This is the third part of a series on winter tree diseases and disorders. This article examines bacterial leaf scorch and bacterial leaf spot.
During the winter months, many of the fungal pathogens that affect trees enter dormancy. The pathogens overwinter on their hosts or in the soil, awaiting spring’s arrival. Despite the frigid temperatures, trees suffering from fungal diseases may still exhibit infection symptoms in winter, especially if the disease has advanced into its later stages. The following describes some of the most common diseases to overwinter on trees, and how they may be detected.
Bacterial Leaf Scorch
Bacterial leaf scorch is a disease of various shade trees, perennials, and crops that is caused by the bacteria Xylella fastidiosa. The bacteria invade the xylem of susceptible trees, and multiply. The xylem rapidly becomes clogged, disrupting the tree’s vascular system. As the infection advances, it causes the tree to experience significant foliar dieback. There is no remedy for the disease. As such, severe infections often culminate in plant mortality.
Bacterial leaf scorch infects a bevy of hosts, including almond, American beautyberry, American elm, blackberry, Boston ivy, English ivy, bur oak, pin oak, red oak, shingle oak, white oak, willow oak, red maple, sugar maple, mulberry, sweet gum, poison hemlock, peppervine, umbrella sedge, dallis grass, wild strawberry, grapes, miner’s lettuce, eastern baccharis, sumac, periwinkle, goldenrod, American elder, peach, and Virginia creeper.
Symptoms of Infection
Diseased plants start to exhibit symptoms of infection in mid-summer. Infected foliage will experience a premature browning. At first, this necrosis occurs along the leaf margins, before spreading inwards toward the leaf veins and petiole. As the infection deepens, yellow or reddish-brown bands form between the healthy and necrotic tissue. Once the leaves become necrotic, they may be cast from the tree.
Bacteria leaf scorch symptoms will appear on the same limbs each year. Over the next 3 to 8 years, the bacteria will gradually spread, until the entire crown has been infected. The lack of healthy growth on diseased plants will result in extensive foliar dieback, followed by plant mortality.
- Bacterial leaf scorch has no known cure. As such, it is pivotal to manage infections in order to prevent diseased plants from rapidly declining.
- Branches that are dead or have declined due to bacterial leaf scorch should be excised from the plant. Sanitize pruning equipment between cuts using a solution composed of nine parts water and one part bleach. Always dry tools after disinfecting them.
- Severely weakened plants and plants that have lost over two thirds of their growth can be hazardous, and should be culled.
- When planting in locations where bacterial leaf scorch has been reported, select plant species that exhibit an increased resistance to the disease, such as ash, beech, or tulip poplar.
- Drought stress exacerbates infections. To limit infections, ensure that plants are sufficiently watered, especially during extended periods of drought.
- Apply a layer of organic mulch around the base of susceptible plants to improve the soil quality, moderate the soil temperature, and retain soil moisture.
- Annual treatments of an Oxytetracycline antibiotic are required to suppress the disease. The antibiotic can be administered via a root flare injection. While it will not cure the disease, the antibiotic will limit bacterial growth for several weeks, delaying infection symptoms.
- Rake and dispose of leaves shed from diseased plants.
Bacterial Leaf Spot
Bacterial leaf spot is a disease that affects various crops and ornamental plants. It is most often caused by the bacterial pathogens Pseudomonas spp. and Xanthomonas spp. The infectious pathogens dwell on the foliage of susceptible plants. They invade plants through leaf and bark wounds, as well as through openings that have been created due to pruning. Infected plants will develop leaf spots, which discolor and kill leaves. The disease is most prevalent on older leaves, but when conditions are favorable, it can also infect new growth. Bacterial leaf spot is highly contagious. Warm, moist conditions can cause clusters of vulnerable plants to be readily infected within a few hours.
Bacterial leaf spot infects a multitude of crops and ornamental plants. The most frequently infected crops include beets, eggplant, kale, lettuce, peppers, and tomatoes. Ornamental plants that are commonly infected include begonia, dumb cane, English ivy, ficus, geranium, hydrangea, laceleaf, philodendron, poinsettia, syngonium, and plants in the genus Prunus.
Symptoms of Infection
Symptoms of infection generally appear within 3 to 7 days. Infected plants will develop lesions, or spots, on their foliage. Lesions may form on the top or bottom portion of a leaf. The lesions are initially small, but can develop an angular shape once the infection has reached the leaf veins. As the infection advances, the lesions will gradually enlargen, growing between 3/16 and ½ of an inch in width. In some instances, a chlorotic halo may form around the spot. Several spots may coalesce, causing infected leaves to die. Some leaves will dry up in the center, and crumble away. This will cause them to assume a “shot-hole” appearance.
Infected shoots, buds, and flowers may blacken, and wither. Infected flowers will become desiccated, and break off easily. V-shaped necrotic areas may form on the leaves of begonia and geranium. On begonia, water-soaked lesions will become visible on the underside of diseased foliage. When infected, hydrangea leaves will turn a reddish-purple. Flowering plants will often wilt.
- Avoid planting in diseased locations for at least a year.
- There are currently no recognized chemical treatments for bacterial leaf spot.
- Registered fungicides may be applied in early spring to control the disease.
- On ornamental plants, remove the affected leaves once infection symptoms become apparent. This will prevent the bacteria from spreading to healthy leaves and plants.
- Rake and dispose of fallen leaves to eliminate potential sources of bacteria.
- When planting, avoid overcrowding trees and shrubs.
- Remove plant debris from gardens to reduce the potential for overwintering bacteria.
- Prune trees and shrubs periodically to increase light penetration, and improve air circulation throughout the crown.
- Avoid mechanical injuries to susceptible plants.
- Adjust irrigation to prevent vulnerable foliage from being doused with water.
- Maintain plant vigor through sound cultural practices. Ensure that plants are sufficiently watered, especially during extended periods of drought.
- Apply a layer of organic mulch around the base of plants to improve soil quality, moderate soil temperature, and retain soil moisture.
- Avoid fertilizing plants that are suffering from bacterial leaf spot.