Nematodes - Small worms that can cause big troubles
By William T. (Billy) Crow

We all know how difficult it is to maintain a pristine landscape in Florida. If the pests don’t get you, the diseases will. Plant-parasitic nematodes can be viewed as either pests or diseases, but either way they can be among the most difficult to manage problems in Florida landscapes.

Nematodes are unsegmented roundworms; most are beneficial, but some species are parasites of plants and animals. Those that feed on plants are microscopic and live either in soil or inside plant tissues. Most plant-parasitic nematodes feed on plant roots and many cause decline of ornamental plants and lawns.

Visible symptoms of nematode damage on perennial ornamentals may include leaf drop, sparse canopy, small leaves, and yellowing (Figure 1).

Annual bedding plants may be stunted and wilt even when there is moisture in the soil. Turfgrasses typically develop declining patches (Figure 2) and have lots of weeds. If these symptoms are present, it is time to take a look at the roots of the plants.

Root-knot nematodes usually cause galls or knots on roots (Figure 3). Other nematodes may cause roots to be stunted, shallow, or rotten. Plant decline is actually caused by drought stress and lack of nutrients because the damaged roots are unable to supply enough water and nutrients to the plant. This is especially problematic when irrigation restrictions are in place (Figure 4). While healthy plants typically do fine under most mandated watering schedules, plants and turf with nematode problems may decline and die.

If roots have any of the symptoms described above, nematodes could be a problem. However, be aware that certain fungal pathogens can cause many of the same types of symptoms. The only way to be certain that nematodes are a problem is to send a nematode sample to a credible nematode diagnostic lab. The University of Florida Nematode Assay Lab provides this service, as do certain private labs.

TAKING SAMPLES

If you are sampling ornamental plants it is important to include both roots and soil in the sample. This is because some nematodes live in the soil around the roots while others live inside the roots. The lab will extract nematodes from both soil and roots in separate procedures.

A good nematode sample will consist of about a pint of soil and a cup of roots. Always take samples from sick plants, but not dead ones. Similarly, turf samples should be collected from declining areas, but avoid sampling in dead patches. Make sure you identify what kind of plant the sample is from. Some nematodes are problems on certain plants and not others, so without proper plant identification, diagnosis may be impossible. It also is important to put the sample into a sealed plastic bag and to keep it in a cool place until it is shipped to the lab. Nematode sample kits with instructions are available at county extension offices throughout Florida.

 One of the first things that people want to know when a nematode problem is diagnosed is “where did the nematodes come from?” Some plant-parasitic nematodes may have been present in the landscape since it was installed and then over time built up to damaging numbers. Nematodes usually do not move much on their own, they are usually spread in contaminated plant material or soil. Often nematodes are moved into a landscape with infected plants. For example, liriope commonly harbors root-knot nematodes, but is not damaged by the nematodes to a great extent. It is when the liriope is planted next to a susceptible plant like bottlebrush that the root-knot nematodes can become a problem. Many times sod is infested with plant-parasitic nematodes, so it comes with its own crop of nematodes when it is planted.

Always inspect the roots of new plants before they are installed in the landscape, do not plant them if they have symptoms of nematodes or other problems. When removing sick or dead plants, clean off your shovels and other tools to avoid moving nematodes or other pathogens that are in the soil sticking to them.

WHAT CAN BE DONE?

 What can be done about nematode problems in a landscape? Unfortunately there are no consistently effective nematicides labeled for landscape use at this time. This can mean trouble for you when your client wants you to treat the problem, but there is nothing to treat the problem with. Sometimes you can replace the affected plants with ones that are resistant to the problem nematodes.

Cyst nematodes are a problem on St. Augustinegrass but not other grasses. So, a cyst nematode infested St. Augustinegrass lawn could be replaced with bermudagrass, zoysia, bahiagrass, etc. Similarly, centipedegrass is more susceptible to ring nematodes than other grasses. A centipede lawn infested with ring nematodes could be replaced with St. Augustinegrass. Bahiagrass is tolerant of all nematodes, so it is often a good choice to plant in nematode infested lawns. 

Table 1 lists annual bedding plants and Table 2 woody perennials that University of Florida research indicates are resistant to root-knot nematodes. These plants may be good choices when replanting root-knot infested sites.

Sometimes it is possible to reduce other stresses on the plants and help them tolerate nematodes better. Composts and other organic materials can be added around ornamentals to help the soil hold onto water and nutrients. If practical, increasing irrigation frequency can help keep water in the rootzone of nematode affected plants and help keep them from wilting. Splitting up fertilizer applications into smaller amounts and then applying them more frequently helps keep nutrients where the shallow roots of nematode damaged turf can get to them. 

 So, what happens if there is a nematode infested lawn that you can’t treat, the deed restrictions mandate a susceptible grass, and water restrictions say it can only be watered once per week?  Is the homeowner up a creek with no paddle?  Unfortunately, while no one likes to hear it, that sometimes is the case.  However, be assured that nematologists at the University of Florida are attempting to remedy the situation.  We are conducting research with a group of bacteria called Pasteuria spp. which show great potential as biological control organisms for nematodes in landscapes.  We also are working with industry to develop effective low risk nematicides. Finally we seek to identify more nematode resistant ornamentals, and are working with breeders to develop nematode tolerant or resistant turfgrasses.  I think that in the next few years we will make great strides in offering practical solutions for nematode problems in Florida landscapes.

Crow is an associate professor of nematology at the University of Florida.

Cutlines:

Fig. 1: Root-knot nematodes have caused this pittosporum to drop most of its leaves and have a sparse canopy.

Fig. 2: St. Augustinegrass with declining patches due to nematode damage

Fig. 3: Root-knot galls on a woody landscape plant

Fig. 4: This bermudagrass lawn has nematode damage compounded by mandatory irrigation restrictions.