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Symptoms and Signs

Leaf scorching in shade trees

Leaf scorch in plants can be attributed to biotic agents or abiotic stresses. Abiotic (or environmental) stresses that can cause leaves to scorch include moisture extremes, wind, salt, air pollutants, toxic metals, and nutrient extremes. In most cases, this type of scorching is fairly uniform around leaf edges, affects newer leaves as well as older leaves, will appear on vast expanses of the canopy, and may also develop soon after a known stress (such as drought or an application of salt) occurs (Figure 2).

Figure 2

Plant infection by living or biotic agents (such as fungi, bacteria, nematodes, and viruses) can also result in leaf scorching, but this type of scorching is not clearly defined on the plant. Symptoms on leaves are often irregular in shape, and frequently a yellow or red "band" will appear between green and scorched tissues (Figure 3). In addition, symptoms may appear first on leaves of one or more branches, and then, over time, appear on other parts of the tree.

Figure 3

Symptoms.Symptoms of BLS vary by shade tree host (Figure 4), but in most cases the disease is identified by a characteristic marginal leaf scorch similar to that described above for biotic agents.


Figure 4a	Figure 4b

Figure 4c	Figure 4d

Symptoms first appear in late summer to early fall. In trees with determinate growth, such as oak, the scorching appears on leaves of all ages at about the same time. In trees with indeterminate growth, such as sycamore and elm, symptoms progress from older to younger leaves. Affected leaves may curl and drop prematurely, and as the disease progresses over several years, branches die and the tree declines. Elms may be killed outright by the disease; other affected species eventually decline to the point where the dead branches pose a risk and the tree must be removed. The process of tree decline may occur quickly or slowly, perhaps depending on the host and the environment. Epicormic sprouts (shoots that arise from adventitious or latent buds) may be prominent on severely diseased trees, and scale insects, borers, Armillaria root rot, and other biotic diseases may be present as secondary pests.

The symptoms and distribution of several important shade tree hosts affected by BLS are described below.

Elm. Leaves of elm (Ulmus americana) affected by BLS have an irregular pattern of necrosis (tissue death) along the margin that is often accompanied by a chlorotic (yellow) halo (Figure 3). This irregular scorch is quite different from the uniform pattern caused by environmental stress (such as drought). Symptoms progress from older to younger leaves (Figure 5).

Figure 5

Elms affected by BLS are also very susceptible to Dutch elm disease (DED) (which is the usual reason such trees die and are removed). DED differs from BLS, however, in that leaves associated with DED flag and curl (not scorch) earlier in the growing season, and xylem discoloration, which is a diagnostic characteristic for DED, does not occur in trees affected by BLS. BLS of elm has been detected as far north as the Niagara Peninsula and is particularly troublesome in the mid-Atlantic United States. For example, in 2001, 30% of 3000 elm trees planted in the monumental core in Washington, D.C. were affected by the disease.

Sycamore. BLS of sycamore (Platanus occidentalis) is a chronic disease, and it may be years before trees affected by the disease die. Symptoms appear late in the season as a papery, interveinal leaf scorch with a narrow chlorotic halo (Figure 6). As in elm, the disease progresses from older to younger leaves (Figure 7). BLS of sycamore may be confused with sycamore anthracnose, which appears earlier in the growing season and tends to affect tissue along the veins instead of between them.


Figure 6	Figure 7

Oak. As mentioned previously, BLS affects many species of oak. Symptoms of BLS on the leaves of red oak (Quercus rubra) appear as a pronounced, marginal discoloration with a dull red or yellow halo between scorched and green tissues (Figure 1). Unlike elm, however, most, if not all, of the leaves on an affected branch will scorch (Figure 8). This is due to the determinate growth habit (all leaves on a branch share the same age) of oak trees. In the early stages BLS, portions of the tree remain unaffected, while other branches exhibit typical leaf scorch symptoms. As the disease progresses, more branches develop symptoms (Figure 9). Within populations of trees, disease incidence usually appears randomly; trees neighboring severely affected trees are often asymptomatic (Figure 10).


Figure 8	Figure 9	Figure 10

Leaf symptoms in pin oak (Q. palustris) are not as distinct (Figure 11) and can be easily confused with abiotic stresses, but the distribution of the disease within the canopy and between trees is the same as for red oak.

Figure 11

BLS of oak may be confused with oak wilt, another vascular disease. Like BLS, initial symptoms of oak wilt appear as scorched leaves. Unlike BLS, however, scorching appears in spring to early summer, and trees defoliate and die within several months after symptoms appear.

BLS of oak is has been reported from New Jersey as far south as Tallahassee, FL, the mid-western states, and in Texas. In some New Jersey municipalities, BLS is known to affect up to 35% of oaks planted as street trees and in landscapes.

Diagnosis

Xylella fastidiosa was not recognized as a pathogen of landscape trees until the early 1980s. Since its symptoms are very similar to those caused by abiotic stresses, it is not surprising that the disease is frequently overlooked or misdiagnosed.

Preliminary diagnosis of BLS is made by interpreting the symptoms described above in late-summer and early fall. Especially useful diagnostic criteria include leaf scorch, premature leaf drop (Figure 12), the random distribution of affected branches around the canopy, thinning of the crown (Figure 13), and the random appearance of the disease within populations of trees.


Figure 12	Figure 13

Definitive diagnosis is most often made in the laboratory using a selective antibody technique known as enzyme-linked immunosorbent assay (ELISA) on symptomatic leaves to detect the pathogen (Figure 14). Other techniques used to detect X. fastidiosa include light and electron microscopy, standard and real-time PCR techniques, and immunomagnetic separation followed by PCR (Table 4) Among these methods, ELISA remains the most rapid and cost-effective method to detect the pathogen in symptomatic tissues. The molecular techniques are more sensitive, however, and are helpful for detecting low populations of bacterial cells in infected, asymptomatic hosts or in insect vectors.

Figure 14

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