Bacterial Leaf Scorch

Bacterial Leaf Scorch (BLS) is a common plant disease that induces the premature browning of leaves in mid to late summer. BLS is caused by the bacterium Xylella fastidiosa. This bacterium infects a tree’s xylem, the conducting tissue that transports water and water-soluble nutrients up from tree roots. Essential materials needed for growth are cut off from leaf tissue. As a result, leaves become brown in mid-summer, appearing as if scorched by the sun and heat. Symptoms worsen towards late summer when X. fastidiosa is most active. This disease cannot be cured and can lead to the death of a specimen after years of infection. This is a progressive disease, and symptoms worsen each year.

Bacterial leaf scorch on oak leaves.

Causes

Leafhoppers, treehoppers, and spittlebugs feed on xylem tissue; thus, they transmit BLS as they feed on new trees. Grafting with infected roots can also spread BLS. The photosynthetic ability of a tree becomes limited as more leaves get infected and die. As a result, the energy demands of the tree are not met. Twigs begin to die, and eventually, branches and limbs die back as well.Because BLS can be difficult to distinguish from other diseases, confirmation from diagnostic tests may be necessary. BLS is not the sole reason leaves appear brown pre-autumn. Leaf browning can occur if there is insufficient water in the soil, when water loss via transpiration is higher than the water gained through precipitation (drought/dry conditions), or if roots have been impacted (like compaction or pathogens). Hot, dry summers and environmental stressors, like high salinity, can increase the severity of BLS symptoms.

A large portion of this tree is visibly affected by BLS.

Symptoms

Many common trees become infected with BLS, but symptoms vary between species. Oaks, like Black, Pin, Scarlet, and Red, are highly susceptible to this disease. BLS is severe for red and pin oaks in eastern Pennsylvania, and BLS is spreading westward. Other trees, such as ginkgo, hackberry, red maple, sycamore and London plane, sweetgum, elm, dogwood, and mulberries, can be infected too. BLS often shows an uneven and undulating scorch pattern, whereas a stress-based scorch has a uniform pattern.

In oak BLS, the dead edges of a leaf are usually separated from the living green parts by a thin yellow border. This does not always occur in every leaf. Browning starts at the leaf margin and progresses toward its midvein and leaf base in an undulating pattern. Defoliation of leaves is not common in oaks. While pin and red oaks are most affected, BLS has been noted in white oaks. Epicormic sprouts will appear on red oaks after years of infection.

Bacterial Leaf Scorch on pin oak leaf. Notice there is no yellow band separating the living from the dead tissue. The yellow band does not occur on every oak leaf, though it is common in oaks.

Treatment and Management

Trees can be treated with an antibiotic called oxytetracycline. Though a tree will not be cured, repeated treatments will alleviate symptoms and help trees live longer. Since BLS is a gradual disease, keeping affected trees healthy is another way to make sick trees last. Regular pruning, provision of nutrients and water, and adequate space are a few ways of doing so. Infected trees can be removed, but in areas where the disease is so widespread, this is ineffective in preventing further transmission. Planting BLS-resistant species is effective for reducing BLS in regions where it is present. Species such as Kentucky coffee tree (Gymnocladus dioicus), lindens (Genus Tilia), black gum (Nyssa sylvatica), and American beech (Fagus grandifolia) are viable alternatives.

Northern catalpa (left image) and black gum (right image) are examples of BLS-resistant trees that can be used in an urban environment.

Urban Issues

In the 1950s, urban environments grew exponentially following the war. With the rise of urban cities, fast-growing red and pin oaks became a popular choice for street trees during this time. As a result, red and pin oaks constitute a large portion of urban forests in southeastern PA. Over time, increased urbanization has led to restricted soil volume and root space for street trees. These stressful conditions have made pin and red oaks highly susceptible to BLS. Because many red and pin oaks were planted around the same time, BLS has led to the mass decline of street trees for several cities in southeastern PA.

Cities with large tree declines lose out on beneficial ecosystem services. This is the case for the City of Phoenixville. In 2021, a tree inventory was performed for Phoenixville to address several dead and declining red and pin oaks. These two species were found to be the most prevalent street trees; many were infected with BLS. Unfortunately, Phoenixville will need to bear the cost of replacing its dead oaks to maintain its urban forest and ecosystem benefits.