This is the fifth part of a series on spring tree diseases. This article examines Diplodia tip blight of pine, and fire blight.
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.
Diplodia Tip Blight of Pine
Diplodia tip blight, also known as Sphaeropsis tip blight, is a common fungal disease caused by two species of Diplodia: Diplodia pinea and Diplodia scrobiculata. Diplodia pinea is the more aggressive species, but both fungi are capable of causing disease. Diplodia tip blight is most common in pines that have two to three needles per bundle. Austrian pine is the most susceptible species. Other conifers, such as spruce, fir, larch and arbovitae may occasionally become infected. Pines that have five needles per bundle are highly resistant to the disease.
The fungus infects more than twenty pine species. It is commonly found on Austrian pine, but is also frequently reported on Scots, Jack, red, ponderosa, and Mugo pines. Infections occur on Monterey pine in California.
Symptoms of Infection
Brown, stunted, or curled new shoots, with short, brown needles may signify the presence of Diplodia tip blight. Symptoms are characterized by resin droplets, which indicate that a new shoot has been infected. Entire new shoots are killed rapidly by the fungus. Needles on infected shoots often turn tan or brown while still encased in their fascicle sheaths. Infection varies among major branches. Damage is first evident in the lower crown, before progressing upwards. Diplodia tip blight can form perennial cankers on mature trees and saplings, inducing sudden branch death. After two or three successive years of infection, the tree’s crown may incur significant damage.
Infection is most common in mature pines. Damage from Diplodia tip blight is generally confined to new shoots; however, the fungus may infect older tissues through damage resulting from wind and hail, or perforations in the wood created by insects. Tissues wounded during pruning or shearing operations may also become infected. Stressed or injured trees remain vulnerable to infection for several days, especially during warm, dry periods. Symptoms on new shoots can be detected in late May. The extent of the infection can be effectively determined in late June or July.
- Infection of new shoots can be reduced by applying a fungicide to pines. Perform the first application as buds begin to swell, followed by a second application two weeks later. Fungicides containing potassium bicarbonate or benomyl as the active ingredient, or the 4-4-50 Bordeaux mixture are most effective. Fungicides applied during late April and early May will not prevent infection of seed cones.
- Prune and dispose of blighted needles, twigs, and cones. Use rubbing alcohol, or a solution comprised of one part bleach and nine parts water to disinfect pruning tools before each cut. This will help prevent the fungus from being spread to other trees or branches.
- Forego pruning or shearing of pines during periods that are favorable for infection.
- When planting, avoid selecting vulnerable conifer species in areas where infection is common. Native species are more susceptible when planted on poor soil sites, or locations prone to environmental stressors.
- Ensure trees are sufficiently watered, especially during periods of extreme heat.
- Fertilizing trees in spring or summer will improve tree vigor; avoid using fertilizers containing high quantities of nitrogen.
- Maintain a layer of mulch around the base of the tree to improve soil quality, and conserve soil moisture.
- Avoid mechanical wounding of pines.
Fire Blight (Erwinia amylovora)
Fire blight is a significant disease of various fruit trees. It is one of the most common diseases to afflict fruit trees in spring and summer. Apples, crabapples, and pears are especially prone to infection. Fire blight is caused by the bacterium Erwinia amylovora. The bacterium can infect the branches, twigs, leaves, flowers, and fruit of susceptible plants. Plants that are severely infected generally experience widespread foliar dieback. Infections that progress into the main trunk often result in plant mortality. Under optimal conditions, fire blight can reduce blooms and fruit yield, or eliminate fruit altogether for a single growing season.
Fire blight infects over 130 plant species of the rose family. It most frequently infects apple, crabapple, firethorn, mountain ash, and quince. Infections also occur on cotoneaster, hawthorn, juneberry, loquat, pyracantha, raspberry, and serviceberry.
Symptoms of Infection
Initial symptoms of infection appear in early spring, as trees break dormancy, and resume growth. The ooze produced by the bacteria is one of the most conspicuous symptoms. The watery, white to tan liquid can be observed exuding from infected branches, twigs, and trunks. The ooze turns black or brown once it has been exposed to the air.
Infected flowers and flower clusters will appear damp, before drooping and shriveling. As the infection deepens, the flowers become discolored, turning brown or black. Flowers that have succumbed to infection will often remain attached to the tree throughout the growing season. Infections may be localized, affecting only individual flowers or flower clusters. Infected fruit turns brown or black, and becomes desiccated. The fruit may cling to the branch for several months, before being shed.
Young leaves, twigs, and water sprouts wilt, turning grayish-green, then bending downwards to form a characteristic crook. The leaves and tips of infected twigs darken to brown or black. Trees with multiple infected shoots may appear scorched. Infected leaves will often linger on the tree for the remainder of the growing season.
Cankers can be identified by small to large areas of dead bark that have been killed by the bacteria. Most cankers are minuscule, making them difficult to detect. The bark on branch and trunk cankers will brown or blacken. As the wood decays, the bark sinks inwards. Eventually, the bark may crack or peel. If the bark is removed, brown staining of the sapwood may be observed.
- When planting, select cultivars that exhibit an increased resistance to fire blight. Resistant varieties of crabapple include ‘Adams’, ‘Adirondack’, ‘Camelot’, ‘Candymint’, ‘David’, ‘Dolgo’, ‘Lancelot’, ‘Pink Princess’, ‘Red Splendor’, ‘Silverdrift’, and ‘Tina’. Varieties of pear that are less susceptible to fire blight are ‘Bradford’, ‘Capitol’, and ‘Red Spire’. Most cultivars of apple are vulnerable to fire blight, though some are especially prone to infection. These include ‘Fuji’, ‘Gala’, ‘Golden Delicious’, ‘Granny Smith’, ‘Gravenstein’, ‘Jonathan’, ‘Mutsu’, ‘Pink Lady’, and ‘Yellow Newton’.
- Diseased twigs and branches should be promptly removed from infected trees. Prune infected twigs and branches in late winter, when the tree and bacteria are dormant. Dormant pruning will prevent the bacteria from being transmitted to other trees or plant parts. If pruning is required during the growing season, sterilize pruning equipment with a solution comprised of nine parts water and one part bleach. After pruning, safely dispose of the infected trimmings. Trimmings may be buried, or burned. Avoid composting diseased trimmings.
- If the main trunk has been infected, cull the tree and stump to eradicate the bacteria.
- Fungicidal applications may provide limited protection to blooms. Applications must be performed on open blossoms. The number of applications required is determined by the length of the bloom period. Administer applications at two to three week intervals until the blooming period has ceased.
- Vigorous plants are favored by the bacterium. As such, avoid excess nitrogen fertilization of plants during the growing season.
- Limit the irrigation of susceptible plants while they are in bloom. This will reduce the incidence of infection.
Photo courtesy of Sebastian Stabinger CC-by-3.0