Pecan Webworms, Honeydew and Cotton Root Rot

Pecan Webworms

Those webs in your pecan trees that keep getting progressively larger are made by the caterpillars of a moth that we know as the fall webworm. How did they get there? Will they harm the tree? Can we control them? Will aluminum foil or metal bands around the tree trunk keep our trees web-free? These are frequently asked questions from homeowners and they deserve some answers. Let's consider each question individually.

1. How do webworms get into a tree? The fall webworm spends the winter as a pupa inside a net-like cocoon. These cocoons are formed in leaf trash around the base of the tree or hidden in bark crevices on the trunk. White moths emerge from these cocoons during the spring months and fly to the foliage of pecan, cottonwood, mulberry, and fruit trees where they deposit an egg mass on the leaves. The eggs hatch within 7 to 14 days, producing the caterpillars that busily begin construction of their web while at the same time, devouring leaves. As the caterpillars (webworms) grow, the web is made larger to surround more leaves that may be eaten. The caterpillar stage requires 4 to 5 weeks before maturity is reached. When full grown, they leave the web for the first time and crawl to a sheltered area to form their cocoon. Moths emerge from these cocoons in mid- to late summer and the cycle is repeated once again.

2. Will webworms harm your trees? Fall webworms can harm trees if they devour most of the foliage. The tree must use reverse or stored food to maintain life, thus lowering the overall vigor of the trees. Trees weakened by defoliation are more susceptible to damage by diseases and other insects.

3. Can webworms be controlled? The answer to this question is simply, “yes, we can”. Two approved methods for controlling webworms are outlined below:

A. Small webs can be pruned from the trees and destroyed. This method is the most obvious, but effective only if the webs are within reach.

B. Insecticide sprays containing Carbaryl (Sevin), Malathion, or Bacillus thuringiensis are highly effective if used as directed.

4. Will aluminum foil and other metal bands around a tree trunk keep webworms out of a tree? Fall webworm moths are accomplished fliers and can easily fly from tree to tree where the egg masses are deposited on the leaves. One can easily see that metal bands will not aid in keeping our trees web-free.
Honeydew

Another problem is honeydew from pecan trees. When honeydew from a pecan tree begins to fall, you quickly realize that there are insects above doing naughty, unmentionable things. "Honeydew" is a classy word meaning "a sweet substance secreted by aphids and other juice-sucking insects." "Secrete" means to be "excreted as a waste" and the last thing that I will tolerate is to be excreted upon by aphids –I don't care if the secretion does have a fancy sounding name like honeydew!

Aphids build to high populations in mid-summer. Although the actual severity of damage from this pest is widely debated, the normal level of leaf performance can be impaired. The primary mode of damage is from leaf shading by a mold that grows over the upper leaf surface of affected leaves. As the aphid feeds on the undersides of leaves, the pest excretes "honeydew" or sugar water. This sticky liquid falls down through the tree and is deposited on the upper leaf surface of leaves over which the aphids fed. This "honeydew" is a perfect substrate for "sooty mold" fungus. Under high humidity, the fungus mold grows on the "honeydew" to form a dark gray to black covering over the upper leaf surface. This effectively blocks the sunlight from the leaf, reducing photosynthesis.

Unfortunately, pesticides do not reduce the numbers significantly, so the best thing to do is to ignore these devils and hope that beneficial insects decrease the aphid population as soon as possible.

For more information about webworms, see:

http://bexar-tx.tamu.edu/HomeHort/F1Column/2004Articles/MAY30.htm

For pictures, see:

http://bexar-tx.tamu.edu/HomeHort/F1Column/2004Articles/MAR21.htm

Cotton Root Rot

You live in the greatest state in the Union. What can you expect down here in Texas? Fresh watermelon! Large tomatoes and sweet corn! Cotton root rot!

Cotton root rot? That is not a very pleasant topic to discuss when considering Texas expectations. It may not be a pleasant topic, but if you live in this area, sooner or later you will encounter this killer.

Cotton root rot, caused by the fungus Phymatotrichum omnivorum root rot, Texas root rot and Ozonium root rot. It is one of the most destructive plant diseases and attacks more than 2,000 species. However, either the fungus infects but does not kill monocotyledonous plants (grasses, etc.) or these plants are all highly resistant. The fungus is prevalent in calcareous clay loam soils with a pH range of 7.0 to 8.5 in areas with high summer temperatures. Therefore, the disease is limited to the southwestern United States.

Cotton root rot has been reported in Texas counties from the Red River to the Rio Grande and from Tom Green County to the Neches River. It has been a problem since the first pioneers came to Texas. In fact, the Texas A&M plant pathology department was founded to find an answer to the root rot problem. We are not much closer to finding the answer today than we were then.

Disease symptoms are most likely to occur from June through September when soil temperatures reach 82 degrees F. The first symptoms of the disease are slight yellowing or bronzing of leaves followed by wilting. Plants die suddenly after the first symptoms of wilting. Leaves remain firmly attached to the plant. Affected plants die suddenly, often after excellent growth. Large trees and shrubs may die more slowly.

Usually, the fungus has done extensive damage to the roots by the time plants have wilted. When roots are pulled from the soil, root bark is decayed and brownish, and wooly strands of the fungus frequently are apparent on the root surface. Affected plants can be pulled from the soil with little effort.

Under moist conditions, spore mats sometimes appear on the soil surface. These mats, 2 to 16 inches in diameter, are first snow?white and cottony, then later, tan and powdery. On large roots and tubers, there are numerous small cushion-like sclerotia, or resting bodies, about the size of a pinhead. At first, they are light tan but later appear dark and warty.

The fungus generally invades new areas by a continual slow growth through the soil from plant to plant. Occasionally, it spreads more rapidly on the roots of infected transplanted plants. The fungus can survive in the soil for many years, and often it is found as deep in the soil as roots penetrate. Affected areas often appear as circular areas of dead plants in fields of infected crops. These areas gradually enlarge in subsequent years as the fungus grows through the soil from plant to plant. Infested areas may increase 5 to 30 feet per year.

Cotton root rot is one of the most difficult plant diseases to control. Fungal behavior in different crops and soils, and its activity from year to year in the same area, are so erratic that it is ineffective to rely on one approach. Development of resistant plants using conventional breeding concepts, has been difficult due to the pathogen's wide host range. Also, in ornamental plants, we have to depend on observations that are not always dependable. For example, some plants that seem immune in the native state may become susceptible to root rot under the continual moisture found around homes. However, a safe index is the behavior in nature under the same conditions that you have where you intend to plant them.

However, there is a list of woody and herbaceous plants which shows resistance or tolerance to cotton root rot and should be considered by the homeowner where the disease is prevalent. See:

http://www.plantanswers.com/cotton_root_rot.htm

There are no confirmed varieties of peaches, apples or pears that are resistant to root rot. Grapes such as Thompson Seedless and the French?American hybrids are very susceptible. Many susceptible grapes can be grown on native vines such as Dog Ridge and La Pryor and resistant varieties such as Champanel and Black Spanish (Le Noir). The wild mustang is immune.

So if you have had plants die of cotton root rot and you didn't know why, now you know. Don't feel like the Lone Ranger. You are not the only one who has had the experience, nor will you be the last.