By Ruth Vaughan

Nematodes are tiny microscopic worms that spread to and within greenhouses through contaminated water, machinery, animals and humans

Ornamental growers are reporting increasing nematode infestations under greenhouse production. According to crop protection experts, many growers do not diagnose the problem until the crop is already damaged. Nematodes are long, thin round worms, so tiny they can usually only be seen under the microscope. They are found in different areas and diverse habitats, but the ones that are of concern to farmers are those that dwell in the soil and feed on plant roots. These are commonly called plant parasitic nematodes that damage plant roots and slow down or block movement of water and nutrients to the growing parts of the plant.

Nematode Infestation

Nematode infestation: Roots of severely attacked (left) and healthy plant (right). Affected plants are normally stunted and eventually wilt and die. The most characteristic symptom is formation of root galls (knots) and these can be seen with the naked eye. Affected roots rot

Symptoms of nematode infestation

Signs of plants infected with nematodes are usually not uniform in the greenhouse but occur in patches along the beds. Above-ground symptoms often resemble nutrient deficiency or drought stress. These include yellowing, wilting, stunting, thinning and flower injuries. Farmers tend to describe these symptoms, mistakably, as a result of nutritional or water deficiencies. It is for these reasons nematodes are called the ‘silent’ enemies. To confirm the presence of nematodes, soil and root analysis is recommended. Visual examination of roots, depending on the types of nematodes present, will show galls or knots on the roots. These are club-like swellings on succulent filamentous roots.

Spread of nematodes, alternative hosts and survival

Nematodes are spread to and within greenhouses through contaminated water, machinery, animals and humans. Once introduced, it is highly unlikely that nematodes will be totally eradicated because they have several host plants . These include the most commonly cultivated vegetable crops such as tomato, carrots, spinach and several weeds such gallant soldier (Galinsoga parviflora ), black nightshade (Solanum nigrum) and pigweed (Amaranthus spp.). In addition, nematodes have a sophisticated adaptation to survive harsh conditions for several month’s or years as eggs or juveniles (young ones) in the soil or within root fragments.

Nematodes

Nematodes are known to be the ‘silent killers’ of crops

Impact of nematodes

When nematode populations in greenhouses are not managed, they significantly reduce crop yields and quality . In addition, nematode entry into plant roots creates wounds that serve as entry sites for other soil borne pathogens like Fusarium spp., Pythium spp., Rhizoctonia spp. and Phytophthora spp. leading to secondary infections. When nematodes and soil borne diseases occur simultaneously on a crop, they complex together, result in a rapid crop health decline and greater yield losses.

Management

Nematodes are mainly spread through movement of infested soil, seedlings, machinery and plant debris. Avoid introduction by planting nematode-free seedlings and using nematode free media especially for propagation. This can be achieved by inspection of nurseries before the seedlings are transferred to the greenhouse. Equipment and tools used on soils should be properly cleaned before use in another greenhouse. Soil conservation measures to check soil erosion also limit nematode spread and introduction.

“Nematodes are so devastating they can lead to total losses requiring replanting; we test frequently to avoid the risk of infestation and ensure our irrigation water is free of pathogens “, Hamish Ker, Oserian Development Company

Balance soil fertility

Soils that have poor fertility, including low calcium levels (low oxygen conditions), will be more prone to infestations. Balancing of soil fertility through soil testing will provide a better environment for beneficial soil organisms to compete with plant parasitic nematodes, reducing the threat to crop yields. A balanced crop nutrition program serves to reduce adverse effects by improving plant health and tolerance to nematodes.

Maintaining proper greenhouse hygiene: Equipment and tools used on soils should be properly cleaned before use in another greenhouse to curb spread of nematodes

Resistant varieties

If the greenhouse is infested, it is highly recommended that farmers adopt nematode resistant varieties

Weed control

Weeds such as Gallant soldier and black/African night shade (Solanum spp.) can habour very large populations of nematodes.

Organic amendments

Incorporation of organic matter and residues into soil including manures and crop residues not only improve soil characteristics, moisture availability and plant nutrition, but also stimulate microbial antagonism by predacious mites, springtails and earthworms. Sandy soils, with lower than optimum organic matter, when fertilized with high nitrogen fertilizers, are susceptible to nematode attack.

Cover Crops

Intercropping vegetables with crops that have antagonistic properties to nematodes reduce nematode numbers by depriving them of food and releasing nematicidal properties.

Antagonistic cover crops

Intercropping vegetables with crops that are non-host or with antagonistic properties to nematodes in alternate seasons reduce numbers by depriving nematodes of food and releasing nematicidal properties. Popular crops cabbage, mustard, velvet bean (Mucuna pruriens), hyacinth bean (Dolichos purpureus), Mexican marigold (Tagetes spp), Sorghum (Sorghum bicolor) and sorghum-sudan grass (Sorghum bicolour subsp. drummondii) When establishing a greenhouse, the antagonistic/cover crop can be harvested before maturity and be mixed or incorporated into the soil (bio-fumigation).

Biological Nematicides

Biologically based nematicides include neem based products, Trichoderma sp., nematophagous fungus such as Paecilomyces lilacinus and Myrothecium verrucaria, reduce nematode numbers without concern for environmental hazards

Chemical Nematicides

Used when populations are severe

About Ruth

Ruth Vaughan is the Technical Director at Crop Nutrition Laboratory Services Ltd. (CROPNUTS). Ruth is also a contributing author to Kenya’s leading horticulture magazines such as the HortFresh Journal, HortiNews and Floricu lture. Ruth is a great believer in soil health, organic matter, biochar and carbon sequestration as a way to alleviate climate change and increase food security. Loves visiting farmers and seeing all the different farming methods