Cedar Apple Rust
Gymnosporangium juniperi-virginianae Schw.
authored by R.C. Pearson, H.S. Aldwinckle, and R.C. Seem.
Cedar apple rust (CAR) is an important fungal disease of apple in North America east of the Rocky Mountains. It can defoliate trees and blemish fruit making them unmarketable. The CAR fungus requires two hosts, apple and eastern red cedar (Juniperus virginiana L.), to complete its life cycle. Spores produced on apple do not infect apple, but only cedar and spores produced on cedar infect only apple.
The CAR fungus overwinters in spherical galls on cedar trees (Fig. 1). Spring rains cause horn-like structures, called telia, to extrude from galls (Fig. 2). When these horns absorb water, they become jelly-like and swollen. Between rains they dry to dark brown threads (Fig. 3). The telial horns are comprised of thousands of two-celled spores called teliospores. Swelling and drying of telial horns may occur 8-10 times during the season. Each time, the horns push out further and expose more teliospores until the supply is exhausted.
During rains, after the telial horns absorb water, the teliospores germinate to produce a germ tube (basidium) from each cell. Four basidiospores are produced on each basidium (Fig. 4). At optimum temperatures, basidiospores are produced within 4 hours of the horns absorbing water.
Basidiospores are forcibly discharged into the air immediately after being formed. They can be carried long distances. Basidiospores that land on young apple tissue may germinate and infect if a film of water is present for an adequate amount of time (Table 1). One to two weeks after infection, orange pustules (pycnia) containing pycniospores appear on the upper side of leaves (Fig. 5) or on fruit (Fig. 6).
One to two months after the appearance of pyonia, the rust produces other fungal structures,called aecia, on the underside of the leaf (Fig. 7) or on fruit (Fig. 8). The aecia produce aeciospores which are released into the air during dry conditions in late summer. Aeciospores that land on young leaves of cedar may germinate, infect, and cause gall formation. Generally, in the second year after infection, the gall matures and produces teliospores, thereby continuing the disease cycle (Fig. 9). Because most galls produce teliospores for only one season, a new crop of galls is required each year if infection of apples is to occur.
INFECTION OF APPLE
Before apple can be infected, adequate moisture must be present in a temperature range of 8-24 C (46-75 F) to allow for formation of basidiospores on cedar galls. Then, the basidiospores will infect apple when susceptible leaf and fruit tissues are wet for certain lengths of time at specific temperatures (Table 1 ). Leaves are most susceptible to infection when 4-8 days of age, and fruit are susceptible from tight cluster through bloom.
Control strategies for CAR are based on fungicides, removing nearby red cedars, and using resistant varieties. Table 2 presents four categories of resistance for 44 cultivars. CAR can be minimized on susceptible cultivars if red cedars are eliminated from their vicinity. Where susceptible cultivars are grown in proximity to red cedars, a fungicide program should be followed from tight cluster through first cover. Use chemicals on a schedule recommended by the local extension service.