Plant-parasitic nematodes in orchards

Have you noticed a decline in your yields without a clear explanation? Experts warn that it could be due to plant-parasitic nematodes. By Jorisna Bonthuys.

Each handful of soil contains hundreds of nematodes, most of which are so small that you need a microscope to see them. The majority of nematodes are free-living, meaning they are active in soils, and feed on a variety of organisms in their environment. A few nematodes infest and damage roots — these plant-parasitic nematodes are ubiquitous in agricultural soils.

For producers, such infestation poses a significant threat. “Although they are microscopic, these pathogens can have a negative influence on the health of a large variety of crops,” says Sheila Storey, director of nemlab, a diagnostic laboratory specialising in soil and root analyses.

Tiny pests of huge concern

Plant-parasitic nematodes are worm-like organisms, 0.4–1.5 mm long. They have a long, thin, hollow tube called a stylet that serves as a specialised feeding apparatus. It is used to inject toxins into plant cells and to extract plant sap.

The damage caused by plant-parasitic nematodes to roots reduces the uptake of water and nutrients. This affects tree growth and fruit yields.

Harmful nematodes are divided into ectoparasites, migratory endoparasites, and sedentary endoparasites. Ectoparasites, such as ring nematodes, feed outside the roots. Migratory endoparasites, such as root-lesion nematodes, feed as they migrate through the roots, while sedentary endoparasites, such as root-knot nematodes, tunnel into the roots, and establish feeding sites from which they do not move.

Plant-parasitic nematodes are not able to move far under their own steam. They are mainly transported by humans, leading to uneven or patchy infestation in a field or orchard.

“In most cases, nematode-related damage is only detected once patches of dwarfed, poor or stunted trees are observed,” says Storey.

Other symptoms include yellowing of the leaves, early wilting, reduced fruit size, and production loss over 3–5 years. Affected areas in orchards can vary in size from a few square metres to several hectares, and will expand over time.

Plant-parasitic nematodes also play a role in replant disease. Replant disease is caused by the build-up of a variety of harmful microorganisms in the soil, including disease-causing fungi. It is a widespread problem in intensive tree-growing areas, affecting stone- and pome-fruit producers.

Table 1: The host-nematode relationship in pome and stone fruit

Type of nematode	Apples	Pears	Apricots	Peaches	Plums
Root-knot nematode*			Unknown	Very important	Very important
Root-lesion nematode	Very important	Rare	Rare	Important	Unknown
Citrus nematode
Ring nematode			Very important	Very important	Very important
Dagger nematode	Very important	Important	Rare	Important	Very important
Stubby-root nematode	Rare	Important		Rare
Pin nematode	Rare		Rare	Rare
Spiral nematode	Indicator	Indicator	Indicator	Indicator	Indicator

*Not applicable to root-knot-nematode resistant rootstocks.

Very important: can cause severe damage. Occurs commonly.

Important: can sometimes cause severe damage in high numbers. Occurs commonly.

Rare: seldom causes a problem. Little knowledge available.

Unknown: insufficient evidence to establish host status.

Indicator: indicator of soil organic matter.

 

Table 1 summarises some of the more familiar nematodes that occur in pome and stone fruit.

Because root-knot nematodes do not attack pome fruit, many producers believe that pome fruit are not attacked by any nematodes. This is not the case. Pome-fruit trees are attacked by root-lesion, dagger, and stubby-root nematodes. Stubby-root nematodes are often the cause, in very sandy soils, of severely stunted pear trees.

Nematode problems in pome and stone fruit primarily stem from the fact that soil is reused, new trees are planted in soils previously used to grow the same crop, and new trees are established within four months of the removal of the previous crop, without fumigation,” says Storey.

The source of the problem

Soil is the most important source of nematode infestation, according to Storey. “The scale of the infestation depends on the previous crop, cover crops, weeds or natural vegetation growing in the soil. If any of these plants were a host for the plant-parasitic nematodes that occur on fruit trees, the nematode population will increase rapidly in the presence of these fruit.”

For this reason, it is essential to minimise the nematode population before planting new trees, says Storey. “Always remove as many of the old roots of the previous crop as possible, as they may still contain nematodes, especially endoparasitic nematodes.”

Storey recommends collecting soil and root samples to test for nematodes before removing trees, vines, or other crops. Once new plantings have been established, wait at least eight months before sampling. Samples can be taken throughout the year in existing orchards, but keep in mind that nematode populations are highest in the summer months.

Rooted plant material is another possible source of infestation, especially of migrating endoparasitic nematodes that live inside roots. Nematodes with long stylets, such as ring nematodes, can become attached to roots, and be transported on infested plant material.

The current plant-material certification scheme requires only visual freedom from nematode infestation. Yet, the symptoms of a nematode attack on pome fruit are not easily seen. In Storey’s opinion, this certification for pome fruit is of no value when it comes to nematodes. “There are, however, plans to ensure that nurseries provide relatively clean plant material in future.”

Water from fast-flowing rivers in areas with significant agricultural activity is considered a minor source of infestation compared to the others, as few nematodes occur in water. The build-up of nematodes in soil due to infested water sources is slow. Trees planted in relatively clean soil will also resist this build-up.

Dealing with nematode infestation

There are four main control options for nematodes in orchards, depending on the severity of infestation and on the time between plantings: treating the existing crop, allowing the soil to rest between plantings, replanting with resistant rootstocks, and fumigation.

More than one treatment might be necessary, especially if the re-establishment of trees is planned for the same year as orchard removal. Decisions about control should be based on a laboratory report.

“The intensive cultivation of nematode host plants creates a mechanism through which nematodes can multiply and quickly reach pest status,” says Storey. “In such cases, let the soil rest for at least a year, and preferably three years.”

Plants that are either poor hosts, or non-hosts, can also help to reduce nematodes. Non-hosts of plant-parasitic nematodes include marigolds, rattlepods, and lovegrasses. Poor hosts include oats, triticale, and wheat. It is important to analyse the infestation again before the crop dies or is ploughed in, says Storey.

Where root-knot nematodes are a risk and soil fumigation is not possible, nematode-resistant rootstocks can be considered. Most stone fruit that are not grown on resistant rootstocks are exceptionally susceptible to root-knot nematodes. Resistance to root-knot nematodes does not confer protection against other nematodes.

Solarisation and biofumigation are other options for controlling disease-causing soil-borne organisms. Solarisation raises the soil temperature above 50°C by covering a moist soil surface with a thin, transparent polyethylene plastic. Solarisation will reduce, but not eradicate, the nematode population.

Biofumigation relies on plants, especially mustards, that produce volatile compounds. The plants are harvested prematurely, finely slashed, and incorporated into the soil. As for conventional fumigation, biofumigation is followed by fallowing for 10–14 days. Fumigation can only be carried out before planting new crops.

“Over the last few years, we have made significant strides in improving our understanding of harmful nematodes in orchards,” says Matthew Addison, crop-protection programme manager at Hortgro Science. For more about the three nematode-related projects funded by Hortgro, turn the page to our next article.

Image: Ring nematodes under the microscope.

Supplied by Antoinette Malan | Stellenbosch University.