Understanding Nematodes and What to Do

Nematodes are microscopic roundworms found everywhere but quite common in managed soils. A fertile soil may contain billions per acre. Most don’t cause plant damage. The ones that do are called plant parasitic nematodes because they feed and rely on the energy and nutrients derived from plants.

Some parasitic nematodes are beneficial such as the so-called entomopathogenic nematodes that parasitize insects. They parasitize many different types of soil insects including so-called “grubs” like white grubs and other larvae of butterflies, moths, beetles, and flies. Some parasitize adult crickets and grasshoppers as well. These can be found marketed under several different trade names.

One of several products that use beneficial nematodes

Nematodes are not a huge problem in most turf and landscapes. Some may never encounter them. But when they are present, they draw a lot of attention because they are difficult to control. Nematode damage falls into that category of “out of sight out of mind”. But once plant damage from nematodes is identified, they are no longer “out of mind”.

Root knot nematode on tomato

There are two primary groups of nematodes that concern us in horticulture; those that feed upon plant roots and those that feed on plant foliage. Those that feed on plant roots live their entire lives in the soil. Those that feed on plant foliage spend most of their time above ground, feeding on leaves and succulent stems.

Root knot nematode on mulberry

Most nematodes do not just attack one type of plant but might feed on a number of different plants. Generally speaking, nematodes that feed on plant roots can damage turfgrass, ornamentals, nursery plants, houseplants or tropicals and greenhouse plants. Nematodes that feed upon plant foliage are, for the most part, restricted to ornamentals, nursery and greenhouse plants.

Sometimes we discover soil dwelling, plant parasitic nematodes when infested roots are exposed during soil preparation. The most common soil dwelling nematode is the root knot nematode. They leave behind root nodules or “swellings” on the roots.

But most of the time we see above ground plant symptoms which cause us to inspect the roots. Aboveground clues to a nematode attack to the roots include leaf yellowing and scorching, leaf drop and poor or stunted growth.

Mulberry infested with root knot nematode but kept going with plenty of water, compost and fertilizer

Sound familiar? Nematode damage can be confused with nutrient deficiencies, drought, salt problems, root damage, under or over fertilizing and plant disease.

However, depending on the type of nematode, root damage may vary from the presence of galls to the stunting and decaying of roots. In some cases, nematode damage might be confused with root disease.

Types of root damaging nematodes include the stunt nematode, lesion nematode, ring nematode, cyst nematode, spiral nematode, and lance nematode which produce other symptoms. These include shortened or stubby roots, malformed roots that are multi-branched, darkened or browning lesions which resemble plant disease which frequently accompanies nematode damage.

Damage from nematodes that feed on foliage are easier to identify since plant symptoms are easier to directly trace back to nematodes. This type of damage frequently occurs in greenhouses. Why? Nematodes need a moist environment to survive and spread. The higher humidity of greenhouses and the presence of surface water on plant leaves contribute to these types of nematode problems.

Most references refer to the presence of “angled lesions” that result from the feeding of foliar nematodes. Perhaps a better description than “angled lesions” is “brown spots on newly attacked leaves that are not round but longer than they are wide”. In advanced stages, severely attacked leaves may turn brown and die which masks the presence of these lesions. In cases like this, search for leaves that are more recently attacked to verify these “angled lesions”.

Nematode damage to turfgrass is common in warm climates and may resemble some turfgrass diseases, soil compaction, nutrient deficiencies, herbicide injury among others. Symptoms from nematode damage may gradually enlarge as much as three feet per year. Machinery that comes in contact with soils, such as aerators and hand tools, may spread nematode infestations with equipment. A common symptom occurring due to nematodes is a lack of a response from applied fertilizers.

How to control nematodes?

Nematodes are nearly young impossible to eliminate using traditional pesticides without killing infested plants. Prevent nematodes from entering the property through exclusion. Most problems develop when soils, composts, soil mixes and plant materials are brought in from unreliable sources. Reduce the spread of nematodes through sanitation. Clean equipment and tools between worksites that have been in contact with infested soils.

Recognize that the presence of nematodes is not always bad. In the past, the general recommendation was to improve plant and soil health so that plants “grow ahead” of their damage. There is quite a bit of evidence that increasing soil organic matter through the use of compost and organic surface mulches, particularly in arid and desert soils, helps keep nematodes in check.

It is thought that compost from organic matter stimulates micro and macroorganisms antagonistic to parasitic nematodes. A population of nematodes antagonistic toward plant parasitic nematodes is an important tool used to keep undesirable nematode populations in check.

Amendments with a low carbon/nitrogen ratio (C:N of 20:1 or lower) seem to be the most effective types of organic matter for keeping nematodes in check. For low organic matter content soils, such as arid or desert soils, there is a direct relationship between controlling nematodes and the nitrogen content of the soil due to additions of compost or chemical fertilizers.

North America is estimated to be the largest market for nematicides; pesticides aimed specifically to control or kill nematodes. Nematicides sales are predicted to dominate the agrochemical industry from 2015 to 2020. That’s the size of our problem.

Chemical control of nematodes is becoming more difficult because of environmental problems associated with their application. There is one traditional pesticide that remains available for nematode control. This product, Nemacur, an organophosphate pesticide, is being phased out. Only the remaining inventory on shelves will be sold. When this inventory is gone, there will be no other traditional pesticides available for use against plant parasitic nematodes.

Bob Morris is an independent consultant for ViraGrow, Inc.

This article appeared in Southwest Trees and Turf, a regional trade magazine.


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