Fungus causes ugly blisters on oak leaves

  • Follow Gardening

The wet spring is causing more woes, with several folks dealing with oak leaf blister, a disease caused by a fungus that occurs in all oak species.

Oak leaf blister causes small, rough (concave-convex) spots as leaves expand in spring. The spots turn pale green and somewhat thick, then turn brown or greenish brown. Leaves with numerous spots will fall prematurely.

If well-established trees defoliate before midsummer, they will sometimes leaf out again later in the season. When defoliation occurs in late summer, leaf loss will have little impact on the tree's health.

Microscopic spores are produced in leaf spots in midspring. These spores are carried by wind and splashing rain to bud scales and twigs. They remain until the following spring, when rain washes the spores onto young leaves. Infection takes place and starts the cycle anew.

Fungicide treatment is usually not needed, because oak leaf blister does not seriously affect the tree's overall health.

There are some steps you can take for limited control:

- Collect and discard fallen leaves.

- On small, newly established or especially valuable oak trees previously damaged by leaf blister, apply a protective fungicide at bud swell in the spring. This obviously cannot be done until next year.

- Fungicides to use for this include chorothalonil (Daconil), and any mancozeb product.

WATCH YOUR STEP

Spurweed is causing a lot of pain for people walking barefoot.

A manager of a neighborhood swimming pool called recently for advice on how to eliminate it before opening day. Unfortunately, I had to give him the news he didn't want to hear: It's too late to do anything about it.

Spurweed (also known as lawn burweed) is a winter annual that grows close to the ground and looks like parsley. The weed germinates in the fall and remains small during the winter. As temperatures warm in the spring, spurweed undergoes rapid growth and begins to form spine-tipped burs in the leaf axis that hurt when you step on them barefoot.

Part of the problem with control is that the typical pre-emergence herbicides used in the fall (Halts, Dimension, Baylan and Crabgrass Preventer) don't prevent spurweed.

The only one that does a good job is atrazine, which can be bought as a liquid or granular herbicide for use in the fall.

Spurweed can be controlled during the winter using post-emergent herbicides such as atrazine or the mixtures of 2, 4-D, MCPP, and dicamba. December, January and February are ideal months to apply them.

The problem is that most people don't take action until it is too late. Once the weed has changed from the vegetative or growth stage to the reproductive or flowering stage, it becomes hard to kill. At that stage, it takes multiple applications, and you risk injuring the grass.

Even when you kill it, burs persist until they naturally decompose.

The best thing to do is to dig them up. This will help in getting rid of potential plants for next fall, but may not be practical in a big lawn with a lot of weeds.

Nature helps, too: Spurweed is already beginning to die, as it always does when temperatures approach 90 degrees.

SID MULLIS IS THE DIRECTOR OF THE UNIVERSITY OF GEORGIA EXTENSION SERVICE OFFICE FOR RICHMOND COUNTY. CALL HIM AT (706) 821-2349, OR SEND E-MAIL TO SMULLIS@UGA.EDU.

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