Nemesis – the inescapable agent of someone’s or something’s downfall. Synonymous with downfall, ruin, ruination, undoing, destruction
Nematodes are microscopic, non-segmented, round worms that come under the Phylum Nematoda. They are very diverse and live in organisms, plants, soils and water (terrestrial and aquatic). 100 cc of soil can contain several thousand nematodes. Nematodes fill many niches of the food cycle feeding on vertebrates, insects, invertebrates, fungus, bacteria, organic matter and of course plants. Plant-parasitic nematodes are of great risk and concern to agriculture and are mentioned in the millennium goals as a direct threat to food security.
Because of their microscopic size and diversity, we are still learning about nematodes, and new nematodes are being discovered all the time! Luckily for us, a great deal of work has already been done on plant-parasitic nematodes and other nematodes that affect agriculture.
Nematodes can be classified by what they eat, and what they eat determines the shape of their mouthparts, which have been specifically adapted to their food source. Nematodes move through moisture films with a whip like motion. The mouth parts, body shape and the motion pattern make it easier for specialists (nematologists) looking under the microscope to identify them.
Plant-parasitic nematodes feed on or in plant roots, and occasionally on leaves and stems, and can be very destructive in several ways. They directly feed on plants, reducing the nutrients and energy of the plants. They damage the roots and root tips that are essential for water and nutrient uptake. They provide an easy entry for plant pathogenic bacteria and fungi. The female root knot nematode, Meloidogyne, enters the plant roots and stimulates the formation of debilitating roots galls. All these actions greatly reduce yields and plant health and can cause cataclysmic collapse of the plants.
Nematode Life Cycle
The life-cycle of plant parasitic nematodes is relatively simple. They have a dormant phase, the egg, that can be very persistent and resistant to drying and chemicals. The eggs hatch in response to a trigger, generally root exudes from plants. They have a mobile juvenile phase that moves, feeds and grows, molting several times, before maturing in an adult. The adults reproduce and lay eggs – and the cycle starts again. The egg to egg cycle can take under 23 days and is shortened in warmer soils and with enough food. Healthy young plants with rapidly growing roots and lots of root exudes can actually stimulate an increase in nematode populations.
How Do We Control Plant Parasitic Nematodes (PPNs)?
First, we need to take a soil sample and send it to a proper laboratory for specific identification and counting. We cannot see nematodes with the naked eye or hand lens. Next, we must ensure accuracy of sampling. PPNs are attracted to and feed off plant roots. The sample should be from the root zone and include some root fragments. The sample should be from an actively growing crop. Nematodes need a moisture film to move in and a crop to feed off. In fallow dry soil the nematodes may have moved into the moist subsoil and not be picked up in a sample or be in a dormant egg or cyst form that is not mobile and difficult to extract and identify (except the potato cyst nematode cyst – that floats).
Finally, we should be aware that we can only count nematodes that are alive – so the storage and transport of the sample are very important. Dried, frozen or waterlogged sample will not give us a good count. The nematodes in the soil sample in the plastic bag on the dashboard would have succumbed to solarization long before the soil gets to the lab.
Nematodes are counted in 100 cc soil and classified as saprofagic (eating bacteria, fungi and rotting plant tissue), predators (eating other nematodes or insects) or PPN’s. The PPN’s are listed under their genus. Each plant species / nematode combination has a THRESHOLD level of infestation (guide high), above which there is economic damage to the crop and treatment is recommended.
With nematodes, prevention is better than cure, as it impossible to reverse the damage that the nematodes do to the plant roots.
Nematodes cannot move very far by themselves. Start clean (check with a nematode test) and stay clean. Nematodes move around in soil, water and plant material. Good hygiene practices, cleaning shoes, implements, tractor tyres, and burying or burning infected plant tissue will reduce the spread. Prevent run-in of soil from other fields. Water can carry nematodes. Check water sources that are at risk (surface water, dams, rivers lakes, and shallow wells). Special nematode filters are available for high risk waters. Always buy transplant and seed material from a reputable source – and make sure it is certified nematode free.
Sadly, most our fields have already been infected with PPNs – so we must try to minimize the damage. A healthy biological soil, with good aeration, structure and organic matter levels and a diverse flora and fauna is the best way to resist nematode attacks. Good agricultural practices help reduce levels of pathogenic nematodes: crop rotation, green manures, antagonistic cover crops (marigolds / mustards / asparagus), building organic matter levels, proper plant nutrition and solarization can all help bring levels below the threshold.
The days of killing the nematodes and everything else in the soil with Methyl bromide and/or broad-spectrum CLASS A poisons are over. This practice was not only bad for us and the environment, but also created a biological vacuum that quickly filled up with PPN’s that had no competition, and created very high levels of PPNs, requiring ever higher levels of poisons to keep them under control. We know better now……………! If PPN levels are too high in your crops, a biological nematicide or nematode specific nematicide can be used (check out http://shambaza.com/nematode for more details).
Pre-plant soil sterilization is a very effective way of reducing nematode populations especially in greenhouses. Wet the soil and cover with transparent plastic for 3-4 weeks in hot weather. (Be sure to re-use the plastic). After this, apply lots of well composted organic matter and biological soil amendments to re-populate and stimulate the soil biology before re-planting.
Not all nematodes are bad, and as more research is done on biological control and nematodes – we can ‘farm’ and use beneficial nematodes to our advantage. There are many insect eating nematode species (EPNs) that are commercially available and are used in biological control.
For more information and to know the nematode status of your soil, please contact us to organize a professional, scientific, independent nematode analysis. Email us on [email protected] today.
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