When old trees infected with citrus nematodes are removed from the soil the roots remaining in the soil and the soil itself are teeming with enormously high numbers of nematodes. Under these conditions, soil environment makes little difference in degree of infectivity. Usually, old grove soil will contain higher amounts of organic materials in the topsoil. This, combined with the great number of citrus nematodes present in the soil, spells problems for a young tree with its limited root system. The roots of young trees planted in this soil are rapidly invaded by nematodes and become heavily infected.
Young trees can support high populations, without apparent damage.
Only when the rains begin, or supplemental irrigation is provided, do these trees respond with new growth flushes. In surveys for the burrowing nematode, samples are taken below 30 inches because this is the zone of greatest nematode concentration. Spreading decline of citrus is rarely found in other than deep, well-drained sands. Citrus nematodes are readily detected on roots in the top 6-12 inches of soil. The effects of spreading decline on the root distribution of citrus.