Plant-parasitic nematodes and weeds can limit agricultural production

NANCY NTIDI, ARC-GRAIN CROPS INSTITUTE, POTCHEFSTROOM

Plant-parasitic nematodes disrupt the physiology 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 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 plant-parasitic nematodes.

Challenges

Generally, 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 efficacy 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.

Nematode control strategies

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. Such weeds include; Cynodon dactylon (kweek or couch grass), Cyperus spp. (nutsedge or uintjie), Hibiscus spp. (wild stockrose or wildestokroos), Nicandra physalodes (Apple of Peru or basterappelliefie), Amaranthus spp. (pigweed or morogo) and Chloris virgata (feathertop or witpluim chloris).

Thus 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.

Such strategies may include one or a combination of 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 plantparasitic 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 root-knot 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 plantparasitic 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. Plant-parasitic nematodes continue to feed and reproduce on root fragments/other plant parts in the soil and build up to damaging levels for susceptible, follow-up crops.

For further information contact Nancy Ntidi (ARC-GCI) at (018) 299-6100.