Above ground or below, fruit and crop production sees its share of microbes, insects and animals that can help or hinder plant health. Among the organisms beneath the soil’s surface is the perceivably notorious nematode. Though the word nematode may strike up fear in growers, many more “good” nematodes exist in the soil than “bad” ones.
These microscopic, unsegmented invertebrates live in soil and water around the world; they can even be found in the Arctic, says Agriculture and Agri-Food Canada research scientist Qing Yu (in photo).
Nematodes can be separated into two main groups: entomopathogenic and plant-parasitic; the former benefits crops and the latter harms plant populations.
Entomopathogenic nematodes (EPNs) help break down organic matter and can infect and kill insects that could harm plants. In berry crops, beneficial nematodes can be used to control insects such as root weevils and white grubs, says Ontario Ministry of Agriculture, Food and Rural Affairs fruit crop specialist Erica Pate.
Plant-parasitic nematodes damage plant root systems by burrowing into roots to feed or by inserting a long stylet, which is a needle-like structure. Root damage reduces the plant’s ability to take up water and nutrients as well as makes the plant more susceptible to bacterial and viral diseases and environmental stressors. The three main plant-parasitic nematodes that impact the tree fruit industry are root lesion, dagger and ring nematodes.
Diagnosing the damage
Damage caused by parasitic nematodes can be hard to diagnose because the resulting above-ground symptoms are non-specific and are similar to other crop-harming issues, including nutrient deficiencies.
“If the nematodes parasitize the crops, then crops will show symptoms of yellowing, stunting and sometimes death,” says Yu.
According to Yu, nematode populations are generally not widespread in a field but instead found in hot spots. As a result, nematodes may cause poor tree vigour in patches of an orchard. Young trees severely infested with parasitic nematodes often become unproductive or die. Replacement trees planted in an infected area become infested and may also die.
Unearthing strategies for success
Since nematodes are microscopic, the best way to monitor presence and population density is with soil and root samples. Nematode populations are highest in the fall when most of the population has moved from the roots to the soil, which is the optimal time to send samples to a diagnostic lab. Producers can also take samples in the spring after the soil has warmed up but fall sampling provides time to plan for the next season if parasitic nematodes are detected.
Monocultures are one contributing factor to parasitic nematode issues, Yu says.
“If you are continuously growing one crop in a field, year after year, you’ll likely have problems with nematodes,” he says.
Producers with long rotation tree fruit orchards, for example, can use integrated management strategies that leverage diversity to improve soil health and thereby increase the number of beneficial nematodes.
“Applying soil amendments like manure, compost, green manure,” says Yu. “They become the food for bacteria and fungi which feed beneficial nematodes. If you have several plant species in a particular field, you usually won’t have nematode problems because you are promoting diversity.
“A more diversified system is more stable. It has been proven scientifically that more diversified nematode communities are healthy and suppress plant-parasitic nematode populations.”
Pate agrees. “Growers can promote beneficial populations by reducing stresses on the EPNs, such as tillage and the application of nematicides, and manage or increase organic matter in the soil,” she says. “EPNs will benefit from soils with a higher organic matter that has less compaction and more pore spaces, and a greater diversity of life in the soil.”
Pate adds that growers should start with clean plants, grow nematode-suppressing cover crops before establishing horticultural crops and control weeds to reduce alternative hosts.
“If plant-parasitic nematodes are present in the soil, be sure to plant crops that cannot be a host,” she says.
Targeting pests with precision
Pre-plant soil fumigation is an option to reduce nematode populations. However, no chemical options are available for use on fruit-bearing trees in Canada. Like many bacteria controls kill off both good and bad bacteria, the broad-spectrum nature of nematicides reduces both beneficial and harmful nematodes. The products are often highly toxic to nontarget aquatic and terrestrial organisms and can radically change the soil flora, fauna, mycorrhizae and the surrounding environment.
In May 2023, University of Toronto researchers developed a new way to target plant-parasitic nematodes through bioactivation. During the study, researchers worked with a chemical compound called selectivin, which kills nematodes with minimal impact on other organisms. The chemical only becomes toxic when metabolized by nematodes; otherwise, the substance is benign on its own.
All nematodes—good and bad — are part of the ecosystem, says Yu. “The ones we focus on most feed on plants and are called plant-parasitic nematodes. For us, they are pests, but for the ecosystem, they are part of the nutrient cycle.”
With sound land stewardship and best management practices, producers can rest assured that organism diversity and healthy soils will contribute to bumper crops without too much interference from undesirable nematodes.
As seen in Orchard & Vine Pre Spring 2024
