Cedar-apple rust and Quince rust
Excerpt from Publication 310, Integrated Pest Management
Table of Contents
Cedar-apple rust, caused by Gymnosporangium juniperi-virginianae, and quince rust, caused by Gymnosporangium clavipes, are two distinct rust diseases caused by different but related species of fungi. They have complex life cycles that take two years, and two different hosts, to complete.
In Ontario, rust diseases of apple are common only in the Ottawa Valley and the east end of Lake Ontario (Quinte area), where red cedar grows abundantly. Occasionally, rust diseases appear in other regions of Ontario where infected ornamental juniper are present and weather is conducive to infection. White cedar (Thuja occidentalis) is not an alternate host of rust diseases.
Like many rust diseases, two alternate hosts - eastern red cedar (winter host) and apple (summer host) - are required to perpetuate the disease from year to year. If not managed, the disease is particularly damaging on apple resulting in complete defoliation and crop loss.
Eastern red cedar (Juniperus virginianae) is the alternate host for cedar-apple rust. However, the symptoms on eastern red cedar look completely different than on apple. Cedar-apple rust infects both the leaves and fruit of susceptible apple cultivars.
Symptoms begin to appear on the upper surface of apple leaves shortly after bloom. Leaves that are three to four weeks old are fairly resistant to infection, but younger leaves and fruit are susceptible, depending on the cultivar. Small, pale yellow spots appear on the upper surface of leaves (Figure. 4-131) and on fruit (Figure 4-132). Eventually, small black spots (spermagonia) appear within the centre of the lesions. The lesions grow larger and more orange-coloured, often with a red margin.
Figure 4-131. Cedar-apple rust symptoms on leaves
Figure 4-132. Cedar-apple rust symptoms on fruit
American hawthorn rust leaf symptoms are very similar to cedar-apple rust lesions on the leaves. This rust disease attacks only the leaves of apple and pear, and affects McIntosh and Cortand apple varieties in particular.
Cedar-apple rust overwinters on its alternate host, red cedar (Juniperus virginanae), or other hosts, as mycelium within rounded, brown-coloured galls 10-30 mm in size (Figure 4-133). Symptoms begin to appear at about the same time apple trees are in the pink stage, after a wet period caused by a rain or heavy dew. Galls on the eastern red cedar produce orange telia horns 10-20 mm long that grow from the galls and become orange-yellow and gelatinous. The telia produce teliaspores on the horns that release windborne basidiospores only capable of infecting susceptible apple leaves and fruit during spring rains (Figure 4-134).
Figure 4-133. Cedar-apple rust gall on juniper
Figure 4-134. Telial horns extending from gall during or shortly after rain
Most basidiospores are discharged in the period from tight cluster of apple through to petal fall. The galls of cedar-apple rust die following the release of the spores, but horns can swell and dry several times, releasing spores during intermittent rains. Spores are discharged as soon as rainfall begins. A short wetting period (compared to scab) of four to six hours at 10°-24°C can result in severe infection, see Table 4-9. Approximate number of hours of leaf wetness required for cedar-apple rust infections on leaves of susceptible cultivars. Generally, long wetting periods around petal fall are particularly damaging. While spores are released during periods of high humidity (>85%), infection only occurs if free water is available. Spores travel a maximum distance of 6-8 km, but most infections occur when alternate hosts are within a few hundred metres. In late summer, cup-like structures appear on the undersides of leaves and on fruit. These structures release spores carried on the wind to red cedars. Spores infect red cedar leaves from mid summer into autumn. Greenish galls are produced the next spring, but these do not mature to release the spores to infect apple until the following spring.
Cedar-apple rust cannot spread from apple to apple or from red cedar to red cedar - the fungus must go through the two-year life cycle, alternating between hosts.
1Source: Aldwinckle, H.S., R.C. Pearson and R.C. Seem. 1980. Infection periods of Gymnoporangium juniperi-virginianae on apple. Phytopathology 70:1070-1073. Assumes cedar-apple rust inoculum orange, (swollen galls) is available at the start of the rain. If inoculum is not already present (dry period prior to rain), add four hours at temperatures above 10°C and six hours at temperatures of 8-10°C. Infection is unlikely at temperatures below 8°C if inoculum is not already present.
Apple cultivars vary in their level of susceptibility to rust diseases
(see Table 4-10).
Estimate the potential for rust fungus infections of apple by examining alternative hosts near the orchard from early May to mid June. Galls producing the orange spore-bearing masses indicate that infection is probable.
Theoretically, it is desirable to remove alternate hosts of rust diseases and wild apple trees from the vicinity of the orchard. In reality, this is usually not practical. Remove and burn rust galls from valuable ornamental junipers, and plant rust-resistant cultivars of ornamental species.
Most of the fungicides registered for scab control also give good control of rust diseases. If disease pressure is light (few rust galls or dry weather), a scab control program using fungicides effective for rust is likely adequate. However, in orchards with a history of rust problems, a protectant fungicide program applied specifically for apple scab control may not be adequate during the period from tight cluster/pink until after petal fall. Scab-resistant cultivars susceptible to rust infection require a specific fungicide program to control these diseases.
Quince rust has a broad range of rosaceous host plants (more than 480 species) including mountain ash, hawthorn, quince and serviceberry. Alternate hosts of quince rust include eastern red cedar, common juniper (J. communis) and several ornamental juniper species. Quince rust infects only fruit, not leaves, of apple and pear.
Young apple fruit is highly susceptible to quince rust infection for about two weeks. Quince rust infects only the fruit of susceptible apple cultivars. The calyx end of infected fruit is greenish brown and distorted (Figure 4-135). Infected apple tissue eventually becomes spongy and brown.
Figure 4-135. Young Red Delicious fruit with quince rust
The life cycle of quince rust is similar to cedar-apple rust. Quince rust overwinters in spindle-shaped enlargements on the twigs and branches of many juniper hosts, including red cedar. These swellings remain a source of infection for several years. Basidia spores are released from an orange gelatinous mass that forms on the galls following periods of rain or high humidity from pink to fruit set stages of apple. Temperatures above 16°C are required for infection to occur. Young apple fruit is highly susceptible to infection for about two weeks.
Quince rust infects only the fruit of susceptible apple cultivars. Spores are not produced from infected apple, and red cedars are reinfected from quince, hawthorn, serviceberry and other rosaceous hosts. Like cedar-apple rust, quince rust has a two-year cycle, alternating between hosts.
See cedar-apple rust above for information
on monitoring and management rust diseases.
For more information: