Protect peaches from nematodes
To lengthen tree life, control viruses and the nematodes that transmit them.
Peach trees, it is often said, love to die and will find any excuse to do it.
That’s a bit harsh. But peach trees, and other stone fruits, are much more susceptible to virus diseases than are the pome fruits like apple, and these viruses wear down orchards. Growers lose a few trees every year until, finally, the orchard is uneconomical. The name of the game is warding off tree death as long as possible. There are no cures for virus-caused diseases or for nematodes that often transmit the viruses. The name of the game is prevention.
Dr. John Halbrendt, a Pennsylvania State University plant pathologist specializing in nematode and virus diseases at the Fruit Tree Research and Extension Center in Biglerville, recommends a step-by-step approach that starts with a soil test for nematodes before planting a new orchard—a test that can be done even before an old orchard is pulled out.
Peaches are susceptible to four different nematodes, and knowing which ones are present determines the next steps. Nematodes are plant parasites that attack roots, causing loss of vigor, reduced yield, reduced winter hardiness, and that may vector viruses that kill trees.
Dagger nematodes are the most severe threat as they vector tomato ring spot virus, to which all peach rootstocks are susceptible. The virus causes peach stem pitting. Dagger nematodes by themselves cause little direct damage from their feeding on peach roots unless they carry the virus.
“Peach stem pitting is the most insidious and potentially costly disease affecting stone fruit in the Northeast,” Halbrendt said. “Infected trees show symptoms of stress and die within two or three years of infection.” Trees may become infected anytime after planting.
The natural hosts for dagger nematodes are broad-leaved weeds like dandelions, plantains, and lambsquarters. Because these weeds are widespread, so are dagger nematodes. These weeds are resistant to the tomato ring spot virus, but the peach trees aren’t.
Not all weeds are infected with the tomato ring spot virus, and not all dagger nematodes are infected. But because the virus can actually be carried in weed seeds, orchards are always at risk from new weeds introduced and growing from infected seed, Halbrendt said. His recommended approach is a combination of nematicides applied before planting and good ongoing weed control to suppress broad-leaved weeds and limit nematode access to the virus.
Grasses are not hosts for tomato ring spot virus, but they are good hosts for dagger nematodes. Grass alleys in an orchard do not pose a threat to the peach trees. The key is to keep these nematodes free of the virus by controlling nongrassy weeds.
Ring nematodes occur on sandy soil, especially in the South, and are a major cause of a complicated disease called peach tree short life.
An orchard can be fine and then collapse completely within two to three weeks in spring.
If tests show that ring nematode is the primary problem on a site, the rootstocks Lovell and Guardian provide protection, but both of these rootstocks are very susceptible to root-knot nematodes. The rootstock Nemaguard, which provides resistance to root-knot nematodes, is highly susceptible to ring nematode.
Root-knot nematode is a cause of the disease called peach tree decline. Infected orchards show a slow decline as they lose vigor and leaves.
Root lesion nematodes are associated with peach replant disease. Infected trees don’t grow or grow only slowly because the nematode kills small feeder roots and starves the trees.
Methods of control
Nematode problems are more likely on replant sites than on new sites, but new sites may be infected, so a test is recommended, Halbrendt said. Here’s the program he recommends:
• Remove tree root residues to reduce population density of nematodes and other soil-borne pathogens.
• Subsoil or deep plow to rework the soil profile and improve internal drainage.
• Rotate to field crops for at least two years to reduce pathogen populations, help eradicate weeds, and increase soil organic matter.
• Lime and fertilize to adjust soil pH and nutrient levels for optimum tree growth and fruit production.
• Submit a follow-up soil sample in the fall before tree planting to determine nematode population densities and the need for soil fumigation.
Soil fumigation is recommended if nematode densities exceed damaging levels, if the site has a history of other soil-borne diseases, or if highly susceptible cultivars are to be planted, Halbrendt said. He recommends using Telone C-17.
Because fumigation is expensive and increasingly fraught with regulations, an alternative approach is “natural” fumigation, sometimes referred to as “biofumigation.” This method involves planting a crop or, even better, two crops, one immediately after the other, of the brassica species Dwarf Essex rape. The rape contains precursor chemicals that release those that actually suppress nematodes, and these are released only when the plant is macerated.
“The crop needs to be thoroughly chopped using a flail mower and the residue incorporated into the soil to work effectively,” Halbrendt said.