involves:

Cleaning up leaf debris and rotted fruits

Removal of these materials eliminates overwintering sites for many of the disease-causng organisms discussed above.  Leaf debris removal is particularly important for management of apple scab, but not particularly important for the Gymnosporangium rusts (because the spores that infect apple trees come from junipers).  Dispose of leaf and fruit debris by burning (where allowed), burying, or hot composting.

Pruning routinely

Pruning removes infected branches that can serve as a source of pathogens (e.g, those that cause fire blight, black rot, and bitter rot).  In addition, routine pruning of a healthy tree opens the canopy and promotes drier conditions that are less favorable for infections to occur.  For branches with probable fungal infections, pruning approximately four to six below obviously dead tissue will likely be adequate to remove pathogens.  If fire blight is of concern however, more aggressive pruning, 12 inches below obviously diseased branch tissue, is needed.  Be sure to prune only when it is dry, and be sure to disinfest your pruning tools between cuts by dipping them in bleach diluted to a final concentation of 0.5% sodium hypochlorite (the active ingredient) or (even better) 70% alcohol (e.g., rubbing alcohol).  Spray disinfectants can be used as a source of alcohol as well.  Just be sure to check the ingredient list of the disinfectant that you select to make sure it contains roughly 70% alcohol.  If you use bleach, be sure to thoroughly rinse and oil tools after you are done pruning to prevent rusting that can be caused by bleach use.  Dispose of branches by burning (where allowed) or burying them.

Eliminating alternate hosts

This technique is most important for managing Gymnospoangium rusts.  Removing junipers near apple trees will help break the life cycle of Gymnosporangium fungi and substantially reduce or even elminate these rust diseases.  Unfortunately removing junipers may not be practical.  I have seen some recommendations for commercial apple producers that advise removing junipers within a two mile radius of apple producing areas!  This would not be possible in a home gardening setting.  For apple scab, removing susceptible crabapple trees may be helpful in controling scab on fruit bearing trees by eliminating a potential source of fungal spores.

Planting resistant varieties

When buying new apple trees, I always recommend that home fruit growers select varieties that have resistance to both apple scab and fire blight.  These two diseases are typically the most problematic for home gardeners.

Using fungicide treatments

I recommend this option only as a last resort and only if you have had your apple disease problems formally diagnosed.  By knowing which apple diseases you commonly encounter, you can develop a fungicide treatment regime tailored to your specific situation.

Now that I’ve spent the afternoon writing about apple fruit diseases, my tummy is rumbling.  It’s time for me to track down a caramel apple.  As always, if you have questions about plant diseases, don’t hesitate to contact me at pddc@wisc.edu or (608) 262-2863.  Enjoy the last colorful days of autumn, everyone!