Groundwork for Aug. 23, 2013

Got a gardening question? Here are a few received by the LSU AgCenter recently. If one of these doesn’t answer what you’re dealing with, call or email the address below.

My water oak tree has a spider web all over the trunk. What is it and will it harm the tree?

Barklice (pscocids) are small insects that suddenly appear on tree trunks and limbs late summer to early fall. They prefer trees with rough bark. The insects are harmless so no control treatment is required.

I have a Chinese tallow tree next to my foundation. How can I kill it so it won’t resprout from the trunk?

Cut the trunk at the soil line and immediately apply a herbicide containing triclopyr to the freshly cut surface. Resprouting will occur requiring a foliar herbicide application using triclopyr or glyphosate. Be careful to read the directions and follow them closely to avoid damage to nearby plants.

How do I kill scale insects and sooty mold on my large holly shrubs?

The plant is infested with soft-bodied scale insects. Their feeding activity will result in sooty mold development if left untreated. When temperatures moderate this fall, apply a horticultural oil. Two or more applications will be required to kill the scale and to help remove some of the sooty mold from the surface of the green leaves. A soil application of the systemic insecticide imidacloprid can provide seasonlong control of soft scales when applied in early spring.

My vinca plants were doing fine until several weeks ago then they started to die, branch by branch, plant by plant. My bed is half dead. What is killing my plants?

Aerial blight is often caused by phytophthora, a water mold. Submit a plant sample to the LSU Plant Diagnostic Center for a positive identification of the problem.

During wet weather or frequent overhead irrigation, water mold pathogens proliferate, especially in poorly drained soils. Once plants are infected, fungicides do not cure disease. Affected plants should be destroyed to prevent the pathogen from building up in the soil. Consider improving drainage and mulching to prevent splash and switching to drip irrigation. Plant genetically resistant vinca (Cora or Nirvana) to avoid this problem.

Got a gardening question? Write to Bob Souvestre, horticulturist with the LSU AgCenter, at Burden Center, 4560 Essen Lane, Baton Rouge, LA 70809, or email to bsouvestre@agcenter.lsu.edu, or call Master Gardeners at (225) 763-3990.