By Larry Duncan
Monitoring population levels of plant parasitic nematodes and phytophthora is best done in springtime prior to the rainy season. Peak population density of the citrus nematode (Tylenchulus semipentrans) occurs at that time, and the accuracy of sample results increases with population density. The sampled population levels of this nematode and those of Phytophthora spp. can be compared to damaging levels (see the 2021-2022 Florida Citrus Production Guide) to help guide management decisions.
Tree and root conditions can also guide management if other economically important nematode species are detected. For example, levels of sting and dagger nematodes do not vary predictably with the season. However, if the nematodes are detected in the soil sample, the occurrence of sparse, stubby fibrous roots (see photo) can confirm the damage they are causing to trees.
There are two basic types of samples — soil and root — for monitoring or detecting nematodes in citrus groves. For either type of sample, first try to stratify the grove according to variables that might be related to nematode abundance — weak vs. healthy areas, or areas with different soil textures, moisture levels, or resistant or susceptible rootstocks.
Within the strata, sample from no more than 2 to 5 acres. Use a soil probe or a shovel to sample soil to a depth of 12 inches beneath the tree canopy midway beneath the tree trunk and the dripline. Put the soil from the probe, or a handful of soil if using a shovel, into a plastic bag. Repeat this on 16 to 30 trees, combining all the subsamples in a single plastic bag. The trees can be selected arbitrarily but should be distributed reasonably equally throughout the sample area.
Most economically important nematodes can be detected in the soil, but root samples more reliably recover endoparasites such as burrowing and coffee lesion nematodes. Collect root samples in the same manner as soil samples, putting a handful of fibrous roots from each of 16 to 30 trees combined in a single plastic bag. As soon as possible, remove the roots from the bag. Rinse all soil from the roots and the bag and return the roots to the bag.
Soil and root samples do not require refrigeration but should never be allowed to overheat in the sunlight. Take or send your samples to a commercial laboratory or to the University of Florida nematode assay laboratory.
Larry Duncan is a professor at the University of Florida Institute of Food and Agricultural Sciences Citrus Research and Education Center in Lake Alfred.