Nematodes, or roundworms, are well adapted to vegetable gardens. They inhabit a large range of soil types, compositions and environments, often surviving extreme temperature and moisture conditions. There are an estimated half million or more species of nematodes and many are detrimental to pumpkins, tomatoes and other giant vegetable roots.
Nematodes are slender, non-segmented worms, commonly less than 2 mm long. Small nematodes are microscopic and resemble earth worms. They have round, slender digestive systems that allow them to munch eagerly on roots, but are often overlooked in garden plots due to their microscopic size.
Many different kinds of nematodes are present in our soils and while some plant-specific nematodes are harmful, there are many that are beneficial for gardeners. They inhabit the soil food web, consuming fungi, bacteria and protozoa. Nematodes in compost piles are gluttonous eaters, gobbling bacteria, viruses and other invertebrates, and then excreting nitrogen and other nutrients back into the soil in a plant-soluble form for easy uptake by roots.
Parasitic nematodes are patch pests and are often a huge unseen problem for many large vegetable growers.Normally they do not reach detrimental levels on area farms, but in a non-rotated, giant vegetable patch, an alarming amount of them can begin to suck away liquids from the plants root systems.
Parasitic nematode species cause damage to roots, leaving galls or what are known as root-knots. Some nematode types can also vector plant viruses in their feeding activity on roots. Some types are endo-parasitic, living within the root tissues. Others are ecto-parasitic, feeding externally through the plant root's cell walls.
Well-drained organic matter and sandy, calcareous and high-porosity soils are thought to be ideal breeding grounds for nematodes, which are are often associated with soil-borne diseases, such as verticillium, or black root rot, and damage caused by root lesion nematodes provides infection sites for disease-causing fungi.
Ecto feeding nematodes can considerably diminish a plant’s uptake of nutrients and water. They leave open wounds on root surfaces that act as a doorway into the plant's vascular system, fostering a large range of plant-pathogenic fungi and bacteria. These secondary microbial infections are often more damaging to yields than the express causes of nematode feeding.
Nematode infestations are thorny to stamp out, as controlling them combines numerous lines of attack. Rotating giant vegetable plants is generally the best line of defense in controlling infestations. Generally it is best to rotate patches to crops not associated with each other – for example, Atlantic giant squash, tomato and other vine type vegetables are closely related, so simply rotating these vegetables in the garden will not keep root-damaging nematode populations under control.
Solarization or covering the infested patch areas with clear plastic tarps in the summer is another effective control method. Growers are advised to frequently turn the soil deeply or double dig two spade depths. This allows delayering of the soil and ensures the sunshine cooks the insects far below the surface, but this is a non-selective treatment that harms beneficial insects as well.
Barren, fallowed patches that are kept free of weeds for a year or two usually have a high degree of reduction in root-knot nematode populations. This plant-free interlude can be completed in one season by using several intervals of tilling the soil all summer long. Both solarization and leaving the area fallow require extensive organic matter replenishment to restore the soil's bio-diversity after completion.
Marigolds grown near the pumpkin plant's roots can also be used to deter nematodes. The symbiotic relationship of marigold host roots and rhizo-bacteria is thought to impede root lesion and other types of parasitic nematodes. This type of nematode control benefits susceptible crops when non-host plants or marigolds are grown in rotation or nearby vulnerable plant roots.
Other treatments include chitosan as a natural bio-control. Derived from the crushed shells of shrimp and crab, chitosan promotes the growth of fungi that consume chitin, which makes up the eggs and shells of nematodes. The bio-control action of chitosan begins spontaneously and promotes defense responses within the plant to resist the attack of insects such as nematodes.
Mustard is also useful as cover crop in annual crop rotations. It can produce a large amount of rich, nitrogen-laden organic matter. Nematode inhibition occurs in the release of a chemical comparable to cyanide that performs as a soil fumigant. Endo-nematodes larvae also pierce the mustard roots and are unable to exit, drastically reducing the number of nematodes in the soil.
Beneficial or predatory nematodes can reduce populations of plant parasitic nematodes in soils. Aerobic compost types of nematodes also consume plant parasitic nematodes when found together in soils. Populations of beneficial nematodes can be raised by adding organic soil amendments that foster bacterial and fungal growth. These predatory nematodes can also be bred by soaking fresh, diverse, composted organic matter in water and brewing it like a compost tea.
The above techniques work best in the environment of a healthy soil food web. Adequate organic matter levels sustain a varied mixture of micro-organisms. A well-balanced soil rhizosphere supports a host of natural biological creatures that together manage infestations at acceptable levels, allowing plants to thrive.