5.7 Fumigants and Nematicides for Tree Fruits

Whenever a new orchard is being planted in an old orchard site, preplant soil fumigation should be considered. Nematodes, particularly Pratylenchus penetrans (the lesion nematode), can seriously injure the roots of a newly planted tree, restricting its growth and future productivity. This type of injury is most common on cherries, but all fruit trees are susceptible to some extent. Nematodes are more likely to build up to damaging levels in sandy and loamy soils than in heavier soil types. The dagger nematode, Xiphenema americanum, can also cause serious losses at population levels too low to cause serious root injury by its transmission of the virus that causes stem pitting of stone fruits, the constriction disease on Stanley prune trees, and apple union necrosis on trees grown on MM.106 rootstock.

Where apples are being replanted following apples, soilborne microorganisms can damage young tree roots and severely reduce the growth of the new trees. This effect can occur in all soil types, regardless of nematode levels. Trees in such situations frequently show a significant growth increase following preplant treatment with a broad-spectrum fumigant that kills fungi and other microorganisms as well as nematodes.

Although fumigants are frequently cost-effective, their potential benefits are often lost when they are applied to soils that are cold and wet or are otherwise poorly prepared. For best results, the site should be plowed as deeply as possible (at least 12 in.) after the old orchard is removed, and all old roots that turn up should be discarded. The land should then be limed, fertilized, fitted, and planted with a cover crop such as creeping red fescue, perennial ryegrass, or sudangrass. The land should be cover cropped for at least 1 yr, preferably 2 or 3. Near the end of July during the year before new trees are to be planted, the cover crop should be plowed under and nitrogen added to improve its decomposition. Poor decomposition of the cover crop or other weeds will reduce the effectiveness of the fumigant. The site should then be disked repeatedly to keep down weeds and work the soil into seedbed condition. The soil should be loose to a depth of at least 10 in. and free of any large, hard clods that the fumigant can’t penetrate. Fumigants are generally most effective if applied while the soil is relatively warm and dry. The best time for fumigation is mid-August to early September, although materials may still be effective if applied until mid-October under some conditions. In NY state soils, best results have been obtained with broad-spectrum soil fumigants (Table 5.7.1).For cost effectiveness, fumigants should be applied in bands 8 ft wide centered over the row. The material should be injected to a depth of 12 in. with shank applicators 6 to 8 in. apart for *VapamHL. Rolling or cultipacking after treatment will help seal the surface and improve the activity of the fumigant. Before the trees are planted in the spring, the soil should be disked, care being taken not to throw unfumigated soil onto the treated band.

Postplant Nematicides. Where soil analysis shows high populations of parasitic nematodes in a young orchard, a postplant application of a nematicide may be justified. Currently, one chemical is registered on nonbearing trees for such a purpose (Table 5.7.2): Oxamyl (*†Vydate L) - mix 2 qt *†Vydate L plus 4 oz of a surfactant/100 gal of water and apply as a foliar spray to the point of runoff. Make 4 applications on a 14- to 21-day schedule, beginning at first full leaf (about the time that petal fall occurs on mature trees).