The world according to Riff.

CEDAR-APPLE RUST Gymnosporangium juniperi-virginianae   F A C T S H E E T

Introduction

Cedar-apple rust is just one of several similar fungal diseases which could be broadly classified as Juniper-Rosaceous rusts. All of these rusts have very similar disease cycles but differ in exactly which juniper and rosaceous species they infect. The fungus spends part of its life cycle on a juniper host and part on a host in the rose family.  It requires both hosts to complete its life cycle.  All of these rust diseases are caused by species in the genus Gymnosporangium.  Cedar-apple rust is caused by the fungus known as Gymnosporangium juniperi-virginianae.  Two other common juniper-rosaceous rusts are hawthorn rust and quince rust, although there are many more.

Examples of juniper hosts include eastern red cedar, southern red cedar, Rocky Mountain juniper, some prostrate junipers, and Chinese juniper. Examples of rosaceous hosts are apple, crabapple, hawthorn, quince, serviceberry, and pear. Some commercial apple varieties are highly susceptible to cedar-apple rust with both direct fruit infection and defoliation of infected leaves.

Symptoms

Symptoms on the juniper hosts can include a swelling of the woody tissue and gall-like growth that may enlarge over time. Characteristic symptoms of a rust infection include bright orange colored growths emerging from the swollen woody tissue or galls. On leaves, lesions may be found on the surface or underside of the leaves depending on the spore stage present. These lesions often also have that bright orange coloration which is very distinct.

Figure 1: Juniper with Cedar-Apple Rust displaying the gelatinous teliospores. (provided by Dr. George Hudler, Cornell University) Figure 2: Apple leaves displaying bright yellow leaf lesions caused by the aeciospore stage of the rust. (provided by Dawn Dailey O'Brien, Cornell University)

Disease Cycle

Cedar-Apple Rust disease cycle.
(provided by George Agrios, Plant Pathology 3rd edition, APS Press)

Rusts spend the winter on their juniper hosts as stem galls up to 5 cm in diameter. Galls can be perennial and survive from year to year. In the spring, bright, orange, gelatinous, horn-like growths 1-2 cm long emerge from the galls (Fig. 1). These orange structures are called telia and they produce bright orange spores, called teliospores, which spread by wind to infect the rosaceous host. On the rosaceous host, the teliospores infect leaves, fruit, and succulent stem tissue and produce bright yellow or orange colored lesions (Fig. 2).

Figure 3: Aeciospores maturing on hawthorn fruit. (provided by Dawn Dailey O'Brien, Cornell University) Figure 4: Juniper with Cedar-Apple Rust displaying the gall-shaped winter survival structure. (provided by Dr. George Hudler, Cornell University)

From these yellow-orange lesions more spore producing structures called aecia develop in mid to late summer. The aecia are papery tubes or cups up to approximately 4 mm in length. The coloring can vary from white, light pink, or orange, to yellow. On leaves the aecia develop on the underside of the leaf. On fruit they develop anywhere on the surface (Fig. 3). Inside these aecia are spores called aeciospores which are powdery in texture and red-brown, bright orange or orange-yellow in color. During dry weather in mid to late summer, the aeciospores are spread by wind and cause infections on nearby juniper hosts. Galls form on the juniper host where the pathogen survives the winter, and the cycle starts over (Fig. 4). In addition to galls, the pathogen can cause dieback in the juniper host.

Management Strategies

In severe cases these rusts can be controlled with fungicides, however registered fungicides will vary by the type of plant(s) to be treated and their use or site. Be certain any formulation(s) of pesticide(s) you purchase are registered for the intended use. In New York State, some myclobutanil products are labelled to treat fruit trees in the home orchard. Several pesticides are registered to treat different ornamental plants in the home landscape and include Kop-R-Spray and some products containing the active ingredients: chlorothalonil, mancozeb, propiconazole, or myclobutanil. Follow label directions for treatment, and be certain the product used is registered for the correct host and site. For information on additional pesticides registered to treat different Gymnosporangium rusts on specific host plants, obtain the latest copy of "Pest Management Around the Home" Part II, Miscellaneous Bulletin S74 (available through Cornell Cooperative Extension).

Additional products may be available for use in commercial plant production. Commercial applicators should refer to the appropriate pest management guidelines for more information.

In addition, there are resistant varieties of juniper, apple, crabapple, and hawthorn available for use in new plantings or when replacing severely diseased specimens. There is potential for some control by eliminating nearby juniper hosts, however, this is not really practical because the spores can travel several miles by wind to infect the alternate hosts.