Bacterial leaf scorch takes a toll on pin oaks

It seems that once again we, meaning the collective gardening public, have disregarded the imperative known as diversity. It applies to more than the plant world, too!

Before I get into the specifics of one current problem (and there are more than one), let me reflect on our past mistakes when biodiversity has been ignored.

Remember the elm-lined streets of days gone by, wiped out by the epidemic of Dutch elm disease. Or your driveway lined with the fast-growing Bradford pear that now looks like a jumble of half-trees, with entire sections torn from the trunk on a windy day. In the West, the anxiety over sudden oak death is great as acres and acres of tan oak trees suddenly die. Elsewhere, the nursery industry is on pins and needles over quarantines, inspections and the fear of infected plant material. Emerald ash borers are taking their toll on over-planted ash in strip malls and subdivisions at this very moment. Ash trees become the darling shade tree of development in the last 20 years, just like the pin oak was decades before. We continually ignore the importance of planting out a diverse selection of species and, as a result, we allow pest problems to spread like wildfire.

So, the disease that now threatens pin oak-lined streets in Kentucky and Indiana has had enough time to actually show itself in a more widespread fashion. The disease is called bacterial leaf scorch, and it is caused by Xylella fastidiosa, a bacterium that essentially clogs up the xylem tissue causing the apparent scorching of leaves and the eventual death of entire branches. The xylem tissue is a part of the tree’s vascular system, transporting moisture to the leaves, which then photosynthesize and return nutrients to the roots. No moisture, and the leaves begin to scorch and cease functioning. This can happen to a healthy tree once in a while without any detrimental effects, but if it happens year after year, the tree will eventually die. This is the long-term effect of bacterial leaf scorch. Clogged xylem tissue means that the corresponding branch structure will die out as it continually gets starved.

Premature browning of foliage marks bacterial leaf scorch symptoms, usually starting with marginal necrosis (browning of leaf margins); then the entire leaf looks scorched and drops. It usually takes about 6 to 10 years for entire trees to die out from the disease. Death from bacterial leaf scorch is gradual, clogged xylem by clogged xylem, scorched leaf by scorched leaf, dead branch by dead branch. Add any other environmental stress, and it may take much less time.

Once we begin to notice the decline of the tree, there is little to do except provide as healthy of an environment as possible to keep the tree alive as long as you can. But here is another problem for the pin oak: The Latin name for pin oak is Quercus palustris, and palustris means swamp loving. Most pin oak are not planted in wet areas, rather most are planted in the verge between the sidewalk and the street, where it’s dry and mostly paved. Alas, keeping the environment as friendly as possible around your declining pin oak may be a tall order.

Bacterial leaf scorch does not restrict itself to pin oaks alone. In fact, most oak species are more or less susceptible, including the red oak, burr oak, white oak, scarlet oak, shingle oak and willow oak. Silver, red and sugar maples, sycamores, mulberries, hackberries, elm and sweet gums have also been affected by the bacteria, but it is not as widespread.

Back to the notion of biodiversity: If we mix up our tree species, both in our municipal, commercial and home landscapes, we are less likely to see the spread of these types of diseases. Plan for replacements that are diverse. The vectors for bacterial leaf scorch are leafhoppers and other treehopper insects. Apparently the spread is pretty slow-going, but if we have a diverse selection of trees out there it will be much more difficult for disease pathogens to spread.