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Nematode Management Strategies

Nematode problems in orchards are difficult to control and therefore good nematode management should focus on preventive measures. In general, nematode control is accomplished with either nematicides or cultural practices. The benefits of each strategy are outlined below.

Benefits of Nematicides and Soil Fumigants
Treating orchard soil before planting trees will reduce replant problems, control parasitic nematodes, and reduce the incidence of soilborne virus diseases, such as stem pitting in stone fruit and union necrosis in apple. Broad-spectrum fumigants may be used for all three purposes and are effective against most replant disorders. Fumigant and contact-type nematicides effectively control nematodes, including species which transmit soilborne viruses. See Table 4-4 for specific recommendations.

Decisions regarding chemical control options should be based on the history of the site and the results of a nematode diagnostic test. Options include treating the entire site (broadcast treatment), treating strips along the proposed tree rows, or treating only individual trees. Broadcast treatments effectively limit contamination and reinvasion by nematodes from untreated areas. If parasitic-nematodes are the only problem, a strip fumigation may be sufficient. Postplant treatment with nonfumigant-type materials may be desirable in areas that have received a preplant treatment with a fumigant-type material.

Soil fumigants should be applied during the late summer or fall and trees planted the following spring. Spring application is an option but raises the risk of phytotoxicity. Poor decomposition of root debris and previous cover crops will reduce the effectiveness of the fumigant. Adequate soil aeration is essential. Nonfumigant nematicides work well if applied in the spring when soil moisture and rainfall are plentiful. Fall application is an option if conditions are good.

Management of soilborne virus diseases in tree fruits requires control of the nematode vectors and the weeds which serve as virus reservoirs. Careful attention should be given to eliminating such weeds from tree rows as well as the groundcover in row middles. Consult Table 4-2 for recommended herbicides

Dagger Nematode Control with Green Manure
In recent years some fumigants have been identified as a threat to public health, and causes of groundwater contamination or ozone depletion. Many fumigants have been banned for environmental protection or withdrawn from the market due to the cost of meeting new regulations. Products that remain on the market have become increasingly expensive; thus, there is a need to develop safe and environmentally sound alternatives.

Over the past few years, we have evaluated a variety of novel rotation and green manure crops for the treatment of replant sites. This work showed that some plants can naturally reduce populations of plant-parasitic nematodes and improve soil structure. Based on these results we recommend the use of selected rapeseed varieties to help control dagger nematodes.

The use of a crop rotation for nematode control can provide several benefits. Rotation crops can reduce weed problems, increase soil organic matter, improve nutrient availability, and help control erosion. In addition, decomposition of rotation crops can improve soil drainage and aeration by creating channels. This improves tree growth and promotes nutrient recycling.

Rapeseed provided all these benefits to some extent. Best results were obtained when rapeseed was incorporated into the soil while still green (a.k.a. green manure), and experiments showed that decomposing rapeseed released nematicidal compounds. The work showed that two years of rotation is desirable, but it may be possible to get the same benefit by growing two crops of rapeseed within one year.

The following timetable is suggested for producing two rotations of rapeseed within one year:

• Prepare seedbed and plant rapeseed by late April or early May. (Plant only recommended winter rapeseed varieties.)
• Incorporate (i.e., turn under) green rapeseed by early September. Prepare seedbed and plant second crop by mid-September.
• The second crop should be turned under in late spring after soil temperatures reach 45°F or higher.
• Ideal conditions for incorporating the cover crop are similar to those required for obtaining the maximum benefit from fumigation (i.e., the soil temperature should be above 45°F and moist).
• Alternatively, planting dates may be reversed so that the first planting is in the fall followed by a second crop planted in the spring. This would end the rotation cycle in fall of the following year.

Which rapeseed varieties to plant?
Some rapeseed varieties are more effective at suppressing nematode populations than others, and some varieties will not overwinter or bloom too early in summer to be useful. The winter varieties Dwarf Essex and Humus work well for both spring and fall planting dates. When planted in the spring, these varieties do not bloom but instead grow vigorously and help crowd out weeds.

Tips:

• Rapeseed requires a firm, smooth seedbed that is free of weeds, heavy residue, and large clods.
• Seed may be drilled or broadcast. Avoid planting too deep! A seeding depth of 3/8 inch is good or if broadcast, a cultipacker may be used.
• A preplant herbicide treatment such as Treflan at 1.5 pt/A can help prevent noxious weeds from becoming established.
• A seeding rate of 7 to 8 pounds per acre works well.
• Rapeseed is sensitive to broadleaf herbicide carryover.
• Fall-planted rapeseed should have 8 o t10 true leaves and a 5- to 6-inch tap root with a 3/8-inch diameter root neck before the ground freezes.
• Sulfur is necessary for rapeseed to produce nematicidal compounds. Although most orchard soil is not deficient in sulfur, a soil test to test the availability of this element may be beneficial.

Biorational Nematode Control
The efficacy of cultural practices such as crop rotation and green manure to control nematodes varies with the nematode species being controlled because of differences in biology, host range, and life cycle. What works for one nematode may not work for others. In some cases, a rotation crop that suppresses one plant-parasitic nematode may actually stimulate an increase in another. Therefore, it is important to know which nematodes are present when developing a nematode management plan. While rapeseed green manure has been shown to effectively suppress dagger nematodes, this practice is not very effective against the lesion nematode and rapeseed green manure is not recommended if there is a potential lesion nematode problem.

The lesion nematode has a broad host range and is notoriously difficult to control with rotation crops. Several plants capable of suppressing lesion nematode populations have been identified but have not proven practical on a commercial scale because of cost or because they are very difficult to establish. Marigold and black-eyed Susan are two examples. Recently, there has been work done in Canada with Canadian Forage Pearl Millet 101 (CFPM 101) that looks very promising; however, more research is needed before commercial recommendations can be made.