Nematodes of ornamentals

Charles Overstreet  |  2/3/2017 3:22:27 PM

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Gardenia root system showing galling and discolored roots associated with the Southern root-knot nematode

Nematodes are a diverse group of wormlike animals found in a number of habitats. Many types live in the soil feeding on bacteria, fungi, or other nematodes. Some types such as the heartworm in dogs are parasites of animals. The group of greatest interest in the landscape and garden feeds on the roots of plants. Nematodes that attack plants are generally so small that a microscope is needed to observe them.

Types

Nematodes that feed on plants can generally be separated into two types. The first type can be found in the above-ground parts of a plant (generally buds or foliage). The summer dwarf and spring dwarf nematodes that attack strawberries are the best examples of these types in the garden. Both cause leaves to be deformed and appear crinkled and distorted. The second type of nematode is generally found to occur in either the soil or roots of plants. Most of our problems are caused by nematodes within this group. Root-knot nematode is the most important and best example of this group. This nematode causes damage to the root system which affects growth and development of many plants in the yard or garden.

Root-knot Nematode

Although there are number of species of root-knot nematode found in Louisiana, the southern root-knot nematode (Meloidogyne incognita) appears to be the most common. This same nematode is worldwide in its distribution and occurs in many countries that are tropical and subtropical. Growing conditions in Louisiana (long, hot summers and mild winters) are considered ideal for this nematode. Southern root-knot nematode is widespread in Louisiana and found in all 64 parishes. It is considered to be a native pest. This nematode does seem to favor a lighter soil and causes the most problems when soils are sandy.

Southern root-knot nematode causes the development of galls or knots on the root system of a plant. These are a distinct symptom on the roots that can be used to readily identify this nematode. Galls are spindle-shaped swellings of the root around the area where the nematode has penetrated the root and begun feeding. The size of galls may vary, depending on the crop. Plants such as tomato, okra, gardenias and boxwood have very large and distinctive galls if this nematode is present.

Severely infected plants wilt readily during the hottest part of the day, even when plenty of water is available. However, during dry periods these damaged plants may not be able to survive very well. If seedlings are seriously injured, then many plants will die. Surviving plants will be stunted and off-color. Plants such as shrubs or trees may show the effects over a period of years and eventually slowly decline. In some cases more than one type of nematode may attack ornamentals or flowers.

Identifying the Problem

Since nematodes may be a problem throughout the state, positive identification is critical to start a good management program. Once root-knot nematode has been identified, then it's probably a good idea to assume that it will be present every year. Nematode populations tend to go up and down each year but are very unlikely to just disappear. Even the best management practices will only reduce the numbers to low levels. Root-knot nematode can be identified through analysis of a soil sample taken from July to about February. This nematode is usually difficult to detect in a soil sample during the spring and early summer.

Management Practices

Once a nematode problem has been identified, then a management program should be developed. No single method is so effective that only one should be used. Try to use several methods that can help you reduce the nematode population. Remember that it is virtually impossible to eliminate a nematode once it has become established. Homeowners should plan to use methods that will keep the populations low enough that the nematodes cannot injure plants. If populations of nematodes are high, then use a method that can lower the numbers to avoid possible damage.

1) Antagonistic plants

There are a number of plants which have been reported to release natural chemicals from their roots which are toxic to nematodes. The most common one grown in Louisiana is the marigold. Although organic nematicides may be produced by marigolds, this does not appear to be the primary method by which marigolds affect nematodes. Root-knot nematode juveniles enter the roots of marigold just like they would a susceptible plant. However, the root reacts to the presence of the nematode and kills it before it can successfully complete its life cycle. This effect depends on the type of marigold used and also the nematode species present. The French marigold (Tagetes patula) has been shown to be effective against both the root-knot and reniform nematode. Marigolds are very common flowers grown in Louisiana landscapes. Plant the marigolds solid in the area where you want to reduce the nematode population. Allow the marigolds to grow for at least two months and till them up as a green cover crop. The effect of the marigolds will be good for only about one crop before nematodes begin building up again.

2) Clean transplants

Although many of our home landscapes or gardens are naturally infested by nematodes, it's easy for them to be introduced on infected transplants. Also infected plants may reduce the effectiveness of other management measures. Most commercial nurseries use clean potting soil, lessening the likelihood that plants will have nematodes present. The greatest danger comes from transplant beds that are in the same area year after year or if non-sterilized soil from the garden is used in a homemade potting mix. If a permanent transplant bed is used, then during the off-season use several cultural methods to reduce nematodes in this area. Small amounts of soil can be made free of nematodes either through drying for 30 minutes to one hour in the oven at low heat or freezing several days in the freezer (0°F). Freezing will not eliminate other soil pathogens.

3) Cover crops

Several crops used as cover crops in Louisiana may be of some benefit in reducing nematode problems or at least will not contribute to the buildup of the ones that are present. Cover crops that can be grown during the winter months include wheat, oats, rye, or ryegrass. Ryegrass may not be suitable since it is so difficult to manage in the home garden. Some winter legumes such as vetch, clover, or Austrian winter peas may cause the populations of root-knot or reniform nematode to buildup in the soil. Root-knot nematode is not very tolerant of a natural chemical released during the decomposition of rye, and this crop may help reduce the population of this nematode. Incorporation into the soil of most of the other winter cover crops may stimulate natural enemies of nematodes or improve growing conditions for plants. Cover crops to use in the summer or early fall are either marigolds or resistant southern peas. These cover crops can directly affect the nematodes by killing them once they enter the roots.

4) Crop rotation

Most of the problems with nematodes arise because susceptible crops are grown in the same area each year. Root-knot nematode does seem to favor some crops over others and may not build high populations on some of these crops. Whenever possible, try to change plant types to less susceptible ones.

5) Escape planting

Most nematodes like root-knot require certain soil temperatures before they are active in the soil. Root-knot nematode is not active until soil temperatures reach 65°F in the spring. Crops grown when soil temperatures are below this level are generally not very damaged by root-knot. Many of our late fall and early spring crops are grown when temperatures are too low for the root-knot nematode to be a problem. If these same crops are planted when temperatures are high enough for the nematode to be active, then the crop may be severely damaged. A good example of escape planting is the Irish potato. It is normally planted in the spring in January or early February and makes without serious injury from root-knot nematode. The times to plant potatoes in the fall are from the middle of August to the middle of September. Root-knot nematode is very active at this time and can severely damage potatoes.

6) Fallowing

Allowing the land to lay idle without a crop has been a practice used for thousands of years. Clean fallow, which involves keeping the soil free of weeds or grass, has been used to starve nematodes of a food source and reduce the populations. Many of the common weeds found growing in a garden such as chickweed, henbit, morning glory, crabgrass, pigweed, or teaweed are also hosts for the southern root-knot nematode. The only major drawback to clean fallowing is that it is detrimental to soil (both through erosion and in structure) and should be used only infrequently. A cover crop is preferred over that of fallowing, assuming the cover crop will not build up the nematode.

7) Fertilization

Most of the influence of fertilization is indirect. The damage caused by a light infestation of nematodes may be reduced by the increase of higher rates of fertilizer. Since you are improving only the growth of the plant, nematode levels may be much higher on these plants at the end of the growing season. If any nutrient is already low in the soil, damaged roots by nematodes will certainly make it much more difficult to obtain. Soil test to determine the levels of various nutrients, and make sure adequate levels are present. However, don't use excessive amounts because of a salt buildup or pH problems from too high levels of fertilizer.

8) Organic mulches

Adding organic amendments to the soil is an effective method of reducing damage by nematodes. These amendments may have an effect on the nematode population or plants in several ways. These organic amendments may stimulate microorganisms in the soil that attack nematodes and reduce the populations of the pest nematode. Many different types of organisms use nematodes as a food source. One group in particular, called the nematode trapping fungi, has been of great interest because of their unique ability to form trapping structures to catch and kill nematodes. Several of these nematode trapping fungi are very common in Louisiana.

Organic amendments may improve soil structure, water holding capacity, and plant nutrition which makes for better growing conditions for plants. Plants growing in a good soil environment may be able to tolerate nematode injury before symptoms begin to develop.

As some organic materials decompose in the soil, they may produce chemicals such as ammonia which can kill nematodes. Organic amendments that are fairly high in nitrogen such as animal manures, shells of crawfish, crab, or shrimp, green manures, and hays from leguminous crops may provide better nematode control than some of the other amendments which are low in nitrogen.

9) Resistant varieties

Since the southern root-knot nematode is widespread throughout the South, considerable research has been spent on developing varieties of plants that have resistance. Resistance has been found for some crops and should be used whenever root-knot nematode is a problem. Most of the varieties that have been developed are vegetables or agronomic crops such as soybeans. Tables 1-2 list some vegetables, annuals, and shrubs resistant to the root-knot nematode.

Table 1. Some annuals that have been reported to be immune or have some level of resistance to root-knot nematode.


AgeratumGlobe amaranthStock
Blue sageLupineSweet alyssum
CoreopsisMarigold (African or French)Thumberiga
CosmosPhloxTorenia
GallardiaRudbeckiaVinca
Berbera daisyScarlet sageZinnia


Table 2. Some shrubs that have been reported as resistant or tolerant of the root-knot nematode.


AzaleaJuniper
Camellia spp. cv, Blue Rug
sasanqua cv. Shore
japonica cv, Spiny Greek
HollyNandina
cv. BurfordiPhotinia
Yaupon

10) Rotavation

Some nematodes found in coarse textured soils (containing a high percentage of sand) may be injured by abrasion if the soil is intensively tilled. Although it may reduce some of the nematodes, it may not be sufficient to prevent damage.

11) Sanitation

Included under this category are such things as weed control, crop residue destruction, and disinfection of equipment. Since many weeds are hosts of nematodes such as root-knot, it is important that management practices include a good weed control program. Plants should be tilled up or removed as soon as they are through producing to prevent any further nematode development. Plants that have badly galled roots should be removed from the garden for disposal. Don't add them to a compost pile unless you are sure that the temperature will get high enough during composting to destroy nematodes. Nematodes can be killed by heat when temperatures reach 111-118°F for a short time. Nematodes in the infected roots can also be killed by exposure to the sun. Equipment can spread nematodes from one area to another. If nematodes are a problem in one area, it may be a good idea to wash off equipment before moving to another location.

12) Site selection

If sufficient land is available, gardeners with nematode problems should try to rotate their plant site every few years. Try to select areas which have been in pasture or grasses. Many gardens or ornamentals can be planted in only one area. Even if you cannot move the site, plan to use a careful rotation of the crops that you plant.

13) Trap Crops

The idea of trap crops is to plant a susceptible crop for a short period, allow the nematodes to attack it, and then till it up before the nematodes have a chance to reproduce. There is great risk in using susceptible crops. Southern root-knot nematode can develop egg-laying females within 20 days of entering a root during warm conditions. The life cycle is longer when temperatures are lower. If the timing of crop destruction is off, then you may end up with a higher population of nematodes for the main crop. The best trap crop to use is the French marigold. It allows the nematode to enter the roots but kills it before it has a chance to reproduce.

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