This is the third part of a series on spring tree diseases. This article examines brown rot blossom blight, and cedar-apple rust.
In spring, as the growing season commences, trees become vulnerable to infection from a bevy of diseases. While some of these diseases are relatively benign, many of them can cripple their hosts. The following describes some of the most common diseases to afflict plants in spring, and how they impact their hosts.
Brown Rot Blossom Blight (Monilia fruiticola)
Brown rot blossom blight is a fungal disease that infects a bevy of stone fruit trees. The disease is caused by the pathogen Monilia fruiticola. Monilia fruitocola invades susceptible trees through various plant parts, including blossoms, branches, and shoots. Once the fungus becomes established within a tree, it induces a blossom blight, which causes the infected blossom to wilt. Infected fruit gradually decay, resulting in a significant reduction in fruit yield. Severely infected trees will exhibit extensive twig and branch dieback throughout the crown. Eventually, infected trees may succumb to infection.
Brown rot blossom blight infects stone fruit trees, including cherries, plums, and peaches. Ornamental flowering trees such as weeping cherry, and flowering plum trees may also be infected. Plum trees are less vulnerable to infection than peach and cherry trees.
Symptoms of Infection
Disease symptoms appear in spring, shortly after bud expansion has begun. Diseased flowers will turn brown and wilt. As the infection advances, diseased petals will appear water-soaked, before collapsing inwards, and becoming laden with masses of spores. Infected flowers will often adhere to twigs into winter. Eventually, the infection will spread from the flowers into nearby twigs and small branches. During this period, extensive foliar dieback may occur. When branches are infected, a sticky or gummy substance will develop on the surface.
Once the infection reaches a twig or branch, it will often incite the formation of a canker. As the canker enlargens, it can girdle the infected portion of the tree. Infected fruit will form small, tan colored lesions that increase in size. When conditions are humid, the lesions will produce grayish brown spores on the surface of the fruit. Diseased fruit will shrivel, and cling to the branch. Peaches may form rings of gray sporulation as the infection deepens.
- Repeated applications of nitrogen-based fertilizer can exacerbate infections, and are not recommended for use around diseased plants.
- Fruit that fails to ripen or is insufficiently pollinated will often be cast from the tree, and plummet to the ground. If not collected and disposed of, cast fruit can increase the amount of inoculum present.
- Remove and dispose of infected twigs, branches, and fruit. Prune periodically to improve air circulation throughout the crown, and promote a rapid drying of damp foliage. Avoid wounding healthy fruit when pruning, as this can create potential openings for infection. Use a solution composed of nine parts water and one part bleach to sanitize pruning equipment prior to performing each cut. Dry disinfected tools thereafter.
- Provide control for insects that damage fruit.
- Trees may be sprayed with registered fungicides to reduce the incidence of infection. Apply fungicides during the bloom period to control the blossom blight phase. Multiple applications may be required to suppress the disease.
- When applying fertilizer, use low to moderate amounts of nitrogen.
Cedar-Apple Rust (Gymonosporangium juniperi-virginianae)
Cedar-apple rust is a fungal infection that commonly affects junipers, apples, and crabapples. It is caused by a pathogen called Gymonosporangium juniperi-virginianae. Infections often occur in spring, when conditions are sufficiently moist. Infected plants may experience minor twig dieback, which can inhibit growth during summer.
Cedar-apple rust requires two hosts to complete its life cycle: a juniper, and an alternative host. During the first stage in its life cycle, the pathogen infects most varieties of eastern red cedar, as well as many other species of juniper. Once this stage has concluded, the pathogen infects an alternative host. Of these alternative hosts, cedar-apple rust primarily affects apples and crabapples. In addition to these plants, quince, hawthorn, mountain-ash, flowering quince, cotoneaster, chokecherry, and photinia can substitute as hosts. Cedar-apple rust is indigenous to North America. It is most prevalent throughout the eastern United States, southern Ontario, and Quebec. The disease is uncommon in the west, though it has been reported in California and Washington.
Symptoms on Juniper
Small brown galls develop on the needles and twigs of infected junipers. Galls produced by cedar-apple rust are often over two inches in diameter. When mature, the galls swell considerably, and produce orange, gelatinous telial horns. The horns form during early spring, following extended periods of rainfall. As spring transitions to summer, affected twigs may die back; otherwise, no significant damage occurs.
Symptoms on Apple and Crabapple
On apples and crabapples, infections occur on leaves, fruit, and twigs. Crabapples are generally more susceptible to the disease. From late April to May, circular, yellow lesions appear on the upper surfaces of the leaves. As the lesions mature, they gradually enlarge, turning a yellow-orange color, with a swollen red border. Drops of orange fluid may ooze from the lesions. Some lesions may develop tiny black dots called spermagonia. These can be observed on the orange spots in the center of the lesions.
In mid to late summer, brown clusters of threads, or cylindrical tubes called aecia develop on the undersides of leaves. They may also appear on susceptible fruit and twigs. The spores associated with the threads or tubes infect the needles and twigs of juniper. Severely infected leaves may drop prematurely, especially during periods of extreme heat.
- Prune and dispose of galls of junipers prior to the formation of the telial horns.
- When planting, consider using disease resistant varieties. Resistant apple varieties include ‘Freedom’, ‘Grimes Golden’, ‘Jonafree’, ‘Linberty’, ‘Narragansett’, ‘Priscilla’, ‘Red Delicious’, ‘Redfree’, ‘Staymans’, ‘William’s Pride’, and ‘Winesap’. Varieties of crabapple that are resistant to cedar-apple rust include ‘Adams’, ‘Beverly’, ‘Candied Apple’, ‘Dolgo’, ‘Donald Wyman’, ‘Eleyi’, ‘Inglis’, ‘Indian Summer’, ‘Liset’, ‘Mt. Arbor’, ‘Red Jewel’, ‘Robinson’, ‘Robusta’, ‘Royalty’, ‘Tina’, ‘Snowdrift’, and ‘Special Radiant’. Susceptible varieties include ‘Golden Delicious’, ‘Jonathan’, ‘Rome Beauty’, ‘Wealthy’, and ‘York Imperial’. Avoid planting juniper and rust-susceptible hosts in close proximity.
- Fungicide sprays are highly effective against cedar-apple rust. Applications to junipers can be made before and while they are in the orange gelatinous state. This will reduce the severity of the outbreak.
- Fungicide applications to apples and crabapples will help protect them from the infectious spores released during mid to late spring. Initial applications should be administered at budbreak. Three subsequent applications can be made at seven to ten day intervals.
- Pesticides can be applied on junipers in July and August to reduce infection. Some of the pesticides registered for use include captan, chlorothalonil, copper, mancozeb, maneb, sulfur, thiophanata methyl, thiram, triadimefon, and ziram.
Photo courtesy of I. J. Holb