Summer Tree Care: Identifying Common Tree Diseases in Summer, Part 2

This is the second part of a series on summer tree diseases. This article examines artist’s conk, and bacterial leaf scorch.  


During the summer months, plants are in the midst of their development. While many plants flourish due to the warmer climate, others can be subject to infections from a slew of disease pathogens. The following describes some of the most common diseases to affect plants in summer, and how they impact their hosts.

Artist’s Conk (Ganoderma applanatum)

Artist’s conk, also referred to as artist’s bracket, red mother fungus, Ancient Ling Zhi, and bear bread fungus, is a bracket fungus that infects a bevy of trees. In Japan, the fungus is known as kofuki-saru-no-koshikake, which translates to powder-covered monkey’s bench. It is called shu-she-ling-zhi in China. Artist’s conk is a polypore: it achieves sporulation through pores instead of gills. The fungus has an ascending, shelf-like appearance that distinguishes it from other polypores. Artist’s conk is initially parasitic. Once the host expires, the fungus becomes a saprotroph, decaying the remaining organic tissue.


Artist’s conk infects most hardwoods, and many conifers. It is most common on alder, ash, beech, Douglas-fir, elm, poplar, buckeye chestnut, horse chestnut, maple, walnut, willow, western hemlock, olive tree, and spruce. Older trees and trees suffering from environmental stressors are more susceptible to infection.

Symptoms of Infection

The fruiting bodies are conspicuous on infected trees. They can measures 12 to 40 inches in diameter, and may be observed on infected tree trunks and stumps. When plucked from infected trees, the fruiting bodies will reveal layers of pores that resemble rings.


The fruiting bodies are inedible in their raw form. However, when cooked, they have a rich mushroom flavor that blends well with various recipes. Slices of the fruiting bodies have been used in fermented foods to enhance their flavor. When dried and ground, the fruiting bodies can be made into a tea or tincture. Artist’s conk can be used for dyeing wool, certain fabrics, and paper. In Asia, the fruiting bodies are blended or cold pressed with water to create ganoderma drinks.

Artist’s conk contains antibacterial compounds. As such, it is often used in traditional medicines. If rubbed or scratched with a pointed utensil, artist’s conk bruises. The bruises become permanent once the fungus has dried. This makes the fungus an ideal platform for artists to paint or sketch intricate designs on. The midge Agathomyia wankowiczii lays its eggs on the fruiting bodies. The forked fungus beetle, Bolitotherus cornutus, dwells within the fruiting bodies for its entire life cycle.


  • Maintain trees through sound cultural practices. Ensures that trees are sufficiently watered. Each year, apply fresh organic mulch around the base of trees. Proper mulching will help to improve the soil quality, moderate the soil temperature, and retain soil moisture.
  • Avoid mechanical injuries to potential hosts.
  • Prune broken or damaged branches. Provide maintenance to trees that are overgrown. Periodically thin out tree canopies to increase air circulation, and promote a rapid drying of the foliage.
  • Culling infected trees may prevent the fungus from spreading, and achieve some control.

Bacterial Leaf Scorch (Xylella fastidiosa)

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.

Similarities to Other Diseases and Stressors

The effects of bacterial leaf scorch are similar to those caused by oak wilt and Dutch elm disease, though there are some notable differences. Oak wilt and Dutch elm disease often kill their hosts within a few months. Bacterial leaf scorch cripples its hosts over a period of years. All three diseases promote a premature browning of infected leaves. This leaf browning is more uniform on plants affected by oak wilt or Dutch elm disease. The browning that results from bacterial leaf scorch infections starts along the leaf margins, and proceeds inwards toward the leaf veins.

Bacterial leaf scorch may be easily mistaken for drought or heat stress. Leaf scorch damage initially impacts older leaves, before spreading toward the branch tips. Environmental stressors tend to affect individual leaves, or entire branches.


  • 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.

Photo courtesy of Henk Monster CC-by-3.0