What is causing odd growths on the leaves of my apple tree?

August 31, 2021 Fruit Trees and Small Fruits, Garden Design and Maintenance, Managing Insect, Diseases, and Animal Pests

Question:

As I was inspecting/removing aphids from our young apple trees, I noticed an odd growth on a few of the leaves of one of the trees (a native bred Black Oxford from our local orchard and nursery). My inclination is to cut off any affected leaf to keep this odd growth from spreading, but before doing so can you identify it?

: Top of leaf

: Close up of underside

: Underside of leaf

Answer:

Rebecca Long, Agriculture and Food Systems Professional

The leaves are showing signs of Cedar Apple Rust. Interestingly, Black Oxford is listed as a resistant variety but disease resistance is never 100%, which could explain why you are seeing symptoms on your tree (plus our unusually humid wet summer). Your plan to remove infected leaves is a good one. Since the fungus also requires a Juniperus species to complete its life cycle, check any nearby (like eastern redcedar or Rocky Mountain juniper) and remove galls from those as well in the fall. It’s unlikely you’ll need to resort to using fungicides, but it’s always a good idea to keep up on pruning to maximize airflow and light penetration in your tree canopy, which will also help reduce fungal diseases.