Diseases of citrus (
Citrus nematode
Only one nematode species, the citrus nematode, Tylenchulus semipenetrans, is important and damaging in Arizona. Surveys indicate it occurs in 90 percent of Arizona citrus. The citrus nematode, an obligate parasite, reproduces only on living roots of host plants. Populations of nematodes in roots were found to be highest in the early stages of tree decline and lowest in roots of declining trees.
Symptoms depend on overall tree vigor. Infected trees growing under optimum conditions may appear healthy for many years. For this reason, the disease is often referred to as "slow-decline." Heavily infected root systems eventually cause a reduction in yield and quality of fruit. Above ground symptoms of nematode damage are non-specific. Root feeding and subsequent damage reduce the overall vigor of infected trees. Symptoms include leaf yellowing, sparse foliage, small, non-uniform fruit, and defoliated upper branches. Dieback is particularly noticeable in the upper portion of trees.
Affected trees appear similar to stress conditions caused by Phytophthora root rot, poor nutrition, and inadequate irrigation. Tree decline, which depends upon care of the grove and overall tree vigor, may not occur for three to five years after heavy infection. Infected roots may appear coarse and dirty. Female nematodes are found in small groups on the root surface. Soil adheres to the gelatinous matrix in which eggs are embedded. Positive identification requires the extraction of the nematodes from soil samples taken in the feeder root zone between the trunk and the drip line of the tree. The nematodes may also be identified microscopically on infected roots in the laboratory.
No chemicals are presently recommended for post-plant control of the citrus nematode. Infested sites should not be replanted with citrus.
More information about citrus nematode and citrus diseases is available
in the "Diseases of Citrus in Arizona” bulletin at: http://ag.arizona.edu/PLP/plpext/publication/publication.html
February 7, 2008