Apples and flowering crab apples are susceptible to several rust diseases, including cedar-apple rust, quince rust and hawthorn rust. Although incited by different species of fungi in the genus Gymnosporangium, they have in common the fact that they must spend part of their life cycle on various trees and shrubs of the Juniperus species, such as Eastern red cedar and common juniper.
All of the rust diseases can result in serious losses if environmental conditions are favorable for disease development. Since they are similar in appearance and life cycles, only cedar-apple rust will be discussed in detail.
Symptoms
Symptoms of cedar-apple rust are found on apples and Juniper spp. On apples, rust occurs on leaves and fruit and, less commonly, on bark tissue. Small yellow spots appear on the upper leaf surface usually within a week or two after bloom. The spots enlarge to one-quarter inch or more, and orange droplets appear in the center. Eventually, small black fruiting bodies are produced by the fungus in the spots. Later in the summer, another type of fruiting structure containing spores of the fungus is produced on the underside of the leaf. These fruiting structures appear as a series of yellowish tubes called aecia. Some infected leaves may drop from the tree, but complete defoliation rarely occurs.
Fruit infections usually occur near the calyx end and are similar in color to the leaf infections but may become much larger–one-half inch or more in diameter. Black fruiting bodies and, occasionally, aecia develop on the fruit lesion.
On the small branches of Juniperus spp., the fungus causes brown to reddish-brown swellings or galls one-quarter inch to two inches in diameter. These galls, commonly called cedar apples, take two years to mature. During rainy periods in May, orange gelatinous tendrils or spore horns, develop on the gall. These may extend from the gall as much as two inches. Each gall may have more than one hundred spore horns. During dry periods in May and early June, the spore horns lose their gelatinous consistency and appear inactive. However, they regain their appearance and function with the return of wet conditions. Sometime in late June, the spore horns dry up and fall off. The gall persists on the tree, but does not function again. Severely infected trees may contain hundreds of individual galls.
Disease Cycle
Spores produced on spore horns on Juniperus spp. in the spring are very lightweight and easily carried by wind to apple leaves and fruit. Billions of spores can be produced on a single spore horn. Release of spores occurs during rainy periods. In Connecticut, spores are released until about the middle of June.
If a spore lands on a susceptible apple leaf and environmental conditions are favorable, infection can occur in as little as four hours. In one to three weeks, rust lesions appear on the apple leaves. Spores produced in aecia on apple leaves are wind disseminated back to leaves of Juniperus spp. The spores germinate, penetrate the leaves and cause galls. The galls produce spores the second spring after infection occurs, completing the two-year life cycle of the fungus.
Control
Removal of Juniperus spp. from the vicinity of apple orchards has been suggested as a means of control. Since spores produced on the cedars can be wind disseminated several miles, tree removal cannot be expected to give complete control. However, removing Juniperus spp. in the immediate vicinity of orchards can reduce the severity of disease.
Apple varieties differ greatly in their resistance to rust. Following is a list of varieties of apples that were grown in the variety test block at The University of Connecticut’s Experimental Orchard located at Storrs, Connecticut and their relative resistance to rust.
| Resistant or Immune | Intermediate | Susceptible |
| Carroll | Barry | Burgundy |
| Delicious | Earliblaze | Esopus Spitzenburg |
| Empire | Golden Delicious | English Russet |
| Holly | Magnolia Gold | Gala |
| Honeygold | Razor Russet | Idared |
| Jonamac | Spigold | Jonee |
| Jerseymac | Stark Bounty | Jono |
| Lobo | Winesap | July Red |
| Mollies Delicious | Jonagold | |
| Macoun | Lura Red Monroe | |
| McIntosh | Ozark Gold | |
| Niagra | Prima | |
| Priscilla | Quinte | |
| Paulared | Rome | |
| Raritan | Roanoke | |
| Spencer | Red Baron | |
| Scotia | Roxbury Russet | |
| Stayman | Summerred | |
| Sungold | Stark Jumbo | |
| Senandoa | Vista Bella |
Native flowering crab apples such as wild sweet crab, Malus coronaria and prairie crab, M.ioensis plus its cultivars, fringe petal crab, M.ioensis ‘Fimbriata’ and bechtel crab, M. ionesis ‘Plena’ should be avoided if rust is serious in a particular location. Some cultivars of crab apple resistant to rust are: Malus cv.‘Adams’, M. floribunda, M. hypehensis, M. cv. ‘Mary Potter’, M. sargentii, M. cv. ‘Snowdrift’, M. cv.‘Winter Gold.’
Rust resistant varieties of hawthorn such as Cockspur Hawthorn and Washington Hawthorn are available.
Several fungicides can be used to protect apples from cedar-apple rust. Contact your local Cooperative Extension center for up-to-date fungicide information and timing.
On Juniperus spp. chemical control of rust is seldom necessary. Hand picking of the galls before the spore horns emerge is a common practice. Juniperus spp. infected with cedar-apple rust will not die from the disease.
Prepared by: David B. Schroeder, Plant Pathologist
Revised by: Edmond L. Marrotte, Consumer Horticulturist, Department of Plant Science
Updated by: Mary Concklin, UConn IPM. 2012
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