Plant parasitic nematodes are microscopic in size which explains why they are often overlooked as causal agents. In addition, the above ground symptoms are typical of a large number of infectious and non-infectious root diseases. Nematodes live on the thin film of water that surrounds each soil particle and are, thus, very sensitive to dry soil conditions. Nematodes move very slowly in the soil but are moved in running water or contaminated equipment. If nematodes are suspected in an area, submit a soil sample to the Texas Plant Disease Diagnostic Laboratory. Control methods are the same as for root knot.
The Pathogen (Meloidogyne spp.): Although different species of root knot nematodes vary in their host-parasite relationships, all have basically the same life cycle. The infectious stage of this nematode is the second stage larvae which occurs free in the soil (The larvae has already molted once in the egg). The second stage larvae will penetrate the plant at or near the root tip and become sedentary. An enzyme is released which causes the plant cells surrounding the head region to enlarge, forming giant cells which serve as a source of nourishment for the parasite. The female nematodes swell until they become pear-shaped or oval. During this time, the nematode undergoes two more molts. Females begin laying eggs around 20 days after penetration of the host. The average female will lay approximately 30 eggs a day for two weeks. The average life cycle is 25 days. Populations will build up rapidly when environmental conditions are favorable.
Control:
(1) Crop Rotation - a three or four year rotation program with resistant crops is an effective program. Most of the cereal crops are fairly resistant.
(2) Resistant or Tolerant Varieties - Some vegetable and field crop varieties have resistance to root knot nematodes, and are advertised as such. Reaction of several ornamental plants is given in a table at the end of the Shrubs section on root knot disease.
(3) Clean Summer Fallow - dry summer fallow with cultivations every 3 to 4 weeks is an effective method of reducing nematode populations. This method may be impractical in some instances.
(4) Selection of Planting Stock and Planting Sites - Select transplants free of root knots. Plant roots should be washed and carefully inspected for signs of nematode injury. Select planting sites free of nematode infestations. An indicator crop of tomatoes, okra or other susceptible plants could be grown in the area if you do not know rotation histories. Submit soil samples to the Plant Nematode Detection Laboratory for analysis.
(5) Chemical Control - Nematicides are effective, well accepted and can give good economic returns on high value crops. Treatment is sufficient for a year and retreatment usually will be required the following year if susceptible plants are to be grown. Application methods are dependent upon the type of material used, along with conditions in the area requiring treatment. Most nematicides are either injected into the soil or washed in by a water drench. Nematicides may be purchased in liquid or granular forms, with application directions dependent upon the type of plants growing or to be grown. When possible, nematode-infested soil should be treated before planting seed or setting transplants.
Nematicide Application Methods:
(1) Preplant-application - First decide whether row or broadcast treatment is desired. If plants are to be cultivated in a row, treat only the potential root zone. Soil should be tilled deeply to prepare a good seedbed. Cover immediately to prevent material loss. For best results, soil moisture conditions should be just right for cultivation or planting and the soil temperature should be between 60 and 80 degrees F. Nematicides diffuse approximately 6 inches from point of application to give a treatment zone 12 inches wide and 12 inches deep. Applying materials on 12-inch centers results in overall or broadcast soil treatment.
(2) Side dressing nematode-infested plants - Make sure the material on hand can be used around living plants without causing injury. Dig a trench to the side of the plants as directed on the label. Cover the material immediately and water to cause diffusion of the material into the root zone. Make sure all label precautions are followed as they relate to chemical rate and plant growth stage.
(3) Drench for established shrubs and trees - Infested shrubs and trees may be treated by building a dike around the tree base that will contain water-diluted materials until they penetrate the root zone. The dike should be large enough in diameter to accommodate a majority of the feeder roots. The nematicide should be accurately measured and diluted in sufficient water to penetrate the root zone. After material penetration, add water to wash the nematicide down into the root zone.
(4) Treatment of established turf - After application of Granular materials, water to leach the material into the root zone.
The use of the fungicide Kocide 101 at the rate of 4-6 pounds/100 gallons of water will control the plant. Applications should be made in the spring (late February, March or April) just prior to the normal spring rainy season. Kocide applied during dry periods will not be as effective. When ball moss growth is dense, an additional application should be made 12 months later. Generally, no more than two applications are required for control of ball moss depending upon how large an area is sprayed and how many trees are in the area that can serve as a seed source.
Spanish Moss (plant - Tillandsia usneoides): This long, whisker-like plant growth hangs from trees in Southeast and East Texas. It is an epiphyte. It grows on hardwoods and conifers along rivers and creeks in the more humid areas of Texas. It is a member of the pineapple family (Bromeliaceae). It has been reported to kill trees where development is extensive. This seldom occurs. It can be removed using mechanical means where growth becomes thick enough to cause tree damage. Chemical treatments are not recommended.
Mistletoe (parasitic plants - Phorandendron flavescens var. villosum, P. flavescens var. pubescens, P. flavescens var. macrophyllum, P. bulleanum var. bolleanum, P. bolleanum var. capitellatum, P. juniperinum): Mistletoes are parasitic plants which derive their food from the host plant. In severe cases mistletoe will kill trees. The seeds of mistletoe are borne as white fruit on female plants. These berries are sticky and are spread by birds. The seed germinate and penetrate young, thin bark. As the seed germinate, a haustorium is formed which penetrates the cambium and eventually on into the wood. Infection often causes large swellings to be formed around the point of entry.
Chemical control has not proven satisfactory, even though a large number of chemicals have been evaluated. Physical removal has been used but is only successful if the haustoria are removed. This may mean removing a large portion of the limbs in some trees. Cuts should be made 12 to 18 inches below the area where the mistletoe is attached. A hoe or rake have sometimes been used to brush the plants off the trees, but this gives only short term relief with new growth springing up from the point of attachment.
Lichens: A lichen is a combination of a fungus and a green or blue-green alga enclosed by the fungal hyphae. The fungus obtains food from the alga, which manufactures food through photosynthesis and the alga receives some of its food and protection from the fungus. Three forms of lichens exist - crustose (flat type of growth), foliose (leaf-like but with prostrate growth), and fruticose (bush-like and erect or hanging growth). The effect of lichens on trees is only slightly damaging. Heavy lichen growth indicates poor tree growth as a result of some other cultural problem. Lichens can restrict gas exchange from the limb or twig and can restrict the amount of light received by a limb.
Materials used in the control of ball moss will kill the lichens for a short period. Regrowth usually occurs within the same year after the tree was sprayed. Chemical control of lichens is not currently recommended. This is due to two reasons, one, chemicals are currently not cleared by EPA and control has not been of long enough duration to warrant spraying. Rather, trees should be encouraged to develop a dense canopy which will shade out the lichen growth.