When orange gelatinized telia appear on the cedar host (Juniperus virginiana), basidospores from it are infecting the apple host (Malus) of cedar apple rust.
Cedar-apple rust (Gymnosporangium juniperivirginianae), cedar-hawthorn rust (G. globosum), and cedar-quince rust (G. clavipes) are closely related rust diseases that require two hosts to complete their life cycle. All three rusts can infect most varieties of eastern red cedar (Juniperus virginiana) as well as many other junipers and an alternate host. Of these alternate hosts, cedar-apple rust is primarily a disease of apples and crabapples. Cedar-hawthorn rust, in addition to affecting apples and crabapples, sometimes infects pears, quince, and serviceberry. Cedar-quince rust has the broadest host range and can infect many genera in the rose family. In addition to those plants already mentioned, mountain-ash, flowering quince, cotoneaster, chokeberry, and photinia are also hosts for this disease.
Symptoms and Diagnosis
Symptoms on juniper: Brown, perennial galls form on twigs. When mature (usually in two years), the galls swell and repeatedly produce orange, gelatinous telial horns during rainy spring weather. The galls of cedar-apple rust are often over 2 inches in diameter, while cedar-hawthorn rust galls are rarely over 2 inches in diameter. Occasionally the twig beyond the gall dies, but usually no significant damage occurs on the juniper host.
Symptoms on apple (or crabapple): Circular, yellow spots (lesions) appear on the upper surfaces of the leaves shortly after bloom. In late summer, brownish clusters of threads or cylindrical tubes (aecia) appear beneath the yellow leaf spots or on fruits and twigs. The spores associated with the threads or tubes infect the leaves (needles) and twigs of junipers during wet, warm weather.
On juniper, galls appear about seven months after infection, and they form gelatinous masses of spores after 18 months. Golf ball-like depressions form on the gall that will give rise to telial horns the following spring. The telial horns are brownish in color, but rapidly elongate and become bright orange with spring rain, shrinking and swelling with intermittent rainfall. After releasing their spores, the horns collapse, dry, and eventually fall off. The galls die at this point but may remain attached to the juniper for a year or more. This rust is very obvious on red cedar and other junipers during spring when the galls are covered with orange-brown gelatinous masses. Rust spores formed on the gelatinous masses cannot infect other junipers but can infect only certain susceptible species of the rose family.
On apples and crabapples, the circular, yellow lesions on the upper surfaces of the leaves appear shortly after bloom. Wet, rainy weather in early spring is conducive for twig, leaf, and fruit infection of these deciduous hosts. Heavy rains within the first two weeks of bud break and leaf expansion cause the disease to be more severe. As the disease progresses, the undersides of the leaves below the yellow spots will develop raised orange structures that will ooze from the center, turn black, and appear as black dots. In late summer, this area will produce the orange and brown rust-colored spores that infect the juniper host, completing the cycle. Severely infected leaves may drop prematurely, especially during a dry summer.
Integrated Pest Management Strategies
1. Prune. Prune out the galls on junipers, if you wish, though be advised that the effectiveness of pruning out the galls or removing junipers is debatable since spores can travel long distances with the wind.
2. Co-exist. Live with the disease. It rarely kills trees. It may disfigure plants when twigs are infected.
3. Preventive fungicide. Use preventive fungicides labeled for use on apples. Fungicide sprays are aimed at protecting developing foliage from infection during the time the galls on the junipers are orange and gelatinous. This usually lasts for several weeks and fungicide applications are not necessary once the galls become dry and inactive. Pesticides registered for use include captan, chlorothalonil (Daconil), copper, mancozeb, maneb, sulfur, thiophanate methyl (Cleary 3336), thiram, triadimefon, and ziram.
4. Mixing plants. Avoid planting juniper and rust-susceptible hosts in close proximity to each other.
5. Resistant apple varieties. Use only resistant varieties in planting. Remove and replace diseased plants with resistant varieties.
Resistant varieties: ‘Redfree’, ‘Liberty’, ‘William’s Pride’, and ‘Freedom’.
Susceptible varieties: ‘Jonathan’, ‘Rome’, ‘Wealthy’, ‘York Imperial’, and ‘Golden Delicious’.
Strategies 1, 2, 4 and 5 are strictly organic approaches. Of the fungicides listed in Strategy 5, consult the Organic Materials Review Institute (OMRI™) for appropriate organic copper products.