Nematodes associated with fruit trees in Louisiana

Charles Overstreet, Xavier, Deborah

Galling on fig tree by the southern root-knot nematode

Louisiana producers can grow a variety of fruit and vine types in the state including many temperate fruits in the northern parishes and many subtropical fruits in the southern parishes. There are a number of different types of plant-parasitic nematodes that can attack these fruits. The most damaging and readily identifiable nematode pest is the southern root-knot. This is the same nematode that attacks a wide range of vegetables, ornamentals, grasses, weeds, and agricultural crops such as cotton, soybean, and corn. Root-knot nematode is particularly destructive to several fruit trees such as fig and peach. Other nematode types such as lesion, ring, dagger, citrus, and bud and stem may be present and can cause problems.

Root-knot nematodes

The southern root-knot nematode is the major nematode pest that a producer is likely to encounter in the home landscape or commercial field. Root-knot nematodes generally prefer a sandy soil and not normally associated with soils with high levels of clay. However, it is extremely easy to move infected plants into an area where they would not normally occur such as a location with high levels of clay. The nematode may still build up in this heavy soil and cause problems to the plant.

Root-knot nematode attacks the root system and causes distinct galls along small and large roots. Galling interferes with the plants ability to take in water and nutrients. Galls can range from being quite small to very large depending on the plant type and how susceptible the plant is to this nematode. Once the root system is damaged, the above-ground part of the plant begins to show visible symptoms. These symptoms include stunting, poor growth, yellowing, and reduced foliage. Reduced yield and early plant death are the major concerns from this pest.

Root-knot nematode is a serious problem to many of the fruit plants grown in our state. Fig, plum, peach, and nectarine are some of plants that seem to be most affected by this nematode. Galling may be very severe on the roots of these fruit trees eventually leading to decline and early death.

Galling on peach roots on the image to the left and sever damage to trees on right from root-knot nematode.

Lesion nematodes

This type of nematode moves through the roots of a plant and can cause serious injury because of its feeding. Lesion nematode attacks a wide range of plants including many of the fruit trees that we plant. The nematode can readily move in or out of the root system of plants. The damage caused by feeding includes dark, discolored lesions of dead tissue on the plant root and killing of small feeder roots. Lesions may also be invaded by other root-rotting organisms. When populations are high enough, the entire root system may be brown and discolored. Damage to the above-ground tree may be stunting, yellowing, and limb dieback.

Dagger nematodes

Dagger nematodes are very large nematodes frequently found in soils that have not been disturbed for some time such as found around trees or vines. Dagger nematode feeds on root tips and can cause them to be stunted or swollen. This nematode is unique in that it can also transmit virus diseases to plants including tomato ringspot virus, grapevine fan leaf virus, tobacco ringspot virus, and several others. The viruses are picked up from the feeding by the nematode and remain within it for up to a year.

Slow decline of peach trees. Very high levels of dagger nematode found at this location and likely a contributing factor.

Ring nematodes

Ring nematodes are very small nematodes that seem to prefer really sandy soils. Although ring nematodes are not normally a serious problem on most crops, they have been linked to peach tree short life. The feeding by this pest can reduce root development and function which puts a stress on plants. This nematode predisposes peach trees to bacterial canker or cold injury.

Peach tree decline. Ring nematode was present in high levels and could have been a contributor to the decline of peach trees.

Foliar nematodes

This nematode type primarily is associated with strawberry. It causes distortion of leaves, flowers, and fruit. Aboveground symptoms also include stunted growth, reddened leaves, small leaves that may be crinkled and deformed flowers. This pest is strictly in the foliage and fruit structures and not found in the soil like the previous nematodes. Good sanitation and clean plants from nurseries limits this disease and is rare within the state.

Damage to the fruit and foliage of strawberry plants by the foliar nematode

Citrus nematode

This nematode has a very limited host range and found only on citrus, citrus root-stock such as trifoliate orange, grapevines, persimmons, and olive. Citrus nematode has been found on citrus grown in the southern parts of Louisiana and occasionally on persimmon. The citrus nematode is associated with a slow decline of trees and often does not show damage for 15-20 years. Trifoliate orange that is used as a common root stock does seem to be somewhat tolerant of this nematode. This nematode does well in some of our soils with high silt and clay content but does require very high populations to cause problems on citrus trees.

Nematode Management

Nematodes are often very difficult to manage once they become established and start causing problems on fruit trees. There are several things a producer or home owner should do to avoid some of the problems caused by these pest. One of the first things to do is be sure that you are buying healthy plants that don’t already have a problem developing. Most nurseries plant into containers that have clean soil or potting mixes and usually don’t have any nematode problems. However, it is always a good idea to take a close look at the root-system of plants coming from pots before you plant. Plants showing any type of galling or swellings on the roots are likely to be infected already. These plants will generally do very poorly when planted and likely not survive.

Mulching is one of the best ways of dealing with a nematode problem once it has been recognized or suspected in the landscape. A heavy layer of leaf mold or other type organic matter should be applied around the root area of a tree or shrub. This method has proven to be fairly effective for us with figs. One of the ways that nematodes injure plants is by reducing the effectiveness of the roots to take up water and nutrients. Apparently, mulching reduces some of this stress to the plant enabling it to better survive and continue growing. Mulches also may help stimulate some of the natural enemies that are in the soil such as fungi or bacteria that can utilize nematodes as a food source.

Some of the fruit trees such as peach have root stocks that are resistant to the root-knot nematode. These include root stocks such a Nemaguard, Nemared, Guardian, Flordaguard, or MP-29. When root-knot nematode is likely to be a problem, try to obtain plants that have one of these root stocks. Very light, sandy soils are prime areas for this nematode. The trifoliate orange has demonstrated good resistance to the citrus nematode although this nematode is not common in home landscapes.

Other management options include proper watering (don’t allow the plants to go through water stress), fertilize correctly to keep plants from experiencing any type of nutrient stress, and keep weeds from growing within the drip line. Weeds are excellent hosts for root-knot nematodes.

There some nematicides that are labeled for commercial producers. However, there are not any chemicals labeled for use by home owners.

If you are not sure if nematodes are likely to be problem or suspect that they may be, a soil sample for nematode analysis could help. The assay will give you some idea of the types and levels of nematodes present.

2/3/2017 9:10:14 PM
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