Apple seedlings grown in soil infested with root-lesion nematodes (on the left) and in soil without nematodes (on the right).
Whenever there is evidence of nematode problems in an orchard or vineyard—such as poor growth or yields—pathogens are usually working in concert with the nematodes, says Dr. Tom Forge, researcher with Agriculture and Agri-Food Canada at Agassiz, British Columbia. “They are rarely the sole issue.”
That’s why Forge advises growers to focus on enhancing soil health in order to address all the various aspects of the disease complex, not nematodes alone.
Nematodes, which are microscopic roundworms, are the most abundant multicellular organisms on earth. There are more than 25,000 species. A cup of typical, healthy orchard or vineyard soil contains anywhere from 1,000 to 5,000 nematodes.
The vast majority of those nematodes feed on bacteria and are involved in decomposition of dead plant material into simple nutrients, but there are also plant-parasitic nematodes in the mix, which are the ones of concern to growers. In certain soil conditions, the plant feeders can become predominant to the point of being damaging to the trees or vines. Nematode problems tend to be worse in poor, sandy soils where trees or vines are suffering water or nutrient stress.
Nematode problems develop very slowly compared with problems caused by microbial plant pathogens, Forge said. It can take years before the symptoms become apparent above ground, especially with perennial, woody plants.
Forge said scientists’ understanding of the impacts of nematodes on trees is limited and tends to focus on replant situations because most experiments are done with young trees in pots, so that plants growing in soil with or without nematodes can be compared side by side. Nematodes are continually feeding on the root system. Consequently, they are a chronic drain on tree vigor, affecting tree health over multiple years, which makes it difficult to make a direct link between high populations of nematodes in an established orchard at any given point in time and the degree of damage in older trees, Forge said. “It’s a very complicated situation.”
Tree fruit and grape growers don’t have the option of taking out the crop and replanting the following year, and as the populations build up year by year, the stresses on the trees become magnified.
Root-lesion nematodes (Pratylenchus species) are the most prevalent plant-parasitic nematodes in the world. These worm-shaped nematodes measure about two thirds of a millimeter long and are known as migratory endoparasites because they wander in and out of roots. They use their piercing and sucking mouth parts to penetrate root cells, and then live and reproduce inside roots, causing dark lesions. The blackened, necrotic areas of the roots are susceptible to invasion by opportunistic soil fungi, some of which might be pathogens. When nematodes are present, these fungi, such as Cylindrocarpon, can more easily get into the root system.
The root-lesion nematode P. penetrans is not a new problem in fruit production. In the 1930s, scientists in California demonstrated its effects on potted apple trees. During the 1940s, nematicides were introduced that provided good nematode control and increased fruit yields. In the 1950s and 1960s, researchers demonstrated the detrimental effects of P. penetrans and developed population thresholds for young trees. They found that a population of more than 300 root-lesion nematodes in one liter of soil could reduce tree growth by 40 percent.
In 1955, a survey in British Columbia’s Okanagan Valley found P. penetrans in 40 percent of apple tree roots and 34 percent of cherry tree roots. In the 1980s, another survey revealed P. penetrans in 80 percent of orchard blocks, and almost a third of those orchards had populations of more than 1,000 nematodes per liter. “Undoubtedly, those orchards were suffering some degree of yield loss and growth suppression as a result of nematodes,” Forge said.
Studies were done on soil treatments, such as pasteurization and biocides, and the research highlighted the important role that nematodes play in replant disease, even though other organisms can also suppress growth in replanted orchards. Research was also done showing the efficacy of nematicides on established trees.
Ecto-parasitic nematodes attack roots from the outside, feeding on the epidermal cells, and are generally not as damaging as root-lesion nematodes. They include the dagger nematode, which is widespread. Dagger nematodes in the Okanagan Valley are not very damaging in their own right, Forge said, but they are possible vectors of viruses, of which tomato ring spot virus is the greatest concern.
Ecto-parasitic nematodes also include ring nematodes, so called because of the annules around their bodies. Ring nematodes were overlooked in the Okanagan Valley until around 2004 because it takes a different soil extraction method to detect ring nematodes than scientists were using, Forge said. “When we switched to a different extraction method to intentionally look for ring nematodes, we started finding them in many different vineyards.”
Ring nematodes are not a concern in apples, but have long been associated with problems in stone fruits, and particularly with peach tree short life, a disease complex that involves winter injury, pseudomonas canker, and cytospora canker, as well as nematodes.
“Nematodes are always associated with this kind of downward spiral as well,” Forge said.
About a decade ago, studies were done in California on potted peach trees in the field to assess the effect of ring nematodes on the trees’ susceptibility to pseudomonas canker. Some trees were planted in soil with ring nematodes and some without. The trees were then inoculated with the pseudomonas pathogen. The effects of treatments with calcium, indoleacetic acid, or urea were also tested. On trees planted in soil with nematodes, pseudomonas and cytospora cankers grew at a much faster rate than on trees planted in soil without nematodes.
“These nematodes can cause growth suppression by themselves in the absence of other pathogens when they’re in high numbers,” Forge said. “But this gives us another level of understanding of their potential role in orchard dynamics.”
This kind of interaction has been observed in peach, prune, almond, and peach, but there is no data for cherries. Forge said he hopes to do research in the future to bring more clarity to the role ring nematodes might have in cherry orchard issues.
In British Columbia, short-term nematode control strategies include preplant fumigation with Vapam (sodium methyldithiocarbamate) or a postplant nematicide application of Vydate (oxamyl) on nonbearing trees. Short-term cultural or biological controls include using nematicidal cover crops, such as marigolds, green manure cover crops, or biofumigants.
Marigolds are among plants that are antagonistic to nematodes because of compounds they emit. However, Forge said planting a marigold rotation cover crop before establishing the orchard is not practical because the flowers are difficult to grow economically, and it means that the land is out of production for a year.
Experiments have been done in other crops with cover crops of Saia oats and Wheeler rye, which look promising. Dr. Mark Mazzola, plant pathologist with the U.S. Department of Agriculture in Wenatchee, Washington, has been researching green manure Brassica cover crops, such as canola, rape, and mustards, as well as using a Brassica seed meal as a planting hole amendment. As the tissues of these plants break down, they give off compounds that are toxic to nematodes.
But Forge said results with Brassica cover crops have been hit and miss. Some Brassica species are hosts of nematodes, so the plants must be incorporated into the soil to get the green manure effect. To be effective, there must be sufficient biomass, and it needs to be incorporated into the soil at the right time.
Mycorrhizae can deter nematodes from attacking plant roots, but that effect has only been observed in fumigated soil, Forge said. Mycorrhizae are present in the soil, and a preplant treatment with mycorrhizae may or may not be beneficial, depending on how degraded the soil is and whether it has been fumigated.
Forge said he sees great potential for long-term management of nematodes, rather than short-term controls. This would involve identifying cultural practices that prevent the build-up of nematodes to damaging levels, even though there will always be some nematodes. Long-term strategies will involve focusing on soil health, which boils down to optimizing conditions for root growth and perhaps fostering the build-up of natural enemies of nematodes, which might minimize the build-up of nematode populations. Ultimately, the development of nematode-resistant and tolerant rootstocks also needs to be considered more seriously.
“It’s time to consider alternatives to fumigation,” he said, noting that fumigants are broad-spectrum biocides that have negative impacts on soil health and the environment and are likely to be subject to increasing restrictions.
Composts and organic mulches can help suppress some plant diseases caused by fungi. For a number of years, Forge has been studying how they might also suppress nematodes on perennial crops under field conditions. Although he’s not seen much nematode suppression, he has seen improvements in root growth. “Regardless of whether you have nematode issues or not, composts are generally good for enhancing root growth and improving soil health,” he said.