Editor’s note: Get Growing, written by the La Plata County Extension Office’s Master Gardener Program, appears during the growing season. It features timely tips and suggestions for your garden and landscape.By Darrin Parmenter
All it took was some rain, and the phone calls started to pour in on our “what is this orange stuff in my juniper tree?” fungal challenge.
Juniper-hawthorn rust, or cedar-apple rust, is caused by the fungus Gymnosporangium. What makes it somewhat unique is that it requires two different hosts to complete its life cycle: junipers and the alternative host, which could be apples, crab apples, hawthorns or mountain ash trees.
Symptoms on the primary host, juniper, look very different from symptoms on the alternate host, with infections typically more harmful to the alternate host than to juniper. Repeated infections over several years can cause reduced fruit size on apple trees and premature defoliation of hawthorn, apple, crabapple or mountain ash. Defoliated trees may suffer winter injury and often fail to set fruit the next season.
On junipers, you typically see these 2-inch, orange balls after a period of rain. These are galls (abnormal growth of plant tissues caused by the stimulus of a disease or insect) that are full of fungal spores. These galls are gross – gelatinous, sticky and covered in little “horns.” If you see them, remove them because when they explode, billions of spores are ejected and transported by the wind up to a few hundred meters.
If they land on the alternative host, they can cause leaf spots that are bright orange-yellow in color surrounded by a red halo. If you flip the leaf over, you may see little hair-like projections that will eventually form spores of their own that can make it back to the juniper, thus, completing the disease’s life cycle.
All in all, the entire life cycle of this disease takes two years, 18 to 20 months on juniper and four to six months on the alternate host. It cannot survive in the absence of one of the hosts.
Design landscapes so that susceptible junipers are separated as far as possible from alternate hosts. Where practical, locate the house or a dense hedge between junipers and alternate hosts. Remove juniper galls in late winter or early spring by pruning them out.
Darrin Parmenter is the director and horticulture agent of the La Plata County Extension Office. Reach him at firstname.lastname@example.org or 382-6464.Darrin Parmenter