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Nematode Problems in Deciduous Fruit Trees
Nematodes are economically important pathogens on fruit crops. They reduce tree vigor and crop yields by parasitizing tree roots; they predispose tree s to disease, reduce winter hardiness , and transmit viruses. Two of the most serious nematode problems in Pennsylvania are peach stem pitting and apple union necrosis and decline. Both diseases are caused by the tomato ringspot virus (ToRSV) transmitted by the dagger nematode (Xiphinema spp.).
The only natural means of infection with ToRSV is by dagger nematode transmission (see Figure 2-5). The nematode acquires ToRSV when it feeds on an infected plant and transmits the virus when it feeds on a healthy plant. In the absence of dagger nematodes the virus does not naturally spread to fruit trees.
ToRSV can be transported over great distances in the seed of some weeds, such as dandelion. See Table 2-12 for a list of common orchard weeds that have been identified as hosts for ToRSV. Dagger nematodes and ToRSV are both common in orchards of the Mid-Atlantic states.
Peach Stem Pitting
Trees with ToRSV have the general appearance of being girdled, and the leaves appear drought stressed. Superficially, the symptoms may be confused with those of a number of disorders and injuries, including root rot, nutrient deficiencies, herbicide damage, mouse girdling, and implement injury. Characteristic symptoms include reduced terminal growth and drooping leaves that may curl upward lengthwise. Leaves of infected trees may turn yellow and drop earlier than those on comparable healthy trees. Generally, infected trees produce a large number of fruits, which tend to ripen early and to be highly colored and small.
Bark from below ground portions of the trunk is unusually thick and corky. When bark is removed from an infected rootstock, pits or grooves may be seen in the wood. The severity of pitting varies with tree variety and stage of disease development. The pitting may or may not extend across the graft union. Some affected trees may break off easily at ground level or below. The tendency to break is apparently correlated with the tree's age at the time of infection. Breakage may be confused with incompatibility of the graft union.
Trees infected with ToRSV lose vigor and eventually die. Once infected they do not recover and cannot be cured. Although feeding by the dagger nematode is the only natural means of virus infection, peach stem pitting may also be spread by grafting and budding. No sources of genetic resistance to ToRSV have been identified for peaches or nectarines. Virtually all other stone fruits, including apricots, cherries, and plums, are likewise susceptible to ToRSV. Therefore, it is important to only purchase trees that have been certified virus free.
Apple Union Necrosis and Decline
Apple union necrosis is an economic problem only in commercial apple orchards. Ornamental crabapples and other Malus species appear unaffected, as do most apple cultivars on seedling rootstocks. ToRSV has been isolated from clonally propagated, size-controlling rootstocks and Malling Merton 106 (MM.106) is the most frequently, naturally infected clone.
Apple trees infected with ToRSV normally begin to exhibit symptoms when they reach bearing age. Foliation is delayed on infected trees; the leaves are small and sparse, and their color is a dull, pale green. Terminal shoot growth is reduced; the stem internodes are short, and infected trees flower heavily and set large crops of small, highly colored fruit. Partial or complete separation of the graft union is common on severely affected trees. Removal of the bark above and below the graft union reveals abnormally thick, spongy, orange-colored bark and a distinct necrotic line at the scion-rootstock union.
The severity of apple union necrosis is influenced by the cultivar-rootstock combination. Red Delicious on MM.106 rootstocks is the most severely affected combination and may exhibit severe graft union necrosis followed by decline and death. The symptoms are generally less severe on other cultivars. See Table 2-13 for a summary of rootstocks and scions that have been reported as resistant or susceptible to apple union necrosis and decline disease. Rootstocks and scions not on the list have not been reported.
Although dagger nematodes are the primary vectors of ToRSV, other factors are important in the spread of the virus in woody fruit crops. Because apples are propagated by grafting, it is important to purchase certified virus-free trees. Infected weeds can also play a major role in spreading ToRSV.
Replant Problems and the Root-Lesion Nematode
Replanted fruit trees frequently have difficulty becoming reestablished, often because of interactions between nematodes and other soil microorganisms. The root-lesion nematode, Pratylenchus penetrans, is often the cause of the problem. It is perhaps the most widespread and best-known nematode pest of fruit trees. It damages roots through feeding and intracellular migration, which destroys tissue in the root cortex. Root damage caused by this nematode promotes infection by root-rotting microorganisms. The resulting damage is greater than that caused by the nematode alone.
Root-lesion nematodes migrate and seek new feeding sites when roots become crowded or decayed. Although root impairment results in a loss in vigor and yield of mature trees, the role of root-lesion nematodes in the development of replant problems is of greater economic importance.
Orchards affected by replant disease never reach their full production potential, and there are no remedial measures that can fully correct problems after the orchard is established. Depending on the extent of the problem, infested orchards force the grower to make tough economic decisions, such as whether to keep trees that are not highly profitable or to reestablish a new orchard at major expense and loss of several years' productivity. However, replant disease can be prevented by assessing the risk of problems with preplant nematode assays and by proper site preparation.
The symptoms of orchard replant disease include stunting, yellowing of leaves, discolored and necrotic feeder roots, and in severe cases tree death within the first few years after planting. Necrotic roots may or may not show obvious lesions. Typically, affected trees show a patchy distribution, and the severity of disease may be quite variable within the orchard.
Nematode management is based on rotating crops, using synthetic soil fumigants or nematicides, or incorporating green manure of rapeseed, which releases nematicidal chemicals. These methods are outlined in Nematode Management Strategies in Part III.
Sampling for Plant Parasitic Nematodes
Nematode assay packets may be obtained from your county extension office. To take samples follow these steps:
1. If the soil in the area to be sampled is fairly uniform and is 4 acres or less in size, one composite sample will suffice. If the field is larger than 4 but less than 8 acres, divide the field into two blocks of approximately equal size and take composite samples from each block. Fields larger than 8 acres should be divided into blocks accordingly, each of which is not larger than 4 acres and has a uniform history and soil type. This is only a guideline, the smaller the area sampled, the more accurately the sample will represent the site.
2. In each field to be assayed, take a sample from each area that has a common cropping history and that will be planted with a single crop. For example, if a 4-acre field is to be planted with peaches next year and if half the field was in apples last season and the rest in woods, collect a sample from each area.
3. If the soil in the area to be sampled is variable, such as having a heavy clay soil in one portion and a sandy soil in another, take one composite sample from each soil type.
4. Preferably using a 1 x 12 inch sampling tube (or a trowel, small shovel, or similar tool if a sampling tube is unavailable), take at least 20 cores of soil from each sampling area. Samples should be taken to a depth of 9 to 12 inches.
5. Feeder roots, found at varying depths, are usually most abundant at the dripline, directly below the outer leaf canopy. Soil samples should be taken from the same area where the roots are growing.
6. Do not sample from dead or nearly dead trees. Nematodes feed on live roots and may migrate away from dying plants. Therefore, when sampling problem areas, the samples should be taken from adjacent trees that either appear healthy or show early symptoms of stress
Since nematodes are not uniformly distributed in a field, a carefully prescribed sampling procedure must be followed to obtain root and soil samples representative of the area surveyed. In addition, the samples must be properly handled and shipped to ensure that the nematodes remain alive until they are processed in the laboratory. If there has been a prolonged dry spell, or if the soil has been saturated with water for an extended period, wait until normal soil-moisture conditions return before sampling.
Handling samples
1. Make certain that all information requested is included on the nematode assay form that you receive with the assay packet. This information is needed to identify the sample and to aid in interpreting assay data. If you collect more than one sample, you must assign a field number to each area sampled and place that number in the appropriate area of the form. Each plastic bag of soil should be sealed tightly. A separate assay packet must be used for each composite sample.
2. Keep samples out of direct sunlight to avoid overheating. Samples may also be damaged by heat if they are stored in the trunk of a car or other hot location. Use a Styrofoam cooler to keep samples cool. Heat kills nematodes, and dead nematodes are unsuitable for identification.
3. When the assay forms are completely filled out and the plastic bags are sealed, place the samples in a suitable container and send or bring them promptly to:
Penn State Nematode Diagnostic Service
P.O. Box 330
290 University Drive
Biglerville, PA 17307
Phone: 717-677-6116
Normally, the U.S. Postal Service is NOT a satisfactory way to send nematode samples.
4. A fee of $15 will be charged for each sample. Payment is by check made out to The Pennsylvania State University and must be submitted at the same time as the soil sample. For more information, contact the Penn State Nematode Diagnostic Service at 717-677-6116.