June 2020: Cherries and Peaches and Plums, Oh My!

FruitThese days, digital photos of diseased plants are arriving fast and furious in the Plant Disease Diagnostics Clinic (PDDC) email inbox.  While clients have been having problems with many different types of plants, I have been receiving a large number of photos of stone fruits.  Ornamental and fruit-bearing varieties of cherries, plums and peaches seem to be having a rather tough year this year.  Several of the diseases that adversely affect stone fruits are fairly straightforward to diagnose by photo.  Below are the common diseases of cherries, plums, and peaches that I have been seeing thus far this season.

Peach Leaf curlPEACH LEAF CURLThe name of this fungal disease is quite descriptive.  Infected peach leaves become curled and puckered, and often have a combination of a green, creamy-white and fuchsia color.  Peach leaf curl seems relatively cosmetic, but repeated occurrences of the disease over time can reduce the quantity and quality of fruit.  Typically with leaf diseases, I recommend good fall clean up of leaves for management.  Unfortunately, this strategy does not work for pearch leaf curl, as the pathogen (Taphrina deformans) overwinters on peach branches.  Management of the disease relies of use of fungicides (e.g., copper-containing products) applied either after leaf drop in the fall or prior to bud swell in the spring.

Peach Leaf curlBLACK KNOTThis disease, which is specific to plants in the genus Prunus (e.g., cherries and plums), is what I affectionately refer to as “poop-on-a-stick”.  It really does look as though some pesky animal has defecated on the branches of affected trees and shrubs.  The fungal pathogen involved (Apiosporina morbosa) induces formation of black, gnarly swollen areas (called galls or knots) on infected branches.

Unfortunately, once the knots form, the only method of management is to remove the growths by pruning.  For fungal diseases, I typically recommend pruning roughly four to six inches below the diseased area.  When pruning, be sure to decontaminate tools between cuts by treating them for at least 30 seconds with 70% alcohol (e.g., straight rubbing alcohol), a commercial disinfectant or 10% bleach.  Spray disinfectants work as well (as long as they contain roughly 70% active ingredient).  Just spray tools until they drip and then allow them to air dry.  When using bleach, be sure to rinse tools completely after pruning and then oil them to prevent rusting.  Dispose of black knot galls by burning (where allowed) or burying them.  In some situations, there will be so many galls in a tree that my recommendation is what I call “basal pruning” or “a single pruning cut at the ground level”.  You remove the affected trees and replace it with non-susceptible plants.

Bacterial CankerBACTERIAL CANKERProbably the most serious of the diseases that I have seen on stone fruits this year is this one.  The pathogens involved (two variants, called pathovars, of the bacterium Pseudomonas syrinage) infect branches causing branch dieback.  From infected areas, sap emerges and gelatinizes on branch surfaces.  For bacterial canker, timely pruning of diseased branches is critical for management, as the pathogens can rapidly colonize infected branches and move into the main trunks of trees where they can girdle the trunks, killing the trees.

Prune at least 12 inches below visible dieback on affected branches and again dispose of branches by burning (where allowed) or burying them.  Decontaminate tools as described above for black knot.  When bacterial canker occurs in main trunks, tree removal and replacement is the only real option.

NEED HELP?

If you need help diagnosing plant diseases, feel free to contact the PDDC.  For the PDDC’s current policy on sample submission, including submission of digital photos, check out the following link.  As always, be sure to check out the PDDC website for timely information on plant diseases.  Also, feel free to follow the clinic on Twitter or Facebook (@UWPDDC) to receive timely PDDC updates.  Or alternately, put in a request to subscribe to the clinic’s new listserv (UWPDDCLearn) by emailing pddc@wisc.edu.

Hang in there, be safe, and stay healthy everyone!