Gail Ruhl, Sr. Plant Disease Diagnostician, Purdue University
Cedar-apple rust is caused by a fungal pathogen called Gymnosporangium juniperi-virginianae. This fungus attacks crabapples and apples (Malus sp.) and eastern red cedar (Juniperus virginianae). In order to survive, many of the rust fungi must "move" from one type of host to another; in the case of cedar-apple rust; from juniper to apple/crabapple.
On a recent walk in the field behind our house I noticed the new, brown, shiny, smooth, cedar-apple rust galls ‘adorning’ an Eastern Red Cedar (Juniperus virginianae). (Figures 1&2) I have watched this tree grow and mature over the years and visit it every spring to collect rust galls I use when I teach about the wonders of fungi. The presence of susceptible apple trees just a few feet away guarantees my source of teaching material.
The take home message at this time of year is that if you remember noticing yellow/orange spots with a red border on the leaves of your apples/crabapples last summer (Fig 3) and also enjoy the presence of junipers in the neighboring landscape, you can check those trees now for the presence of new, shiny, brown rust galls and can help manage this fungal disease by picking off the young galls from nearby cedars. Granted, you will not completely eliminate the disease with gall removal since spores can travel long distances by wind from other sources, however, you will help reduce the overall disease pressure.