The trumpet tree (Catalpa bignonioides) - not to be confused with the angel's trumpet (Brugmansia) - is a medium-sized tree cultivated primarily for its white flowers and large, heart-shaped leaves. If the latter turn yellow, this is usually due to incorrect care.
Yellow leaves on the trumpet tree can have various causes
Trumpet tree is sensitive to improper care
Catalpa reacts very sensitively to care errors or insufficient culture conditions. Yellow leaves are not uncommon if the tree is watered and/or fertilized too seldom. Waterlogging or a location that is too shady can also make things difficult for the trumpet tree.
Fungal diseases are rare but possible
Unfortunately, Catalpa bignonioides is also very sensitive to Verticillium wilt, which occurs mainly in very humid and warm weather (e.g. in very rainy summers) and when there is waterlogging. This fungal disease cannot be treated with fungicides, but only symptomatically. To do this, the affected parts of the plant must be cut back far into the healthy wood and carefully removed.
tips
Verticillium wilt is indicated by the tree gradually withering or dying over a few days or weeks. The yellow leaves first appear on one branch and only later in other places.