Nematodes may damage forest plantations which are usually established on poorly stocked natural forest areas, or planted on areas which have been recently cleared out, or on abandoned farm-land. Changing to closely spaced planting of tree species susceptible to nematodes may promote an increase in the nematode population that result in an unacceptably large decrease in rate of growth. Nematode diseases are one of the causes of poor crop yield on the old farmlands and contributed to their abandonment.