Nematodes : alternatives to chemical control

In a wide range of crops the average yield loss caused by nematodes is estimated to be 10-15%. Sometimes it can be total. Food and cash crops are susceptible to one of the many types of these tiny soil-borne, thread-like parasitic roundworms. They feed and reproduce within the host plant, affect growth, and can cause distorted seeds, and rotten or malformed roots and tubers. Damaged tissues cannot get enough nutrients and water from the soil to maintain normal growth, and a poor harvest results. Often farmers attribute this disappointing performance to poor soil, water stress, or salinity rather than to the true cause Once a soil is infested it is virtually impossible to eliminate the nematodes, removing the host plant for long periods by crop rotation is rarely adequate. However, damage can be limited. Since most plant-parasitic nematodes spend part of their life-cycle in the soil, they can be killed there by nematicides, but two major disadvantages of these are their toxicity, and the fact that their effect is short-term. Thus repeated use is necessary to prevent a resurgence of the problem later in the growing season. Despite the widespread and serious damage caused by nematodes in the Tropics, the use of nematicides is not common because farmers lack financial resources. While removing the host plant for long periods by crop rotation is inadequate to control nematodes, appropriate techniques exist which are less costly and less damaging to man and to the environment than the repeated use of chemicals. Under slash and burn systems or shifting cultivation, land was unlikely to become nematode-infested; however, the natural balance is easily upset by fixed farming and the introduction of new crops and cultivars. Identifying nematode damage The detection of nematodes is the first step to control. Relatively few nematodes can be passed on in seed but, of those that are, some are easily detectable by their effects such as the blackened seed galls caused in wheat and distorted and discoloured seeds of beans. The peanut/groundnut seed nematode found in Nigeria makes the seed shrivelled and dark brown, and is seed transmitted if the seeds are left in their pods or are not thoroughly dried. A rice nematode, Ditylenchus angustus, is found in freshly-harvested seeds, and can also be killed by sun-drying but, like a number of foliar nematodes, can still be carried in plant debris mixed with seed. In yams and potatoes, nematodes produce small yellow lesions just under the surface which develop into a dark brown dry rot and some cause knobbly tubers. Infected tubers must not be used for planting. Nematodes are common causes of damage to dessert and cooking bananas. They are spread in the vegetative corms used to grow new plants, and the symptoms are most noticeable in the roots of suckers immediately after lifting - as purple or dark brown lesions seen when the root is split lengthwise. There may also be dark areas on the root surface. The first signs of nematode infection in taro are red streaks in the centre of the corm; these allow bacteria and fungi to establish, and the corm can rot away completely. All seedlings should have their roots checked for swellings, or galls or distorted shapes prior to transplanting. Unlike Rhizobium nodules in leguminous plants, nematode galls are an integral part of the root. Damage limitation Once a farmer notices nematode damage to root or leaf, there is little to be done that season other than to remove and destroy infected plants and increase amounts of water and fertiliser to compensate. In alleycropping, perennial trees and food and cash crops can even exchange nematodes, but the problem can be contained by careful choice of planting material and land use. Nematode-free stem cuttings can be taken from sugar cane and cassava, and also from kumara, though tubers and roots can pass on the pest. Using part of a yam tuber for planting means that farmers can check for damage. Citrus can be propagated by marcotts or cuttings. Extension services must avoid distributing nematodes with cocoa, coffee or other tree seedlings. Seedbed soils can be kept pestfree by annual or seasonal rotation of the seedbed site, always using previously uncultivated ground. Dry season crop seed beds can be allowed to flood; wet season crop seedbeds should be kept free of weeds; soil can be turned frequently in dry periods since exposure to the sun kills a large proportion of nematodes; brushwood, crop, seed residues, and dried dung may be burned to kill the nematodes Nematode populations can also be reduced by incorporating chicken manure, green manure, sawdust, cocoa pods or cassava peelings into the soil. Some plants, such as mustard and marigold (Tagetes spp) have nematicidal properties. Strict plant hygiene and early detection will help prevent the disastrous damage wreaked by nematodes, and are far cheaper and less hazardous than the use of chemical nematicides. Reference: 'Principles and Practice of Nematode Control in Crops', edited by R.H. Brown, Plant Research Institute Department of Agriculture and Rural Affairs, Burnley Gardens, Burnley, AUSTRALIA and: B.R. Kelly, Nematology Department, Rothamsted Experimental Station, Harpenden, UKReference: 'Principles and Practice of Nematode Control in Crops', edited by R.H. Brown, Plant Research Institute Department of Agriculture and Rural Affairs, Burnley Gardens, Burnley, AUSTRALIA and: B.R. Kelly, Nematology Department, Rothamsted Experime