Symptoms depends on the age and vigor of the tree, the number of nematode in the soil, and the susceptibility of the roots. Established orchards in which trees are stressed by suboptimal growing conditions may be more vulnerable. Heavy infestations are first visible on the above-ground parts of the tree or plant as leaf curling, wilting, lack of vigor, yellowing of foliage, shoot dieback and poor fruit growth. What's more, these symptoms do not get amended in response to watering and fertilization. When symptoms are observed on leaves, the disease may already be at an advanced stage, as the unearthing of roots will show. These are thicker and have a fibrous and stunted aspect. Their encrusted appearance is due to the adherence of soil particles to the gelatinous eggs masses laid by the females on the surface ("dirty roots").
The symptoms are caused by the nematode Tylenchulus semipenetrans, a major plant-parasitic pest that can cause 10-30% losses on citrus trees. It is an obligate parasite, meaning that they need living plant tissues to live and reproduce (in that case in the roots). The females enter the rootlets, mainly through tiny lesions, and lay their eggs there, embedded in a gelatinous matrix. The adhesion of soil particles to this matrix explain the encrusted aspect of roots. After hatching, the juveniles start to attack the internal tissue of the roots, resulting in damage and poor root development. This hinders the transport of water and nutrient to the aerial tissues and triggers the symptoms. Root damage is more severe in arid areas with high soil salinity. By contrast, well fertilized orchards with optimal levels of potassium in roots and leaves are less prone to be infected. It has a very narrow host range and, beside citrus, also infects grape, olive, and persimmon. Temperatures of 24-30°C are optimum for their growth.
In case of container-grown citrus, soils can be treated with steam or soil solarization, the process whereby soil is tilled and exposed to solar radiation to kill eggs and nematodes.
Always consider an integrated approach with preventive measures together with biological treatments if available. Preventive measures are recommended to avoid infestations because the chemical control of nematodes on established trees is expensive and no strategy gives an adequate control. Fumigation and nematicides can be used to reduced initial population densities, with methyl bromide, 1,3 dichloropropene, and chloropicin are the most effective. Application techniques are important for a successful outcome.