Thanks for your question.

Typically leaf spot fungal diseases in catalpa are caused by species in the genus Cerecospora or Phyllosticta. Either disease leads to leaf spots manifested by discolored areas on the leaves. These areas eventually turn into holes as the infected tissue dies and falls out.  Generally these consequences pose no serious threat to the tree and chemical treatment is not usually employed.  This seems to be most likely in your case.

Anthracnose is another fungal disease of found in catalpa. It can cause irregular spots that merge together, leading to large dead areas that may fall out, creating holes. This seems less likely for your catalpa.

Generally when dealing with these fungal diseases, the following procedures should be done:

1). Removing and destroying all fallen leaves reduces the potential for future infection. These leaves can be a spore overwintering site and the source of some spores for early spring infections.

2). Infected twigs and branch cankers, a source of spores, should be pruned out.

3). All diseased plant parts should be buried, burned, or removed from the site to prevent reinfection.

Also in catalpa, leaf spotting can be caused by various bacteria. These spots often start as water-soaked lesions that can turn brown or black. Severe infections can result in leaf tissue dying and falling out, leaving holes. I also consider this to be less likely in your case.

See the following for further information on these and related points:

https://cropwatch.unl.edu/plantdisease/sugarbeet/cercospora-leaf-spot

https://plantpath.caes.uga.edu/content/dam/caes-subsite/plant-pathology/extension-pdfs/Extension-Fungi-Cercospora v2.pdf

https://plantdiseasehandbook.tamu.edu/landscaping/trees/catalpa/ - :~:text=Leaf Spot (fungi – Cercospora sp,Chemical control usually not necessary.

https://extension.psu.edu/catalpa-diseases

https://www.picturethisai.com/disease/Catalpa-speciosa-Dark-spots.html

https://www.maine.gov/dacf/php/gotpests/diseases/factsheets/anthracnose-purdue.pdf

Distinguishing among these different types of diseases cannot be done via pictures. An onsite inspection and assessment should be done by a licensed, professional arborist. Trying to locate such a person can be challenging. I suggest that you contact the Michigan State University Extension Office for suggestions:

https://www.canr.msu.edu/outreach/

Finally, larvae of the Catalpa Sphinx Moth (Ceratomia catalpa) feed on catalpa leaves, creating large holes and significant defoliation. These caterpillars are a primary pest for catalpa trees. I don’t think that this is causing your situation. I mention it only for the sake of completeness. See:

https://extension.psu.edu/catalpa-worm-catalpa-sphinx-moth

https://www.butterfliesandmoths.org/species/Ceratomia-catalpae

Here’s the bottom line. From pictures alone, the exact causes of this leaf spotting and holes cannot be positively identified. This is why an arborist must be consulted. The good news is that your catalpa tree should not be adversely affected by what is going on.

Good luck. Thanks for reaching out to us and using our forum.