Week of August 12, 2002
Submitted by Calvin Finch, Ph.D., Manager, Conservation Division, Water Resources & Conservation Department, SAWS, and Horticulturist

  Oak Leaf Rust and Purple Caterpillars

 

The cool weather and rains were great for most plants. The lawns are filling in the dead areas that resulted after several years of hot, dry summers. Many live oaks put on a flush of new growth. Unfortunately, the weather this summer has also been ideal for rust and some live oaks are defoliating as a result of the disease. Losing leaves to rust is not a major problem; they will come back. No treatment is necessary.

Recognize rust by red-brown pistules on the underside of the leaf. The rust version on live oaks looks more fealty than the rust we are used to on peach trees, zoysia grass, and other plants. Oak wilt, which also defoliates but kills the tree, has a completely different disease pattern. The tree dies in sections, one big branch at a time. The symptom is called flagging. Oak wilt also has a classic leaf symptom. The area around the veins become bright red, orange, or yellow while the interveinal area is still green.

            Galls are also prevalent at this time on live oaks. Galls are the growth of plant tissue due to fungal secretions or injections of a chemical by insects, usually wasps that also lay their egg in the hole. The plant tissue protects the egg until it hatches and may even serve as a larval food source. Some galls are ugly masses of tissue but many are perfectly round and colorful. Some leaves will be covered with galls. Galls are generally not seen as a direct threat to the tree. Besides, once they are in place there is nothing that can be done.

            Oak blister fungus fits its name. It is another fungus that attacks oak leaves, usually red oaks. The blisters make the leaves gnarled and deformed but do not provide a long-term threat to the tree. Treatment is not necessary.

            Sunflowers are a great flower for the heat of summer. The seeds from our bird feeders work but there are also more disciplined growers with flowers of different colors. Whatever the color or size, sunflowers have been blessed this summer with the blue caterpillar-looking "worms" are actually sawfly larvae. Their color is usually greenish to pale white, not blue. The variation in color could be a result of something the insects ate, typically foliage. The 3/4-inch long insect larvae are of the same order as ants, bees and wasps. The order is Hymenoptera, the family is Cimbicidae, the genus Cimbex and the species is americana. The identification of this insect can be done by comparing the shape and counting the legs. The sawfly larvae have a pair of legs on every segment or about 11 pairs. Common caterpillars have eight legs or less. Also, the sawfly larvae doesn't have crochets (little hooks) which are used to hold onto plants. The larvae take six to eight weeks to mature and then form a pupa or cocoon stage, and don't typically reach the stage where they become sawflies, or wasp-like, until spring. Although not very common, their range extends from Arizona to New England. The caterpillars on my sunflowers were all orange yesterday. They were moving fast, chomping one leaf after another. Every few minutes a wasp would show up to sting and subdue a caterpillar to carry it off to the nest—high drama in nature. Contributing to the action was that lesser goldfinches and cardinals were plucking seeds from mature heads on the plants.

            Crepe myrtles reacted to the rain with a great flush of blooms. In some varieties the bloom period is over for a while, but there is a good chance that they may make another show this autumn.  Some newly planted crepe myrtles and plants in containers were showing symptoms of nutrient deficiency. The leaves turn blotchy with a red color just like it was autumn.

            There are two causes that may be at work. If the plant received root damage from the rains, the symptom will appear. Expect root damage if water sat on the surface of the soil or in the planting hole. If that is the case, you must make a choice. If the location is a low spot that will be soggy quite often, move the plant. If not, just let the roots repair themselves with time. Do not fertilize.

            Some newly planted crepe myrtles used all the available nutrients that their limited root system could provide and now are hardening off. They can be left to their own means until spring (February) when they should be fertilized or you can gamble a bit and provide a cup of slow release lawn fertilizer that may allow them to bloom one more time this autumn. Select this option if the drainage is good and you are certain there is no root damage.