As soil fumigation becomes more restrictive, it will take new ways to control a growing problem of plant parasitic nematodes. Although most nematodes—microscopic roundworms that live in soil and water—are beneficial, plant-parasitic nematodes are not and cause economic damage by feeding on plant roots.
Plant parasitic nematodes were not always recognised as major pests but have recently come under scrutiny as they possess a highly toxic nature and pose a constant risk to applicators. Plant-parasitic nematodes interrupt the make-up of plants and decrease crop yields as well as the quality of various agricultural products substantially, resulting in economic losses for producers and related industries.
In an article by ARC-grain Crops Institute, Nancy Ntidi, explains that in general, root-knot nematodes are the most important and widespread group among plant-parasitic nematodes that attack and infect crops, “Damage due to plant-parasitic nematodes parasitism is usually more serious in subsistence farming communities than in first world countries. This is mainly due to knowledge gaps as well as the limited availability of infrastructure and finances in the subsistence agricultural sector. Weeds do not only compete with crops for space, light, nutrients and water, but also serve as alternative hosts for plant-parasitic nematodes during growing seasons as well as after harvesting. Certain weeds that serve as a supplementary source of human food (e.g. Amaranthus spp. “Morogo”) are for example prone to infection by plantparasitic nematodes.”
Furthermore Ntidi explains weeds that occur in agricultural cropping systems are not perceived as good
hosts of plant-parasitic nematodes and thus make it difficult for researchers or scientists to identify the effective and compatible integrated pest management strategies that will address both weed and nematode management collectively, “weeds reduce the efficiency of crop rotation aimed at nematode management since weeds are often neglected in nematode management plans. Weeds that serve as a supplementary food source, may not be intentionally removed by producers, but rather be semi-cultivated along with a given staple food crop. This inevitably leads to a build-up of plant-parasitic nematode populations and eventually the main crop suffers damage while producers are unaware of the situation.”
Chemical companies have invested heavily in product stewardship and applicator training, as well as reducing the risk of applying highly toxic chemicals. Although at the 6th International Congress of Nematology (ICN) in Cape Town in 2014, two products containing two new active ingredients with different modes of action were launched. This, according to ICN, was the first new active ingredient to be registered against nematodes since 1992. The new products are safer and add far less of the active ingredient to the soil. Says the organisation “There is also a far greater focus on integrated pest management – more biological control agents in combination with conventional nematicides. Products containing more than one active ingredient are also being developed and include a product containing an insecticide and nematicide currently being investigated by the South African Sugarcane Research Institute (Sasri).”
Ntidi adds that the main objective of nematode control is to grow crops economically in the presence of plant parasitic nematodes, “However, keeping plant-parasitic nematode population levels low and manageable over seasons to enable the sustainable production of crops in the long term should be the most important objective. The most accurate way of diagnosing plant parasitic nematode problems in crop fields, is to send both plant tissue (i.e. root/tubers/seeds) and soil to a nematology laboratory for analyses. Weeds and nematode surveys conducted throughout South Africa indicated that weeds that commonly occur in agricultural cropping systems can be good hosts of plant-parasitic nematodes.Therefore an urgent need exists for the development and application of integrated, but effective nematode as well as weed management strategies to enable sustainable food production.”
Ntidi suggests the following:
• Timely weeding of food plots:
necessary to limit infection of crops by plant-parasitic nematodes since weeds may serve as hosts and support the development and reproduction of these parasites.
• Addition of organic matter: helps retain soil moisture and adds to the available plant nutrients. Increased water and nutrient uptake by plants help to withstand nematode attack. Manures, peats or compost amendments will also increase the level of microbes in the soil and thus favours the build-up of other beneficial microorganisms that feed on all soil microbes, including non-parasitic nematodes. However, it is essential to ensure that compost used should not include partially decomposed roots/tubers that are infected with plant parasitic nematodes or other soil-borne pathogens. Previous research showed that decayed kraal manure treatments reduced root-knot nematode numbers between 41% and 71% in tomato trials and between 49% and 99% in maize trials planted in resource-poor areas.
• Soil solarisation: effective for small plots and entails covering the soil with transparent plastic during the summer season when high day temperatures are experienced. This strategy was also successfully applied in ARC trials and reduced general root-knot nematodes substantially. This strategy is based on the solarising effect of the heat from the sun that is shining through plastic together with the soil moisture to kill soilborne nematodes. It is particularly suited for areas where daily air temperatures are high, resulting in a high solar radiation of the soil where no plants are growing at that stage.
• Crop rotation: Plants that are related usually are susceptible to same pests and diseases and should not be planted close to each other or follow each other in a rotation cycle. Root crops in particular should not be planted in the same area of the garden in succeeding years because they are highly susceptible to plant parasitic nematodes and other pests and diseases.
• Host plant resistance: another option for the prevention of general root-knot nematodes population build-ups in cropping systems. This method is a good management choice because it involves minimal effort and expense. However, resistant crop varieties are not available for all vegetable crops.
• Use of green manure and or cover crops: vetiver grass and the Brassica cultivar, Nemat, reduced general rootknot nematode populations in both greenhouse and field trials. Vetiver grass can also add value for a producer where livestock forms an integral part of the farming system.
• Early season cropping: lettuce, onions, leafy green crops, green pea, bean and cabbage can be planted early in the growing season and during colder months to escape serious damage by plant parasitic nematodes. This is particularly recommended for areas where low temperatures prevail and in this way, prevent or limit general root-knot nematode reproduction and activity.
• Physical destruction of roots/other plant parts: destroy roots/other plant parts as soon as the plants are no longer growing in the garden. Plantparasitic nematodes continue to feed and reproduce on root fragments/other plant parts in the soil and build up to damaging levels for susceptible, followup crops.
Dr. Inga Zasada, research plant pathologist U.S. Department of Agriculture is of the opinion that a new approach with regards to nematode management, “We keep losing nematode management tools. We don’t have the nematode management tools that we had 40 years ago, and because of that, nematode management practices in the future will probably not be as effective as they were in the past.” Zasada is working to develop sustainable plant parasitic nematode management systems for small fruits and grapes.
Research encompasses developing production systems that integrate a range of tools to promote root health and suppress nematodes, as well as providing information on management strategies. Furthermore she believes that a better understanding of nematode biology—knowing when they are most active, where they reside in the soil, and what host plants they like—will help growers improve management strategies, especially if soil fumigation becomes more restrictive in the future. Says Zasada referring particularly to wine grape growers, “Knowing when nematodes are most active in the soil can help growers collect more accurate samples and better time nematicide applications. A longterm project is under way to learn how nematodes impact vine establishment and productivity of a vineyard. Three white and three red varieties have been planted into areas of a field either fumigated or not. Population dynamics of plant-parasitic nematodes, including root knot, as well as their impact on vine productivity, will be monitored during the coming years.”
Life cycles of plant-parasitic nematodes include the egg, four juvenile stages,and one adult stage. Most nematodes complete their life cycle in 20 to 40 days. For some nematodes, two to three generations are possible per year.
Knowing when nematodes are most active in the soil can help growers collect more accurate samples and better time nematicide applications. Root knot nematode is a sedentary endoparasite that lives most of its life within plant tissue. Some 300 to 400 eggs are laid on the root surface or in the root. Migration in the soil occurs at the second juvenile stage. More than 550 host plants are known, including potatoes, alfalfa, dandelion,and lamb’s quarters. According to U.S.Department of Agriculture research, root knot nematodes are concentrated in the top 18 inches of soil in clearly defined areas that follow water (drip lines) and roots. Dagger nematode is a migratory ectoparasite, spending all of its time in the soil living on the exterior of roots. Dagger nematodes can transmit tomato ringspot virus and grape fanleaf virus to grapes. The species that transmits tomato ringspot virus, X. index, has not been found in Washington vineyards.