Cedar-Apple Rust and Quince Rust

Table of Contents

1. Introduction
2. Description
   • Cedar-Apple Rust
   • Quince Rust
3. Monitoring and Management
4. Related Links

Introduction

Cedar-apple rust, Gymnosporangium juniperi-virginianae, and quince rust, Gymnosporangium clavipes, are two distinct diseases caused by related species of fungi. They have complex life cycles that take tow years, and two different hosts, to complete.

Red cedar (Juniperus virginianae) is its alternate host. Cedar-apple rust will infect both the leaves and the fruit of susceptible apple cultivars.

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 red cedar, common juniper (J. communis), and several ornamental juniper species.

Note that white cedar (Thuja occidentalis) is not an alternate host of rust diseases. 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.

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Description

Cedar-Apple Rust

This fungus 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 1). Beginning approximately at the same time that apple trees are in the pink stage, orange jelly-like horns 10-20 mm long grow from the galls and shed billions of spores during spring rains (Figure 2).

Figure 1. Cedar-apple rust gall on juniper.

Figure 2. Telial horns extending from gall during or shortly after rain.

Most are discharged in the period from tight cluster of apple until early June. The galls of cedar-apple rust will die following the release of the spores; however the horns can swell and dry several times, so that spores are released 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 1): generally, long wetting periods around petal fall are particularly damaging. While spores can be released during periods of high humidity (>85%), infection will only occur if free water is available. Spores may travel a maximum distance of six to eight kilometres, but most infections occur when alternate hosts are within a few hundred metres.

Table 1. Approximate number of hours of leaf wetness required for cedar-apple rust infections on leaves of susceptible cultivars.

Average Temperature (°C)	Degree of Infection1
	Light	Severe
3	24	-
4	12	24
6	8	10
8	6	7
10	5	6
12	4	5
14	3	5
16	3	4
18	3	4
20-24	2	4
26+	-	-

 

¹Based on the data of Aldwinckle, Pearson, and Seem, Cornell University. Assumes that 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 4 hours at temperatures above 10°C and 6 hours at temperatures of 8-10°C. Infection is unlikely at temperatures below 8°C if inoculum is not already present.

Leaves three to four weeks old tend to be fairly resistant to infection, but younger leaves and fruit are susceptible, depending on the cultivar. Approximately two weeks after infection, yellow spots appear on the upper surface of leaves and on fruit (Figure 3). The lesions grow larger and more orange-coloured, often with a red margin (Figure 4). Eventually small black spots appear within the centre of the lesions. In late summer cup like structures appear on the undersides of the leaves and on the fruit. These structures release spores that are carried on the wind to red cedars. Spores infect red cedar leaves from midsummer into autumn. Greenish galls are produced the next spring, but these do not mature to release the Malus infective spores until the following spring.

Figure 3. Cedar-apple rust symptoms on leaves.

Figure 4. Fruit symptoms of cedar-apple rust.

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.

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Quince Rust

The life cycle of quince rust in similar to that of cedar-apple rust. Quince rust overwinters in spindle-shaped enlargements on the twigs and branches of red cedar. These swellings remain a source of infection for a period of several years. Spores are released from an orange gelatinous mass following periods of rains of high humidity from pink to fruit set stages of apple. Temperatures above 16° are required for infection to occur. Young apple fruit is highly susceptible to infection for about two weeks.

Quince rust infect only the fruit of susceptible apple cultivars. The calyx end of infected fruit is greenish brown and distorted (Figure 5). The apple tissue eventually becomes spongy and brown. Spores are not produced from infected apple; red cedars are reinfected from quince, hawthorn, serviceberry, and other rosaceous hosts. Like cedar-apple rust, quince rust must go through the two-year cycle, alternating between hosts.

Figure 5. Young Red Delicious fruit with quince rust.

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Monitoring and Management

Apple cultivars vary in their level of susceptibility to rust diseases (see Table 2).

Table 2. Apple cultivar susceptibility to rust diseases.

A more complete rating of disease susceptibility can be found on the Scab Resistant Cultivars page.

The potential for rust fungus infections of apple can be estimated by examining red cedars near the orchard from early May to mid June. Galls that have produced the orange spore-bearing masses indicate that infection is probable.

Theoretically it is desirable to remove the alternate hosts of rust diseases, and wild apple trees, from the vicinity of the orchard. in reality this is usually not practical. Rust galls may by physically removed from valuable ornamental junipers and burned. Rust-resistant cultivars of ornamental species are available and should be planted.

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 will likely be 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 mid-June.

Scab-resistant cultivars susceptible to rust infection will need a specific fungicide program to control these diseases.

Related Links

• Scab Resistant Cultivars

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