Saturday, July 9, 2016 By: Ask A Master Gardener

Wilting Plants and Fall Webworm Alert

Wilting in plants has many causes

Brian Jervis: Ask a Master Gardener

Saturday, July 9, 2016

Q: What would cause a shrub in my yard to wilt? I am watering it a lot, and it is still wilted. Carl, Tulsa.

A: Most gardeners associate wilted leaves with too little water and often that is the case. However, there are other causes of wilting for plants in the landscape and in houseplants.
To understand wilting, one must understand a plant’s circulation. Absorption of water in soil depends on having an adequate number of tiny hair-like roots. These absorb water, as well as nutrients and oxygen. After absorption, water is transported to the leaves by the cambium, a layer of tissue just under the bark. In the leaves, the pressure of water supports the leaf structure. Any interruption of the flow of water to the leaves results in loss of structural support, resulting in wilting.
It is not often recognized that too much water can produce the same effect as seen in a drought. Excessive water forces oxygen from the soil, which results in suffocation of roots which, oddly enough, prevents water absorption. Initially, this results in wilting, followed by yellowing of leaves and, if prolonged, death of the plant.
Protracted wet soil may be associated not only with changes described above, but also a higher incidence of root fungal diseases, diseases which love wet soil. These diseases, collectively called “root rot,” can kill the plant or lead to poor performance in the future.
Other conditions interrupting a plant’s circulation may produce wilting. These include temperature-related problems, either too hot or too cold. Hot weather causes water loss to exceed the ability of the plant to keep up with demand. Hot and cold weather may also damage roots in some plants. Mulch will help prevent this.
Excessive salt in the root zone, such as is seen from overdoing fertilization or from de-icers in winter, it will damage roots and cause wilt. Salt accumulation from fertilizers in houseplants is also a common cause of wilting. Periodic flushing of the potting soil with water will prevent this.
In addition, any condition that interrupts the flow of water in the above ground portion of plants will result in wilting. This includes circling roots, which strangle the plants, grass trimmer damage and string and wire bindings used to stake a tree or hold a root ball together.
One type of wilting not mentioned and that is reversible and insignificant is the wilting occurring in evergreen plants when it is cold. This is a way some plants have of protecting themselves from the cold, and it reverses with warming.
All of the above may cause wilting with or without progressive damage to plants. It is important then to try to determine what the problem is before launching into a vigorous watering program.
Master Gardeners can help you sort out the problem. Call their office helpline at 918-746-3701 or bring samples of plants into the office at the OSU Extension Center at gate #6 at the Tulsa Fairgrounds, 4116 E. 15th St.

Webworm alert
We are beginning to see large numbers of fall webworm nests in pecan, persimmon and occasionally redbud and other trees. These pests, as the name suggests, are usually more prevalent in the later part of summer and into the fall. This year’s early outbreak has prompted Eric Rebek, entomologist at OSU, to issue a “Pest e-alert” document, accessible online.
We had the same problem last year, and Rebek feels the large numbers of webs both years is related to a mild winter and wet spring.
These caterpillars build dense web nests around leaves on the ends of tree limbs. They devour the leaves within the webs and after leaves are eaten, expand locally to include new leaves. Even with large numbers of webs in pecans, the pests usually do not damage mature trees (in pecan orchards nut production may suffer). Although they are unattractive, mature trees will survive, but heavy infestations may injure or kill smaller recently planted trees.
Control is difficult. There is no effective way to predict or prevent the pests. Even though there are large numbers of parasites and predators, they are unable to control large outbreaks.
On smaller trees, the webs may be pruned out or removed by hand. Insecticides are usually not warranted. If a synthetic general purpose insecticide is used, it needs to be sprayed with enough force to enter the web. However, this is not practical or safe, especially in an urban setting. An alternative is to use one of two relatively safe organic insecticides — Bacillus thuringiensis (Bt) or spinosad. They are sold under several different brand names. These are sprayed on leaves next to the webs and will be consumed as the webs expand
Contact the OSU Tulsa Master Gardeners for more recommendations. Call 918-746-3701 from 9 a.m. to 4 p.m. Monday-Friday.


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