Nematodes are tiny “wire worms” that abound in the soil. The root-knot nematode, Meloidogyne incognita, is one of the most infamous, both for its devastating effect on crops and the ease with which its presence can be identified. It causes knots to form on the roots, in some cases making roots look something like a string of beads. Other kinds of nematodes cause major crop losses, but require a nematologist to identify them.
The increasing use of agroforestry systems in which trees and shrubs are permanently grown in close association with annual crops raises an interesting question. How do these associations affect nematode damage, especially if the trees are themselves hosts for nematode survival and population build-up? This question is addressed in an article in Agroforestry Today by Mia D'Hondt Defrancq (April-June 1993, pp 5-9), from which the following is abstracted.
“Two types of interaction between trees and crops affect nematode populations. Direct interactions take place where the nematode population is directly influenced by the introduction of a species of plant new to the area or a new species of nematode.” Indirect influences occur when the nematode population is altered by the local environment.
Direct Influences: Some species of trees and shrubs actually reduce the number of certain species of nematodes. This might be due to a chemical that is exuded which kills nematodes. In other cases the tree or shrub acts as a trap-host (it attracts nematodes but prevents their reproduction).
“In Nigeria, for example, the deliberate planting of Leucaena leucocephala in a fallow period dramatically reduced parasitic nematode populations in the soil. When the fallow was converted to leucaena alley-cropped with maize, the population of parasitic spiral and root lesion nematodes remained low. In West Africa, Sesbania rostrata acts as a trap host for the Hirshmaniella species of nematode that are prevalent in flooded areas where rice is grown.”
“In cases where trees and shrubs are suitable hosts for harmful nematodes … [the damage] may increase drastically. This is because the host will not only allow continuous build-up of the nematode population but will become a very efficient reservoir from which attacks can be made [on future crops].” For example, there were many more nematodes within 2.5 meters of a sesbania hedgerow in the Rwandan highlands than there were 5 meters from the row. In Malawi studies suggest that Acacia, Leucaena and Sesbania species can act as good hosts for root-knot nematodes. Presumably crops susceptible to this nematode will be more seriously attacked when grown in alleys with these trees. “Similar problems can be expected if Tamarindus indica or certain species of Acacia, Albizia and Casuarina are planted where the burrowing nematode is a threat to crops such as banana or vegetables.”
Indirect Influences. Trees can reduce nematode problems by indirect interactions. For example, many crop plants have some natural resistance to nematode attack, but this is reduced by high air and soil temperatures (both of which are reduced by shade). Trees and shrubs can also reduce soil erosion and hence prevent the spread of nematodes that are attached to soil particles. To the extent that trees reduce growth of weeds that harbor nematodes, crop losses may be reduced. If benefits of the trees cause crops to be more vigorous, this in itself can reduce nematode injury. “There is also evidence that leachates from the litter of certain trees and shrubs [Ed: water that has soaked through the litter] have nematicidal properties, e. g. Azadirachta indica (neem), Ricinus communis (castor bean) and Leucaena leucocephala.”
Indirect interactions can be negative. Plowing reduces nematode density. Reduced cultivation in an alley crop system can thus enhance nematode populations.
I have often wondered if knots caused by nematodes might not sometimes be confused with galls caused by nitrogen-fixing rhizobia. How can you tell them apart? “The nitrogen-fixing galls are readily identified because they are easily rubbed off from the roots and are often pink-red inside.”
Many leguminous trees are also good hosts for nematodes. Nematode infection may reduce rhizobial colonization and, hence, nitrogen fixation. “
The following trees have been found to be resistant to the widespread Meloidogyne incognita (rootknot nematode): Acacia senegal, Acacia tumida, Anacardium occidentale, Azadirachta indica, Cassia obtusifolia, Cupressus sempervirens, Eucalyptus camaldulensis, Leucaena leucocephala (found resistant in most countries), Sesbania tetraptera and varieties of Sesbania macrocarpa.” The author did not provide a list of trees that definitely are harmed by nematodes. He did mention that Sesbania sesban failed in east Africa due to nematodes. Sesbania grandiflora is badly damaged by them at ECHO.