Bacterial Leaf Scorch

by Curtis Helm

 

Bacterial leaf scorch (BLS) is a devastating infectious disease that is slowly killing many of our urban trees. In Princeton, examples can be see on Hamilton Avenue. They have a sparse canopy, many dead branches and leafy growth or “suckers” along some of the larger stems.

 

The bacterium (Xylella fastidiosa) clogs the tree’s water transporting tissue (xylem) and is transmitted by xylem-feeding insects such as leafhoppers, treehoppers, and spittlebugs.

 

The pin oak (Quercus palustris) is the primary species affected, but others are susceptible:  red oak, scarlet oak, black oak , shingle oak, American elm, London plane tree and red maple. BLS also appears in “weedy” plants such as goldenrod, clover, blackberry and grasses, which serve as reservoirs from which insects carry the disease.

 

BLS first appears in the crown and then progresses slowly throughout the tree. Small trees succumb in several years while large trees may not die for 10 years. BLS infected trees are vulnerable to other diseases and insect attack. Drought and heat exacerbate the symptoms. Although “wild” trees are susceptible, they seem to be less affected than their urban counterparts.

 

BLS is also costly news for homeowers who may have to prune dead branches or remove dead trees. Jim Irish, owner of a local tree care company, has observed a drastic increase in the number of afflicted pin oaks and notes that if homeowners delay dealing with the problem tree removals become more dangerous for workers and consequently more expensive.

 

Symptoms:  brown, scorched, irregular shaped margins on leaves that may extend to the mid-rib; yellow or reddish brown areas appear between scorched and healthy portions of the leaf. Although the canopy of BLS infected trees will leaf out, the leaves may be smaller and discolored. Infected branches will leaf out for several years before they die back to the main stem. BLS can be distinguished from drought stress which produces more regularly shaped brown margins and no discolored zone.

 

No cure exists for BLS. The NJ Division of Parks and Forestry and Bartlett Tree Research Labs recently performed trials using the antibiotic Oxytetracycline (OTC) and the growth regulator Paclobutrazole (PBZ) on infected pin oaks and red oaks. OTC was injected directly into the trunk, while PBZ was applied to the soil. The study found that OTC only suppressed BLS symptoms during the year of treatment. OTC injections were effective only in the early stages of disease. 

 

The Princeton Borough Shade Tree Commission recommends that homeowners avoid planting trees which are susceptible to BLS, such as those mentioned above, but select other species such as the blackgum (Nyssa sylvatica).  Princeton Boro has recently planted a number of black gums within the street right-of-way. 

 

To learn more about this issue, the public is welcome to attend the Princeton Borough Shade Tree Commission meetings at Borough Hall (1 Monument Drive) on the third Tuesday of each month beginning at 5:30 PM.