The symptoms you describe in your northern catalpa (Catalpa speciosa)—brown spots forming on the leaves, spreading, and then causing the spots to fall out—suggest a potential leaf spot disease. Given the recent wet weather, it is plausible that a fungal or bacterial infection could be at play, as these conditions often favor the development of such diseases.

Potential Causes:

1. Fungal Leaf Spot Diseases: These include diseases caused by fungi like Cercospora or Phyllosticta species, which can thrive in wet conditions and lead to the symptoms you're observing.
2. Bacterial Leaf Spot: Bacteria such as Pseudomonas or Xanthomonas can cause leaf spot diseases, especially in humid and wet conditions.

Steps to Diagnose and Manage the Issue:

Diagnosis:

1. Examine the Spots:

   
   

   • Fungal spots often have distinct margins and may exhibit a variety of colors, including brown, black, or purplish.
   • Bacterial spots may appear more water-soaked or greasy and can have a yellow halo around them.
2. Leaf Tissue Testing:

   
   

   • If possible, take a sample of an affected leaf to a local extension service or plant pathology lab for precise identification of the pathogen.

Management:

1. Cultural Practices:

   
   

   • Improve Air Circulation: Prune nearby plants and avoid overcrowding to increase air flow around the catalpa.
   • Water Management: Water the tree at the base, avoiding overhead irrigation which can keep the foliage wet and promote disease.
   • Clean Up Debris: Remove and destroy fallen leaves and any other plant debris to reduce the presence of infectious spores in the area.
2. Chemical Treatments:

   
   

   • Fungicides: Continue using fungicides, but make sure you are using the right type for the specific pathogen. Copper-based fungicides are effective against both fungal and bacterial leaf spots. Follow the label instructions carefully.
   • Timing: Apply fungicides preventatively or at the first sign of disease. Reapply according to the product’s instructions, especially after rain.
3. Soil pH and Health:

   
   

   • While the soil pH of 7.5 is slightly alkaline, catalpas can generally tolerate a wide range of pH levels. Ensure the soil is well-draining and amend with organic matter if necessary to improve soil health.

Additional Tips:

• Monitor Weather Conditions: Be especially vigilant during wet periods. Consider using protective fungicides ahead of forecasted wet weather.
• Healthy Practices: Ensure the tree is not stressed by providing adequate water during dry periods and avoiding mechanical damage to the roots and trunk.

By combining proper cultural practices with targeted chemical treatments and accurate diagnosis, you should be able to manage the leaf spot issue on your northern catalpa effectively. If the problem persists, consulting with a local arborist or extension service can provide further tailored advice.

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