Hello Gabrielle,

Thank you for your question.  To narrow down the root cause without paying for an arborist's diagnostic visit, you can look closely at the texture of the sap and the wood inside the cavity.  Generally what you describe is a symptom rather than a specific disease.  The oozing is the tree’s immune system response to try and resolve a problem.  

There are a few possibilities:

1.  Cytospora Canker: Canker is a fungal infection that enters through wounds (like a broken branch, frost cracks, or a lawnmower scrape). However, canker typically produces sunken, dark, oval lesions on the bark, and the wood underneath turns soft, brown, and foul-smelling. Since your exposed cavity wood feels hard, a deep, active fungal canker rotting the core is less likely, though fungal pathogens can still take advantage of borer entry wounds.  This also typically would happen in the spring, not in September.  

2.  Peachtree Borers: If the gummosis occurred specifically at or just above the soil line, peachtree borers (Synanthedon exitiosa) are the primary suspect. Look closely at the area where the gum oozed. If the jelly contains fine, sawdust-like wood shavings (called frass), it is a definitive sign of borers. The larvae chew through the inner bark right at the base, causing the tree to ooze sap to try and drown them out.  They also tend to attack trees that already have injuries or previous trunk wounds.   

3.  Lastly, physical damage or weight stress: Peach trees are brittle. The combination of a broken heavy branch this summer, professional pruning cuts, and urban stress can trigger general gummosis. The tree may simply be bleeding sap from structural stress from having produced strong yields of fruit over the past several years.  

If you suspect borers, take a thin, flexible piece of wire (like a guitar string or a long wire tie) and gently poke it into any visible entry holes around the base. Gently fish around to mechanically kill the burrowing larvae without further slicing the bark.  Pulling back the mulch was a good thing to do.  Mulch should never touch the bark of a fruit tree.   Keep it three to four inches away from the trunk of the tree to help prevent spread of things like fungi as well as borers.  Both love damp conditions.  

Because you said it has had leaf curl in the past, I would buy a bottle of Liquid Copper Fungicide or Lime-Sulfur.   Apply a thorough spray to the entire trunk, cavity, and branches after all leaves have dropped. This kills overwintering fungal spores harboring in the bark crevices and the open cavity.  Spray the tree a second time in late winter or early spring before the green leaf buds begin To open.  Spraying after they open will be ineffective for leaf curl.  

You also may consider manually thinning the fruit in May or early June to prevent mechanical stress on the branches.  You can pick off green peaches so the remaining fruit is 6 to 8 inches apart.  The remaining peaches will also be juicier and sweeter too.

I hope this helps.  

Doug Kilburg