Root-knot nematodes can cause serious problems on flowers and bedding plants. Root-knot, which is characterized by swelling of the root (Fig. 1), is caused by the feeding activities of root-knot nematodes (Meloidogyne spp.). Different species of root-knot nematodes may be present in the soil, and different races may occur within these species. These root-knot nematode races may differ in their ability to infect some plant species and cultivars. Different species or cultivars of flowers may have different susceptibilities to these species and/or races. Selecting the right flower or bedding plant for a site may help to prevent losses due to root-knot nematodes. This publication summarizes some recently published work on this subject and provides an overview of flower cultivars and their susceptibility to different species and races of root-knot nematodes, particularly M. incognita, M. javanica, and M. arenaria, all of which are common in Florida.
Selecting a bedding plant – Identifying a possible root-knot problem
Before selecting any bedding plant, it is beneficial to obtain a soil nematode sample to determine which nematodes are present in the soil. More information on how to take a soil nematode sample and where to submit it can be found here: https://edis.ifas.ufl.edu/sr011. However, identification of root-knot nematode species is difficult and often requires molecular techniques and is not performed on routine nematode samples. Submission of a soil sample will only help to clarify if root-knot nematodes are present in the soil. If root-knot nematodes were a problem in the site before, selecting the right plant may be easier, but it is still often based on trial and error, eliminating or selecting flower species and cultivars according to the degree of damage observed in the previous crop. Eventually it might be possible to identify which root-knot nematode is present or if there is a mixture of species and races. As techniques for root-knot nematode identification improve, new species and races might be found. For example, M. mayaguensis and M. floridensis have only recently been identified from Florida (Mendes et al. 2007, Stanley et al. 2006). Relatively little is known about the susceptibility of flowers and bedding plants to these species.
An important tool in managing root-knot nematodes is rotation of plants that are susceptible, but are desired plants, with plants that are resistant. Root-knot nematodes, which are plant parasites and require a host for long-term survival, will be either unable to reproduce on these resistant plants or may only produce relatively small numbers of offspring. As numbers of offspring decrease, so does the potential for damage to a following susceptible flower planted in the site. Once the susceptible flowers are planted, nematode numbers build up again. Successful use of rotation requires knowledge about the degree of susceptibility of different plants. Tables 2 and 3 below might be useful in making the decision about which bedding plant to select. It can also serve as a guideline for choosing more suitable plants for sites that are infested with root-knot nematodes.
How Tables 2 and 3 were prepared
These tables combine and summarize the research efforts of several researchers over a number of years (McSorley 1994, McSorley and Frederick 1994, McSorley and Frederick 2001, Mendes et al. 2007, Om et al. 2008, Wang and McSorley 2005, Wang et al. 2004). In all of these studies, the researchers used similar methods, so it is possible to compare results among the different studies. An older study from the 1930s used different methods, and the identification of the root-knot nematode species was not clear at that time. Crow (2007) gives a good summary of this older study as well as some other work: https://edis.ifas.ufl.edu/in470.
In the studies used to prepare Tables 2 and 3, all researchers used a root gall index to rate the severity of root-knot infestation on the roots of a plant. Several studies also reported the number of nematode eggs produced per plant, and/or numbers of hatched mobile juveniles (J2) that were produced per plant. We wanted to develop a rating scale for flower cultivars that included root gall indices as well as the numbers of eggs or juveniles produced per plant. Ratings were assigned based on the categories shown in Table 1. A rating was given in each category (gall index, eggs per plant produced, and J2 per plant) for each species/race of nematode and each plant cultivar. In most cases, ratings were identical or similar in all categories, so the corresponding descriptive term was used in Tables 2 and 3. If ratings were close they were averaged, but if they were far apart, the result was described as "variable." If a nematode has not been tested on a particular cultivar, then the result is listed as "unknown."
Explanation of different susceptibilities
A wide range in susceptibility is seen among the flower species and cultivars (Tables 2 and 3). Some of these results come from single tests while others were averaged across several studies. Snapdragon is consistently one of the most susceptible flower crops (Fig. 2). Marigolds (Fig. 3) generally show good levels of resistance. The use of resistant marigolds against root-knot nematodes is well known, and additional information can be found here: https://edis.ifas.ufl.edu/ng045. Cultivars designated as "high" or "susceptible" could be expected to develop problems if root-knot nematodes are present, but even cultivars designated as "intermediate" or "variable" should be used with much caution. Several different cultivars were classified as "variable," such as Periwinkle cultivars, Dianthus 'Baby Doll Mix', and 'Qis White' larkspur. 'Qis White' was inconsistent, making its responses difficult to predict. Because dianthus 'Baby Doll Mix' is technically a mixture of different cultivars, it also has a variety of susceptibilities, and, therefore, variable results are expected, unless all the cultivars contained in the mix respond similarly to root-knot nematodes. Periwinkle, which produces high galling indices, does not support a high degree of egg production. The same is true to some extent for 'Scarlet' Zinnia. This indicates that although the plant is subject to infection, it has some degree of tolerance to root-knot nematodes. It may initially produce galls but possibly will grow out of it because future egg production on that plant will be low.
On the other hand, a plant that has little galling, but high egg production, is not a good candidate for rotation. Even if this type of plant can tolerate some damage, it does not minimize nematode numbers in the soil, which is the purpose of a rotational plant. Nematode population density will continue to increase on this plant, and will threaten the next crop planted if that plant is susceptible. Ratings of Lisianthus in response to M. incognita are favorable but should be used with caution. Although it showed little galling in greenhouse tests, considerable galling can be observed in the field. It is possible that galling in the field resulted from another species of root-knot nematode that was not evaluated in those greenhouse tests. For example, M. mayaguensis is a relatively newly discovered species in Florida soils, as is M. floridensis. Recent work by Mendes et al. (2007) evaluated the susceptibility of many petunia cultivars to M. incognita race 4 and M. mayaguensis. All of the petunia cultivars in Table 3 received a rating of "high" for susceptibility to M. mayaguensis and all cultivars except for 'Supertunia Lavender Pink' were rated "high" for susceptibility to M. incognita race 4. 'Supertunia Lavender Pink' was rated "susceptible." From these findings it becomes clear that most petunia cultivars should be avoided if M. incognita race 4 or M. mayaguensis are present. Greenhouse pots from the tests summarized in Tables 2 and 3 were inoculated with reared root-knot nematodes whose identity was firmly established, and not with nematodes found in the soil of field plots or gardens. Nematodes found in the soil from a sampled site might not be accurately identified for root-knot nematode species or races, or could be a mixture of different species and races. For this reason, plant cultivars that show resistance to multiple types of root-knot nematodes are generally more useful than those showing resistance to only one species or race.
Because of the difficulties mentioned above in identifying root-knot nematodes, it is highly unlikely that anyone planting flowers or bedding plants will know which species they will have. Meloidogyne incognita is very common throughout Florida, but other species, even M. mayaguensis, could occur throughout the state as well. It is interesting that whenever more than one nematode species or race were tested, relatively similar results were obtained for the species and cultivars summarized in Table 2, rarely differing by more than one rating classification. It is hoped that the table will provide some general guidelines about which plants are typically quite susceptible to root-knot nematodes and which ones tend to show high levels of resistance. In practice, growers, landscapers, and home gardeners should carefully check to verify that these responses are consistent in their own sites, recognizing that in some cases results could be very different in the event of unusual responses to other root-knot nematode species.
Crow, W.T. 2007. Nematode management for bedding plants. ENY-052, Entomology & Nematology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL. (https://edis.ifas.ufl.edu/in470 )
Crow, W.T., Woods, F.E. 2007. Nematode Assay Laboratory. ENY-027, Entomology & Nematology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL (https://edis.ifas.ufl.edu/sr011 )
Krueger, R., Dover, K.E., McSorley, R. 2007. Marigold (Tagetes spp.) for nematode management, ENY-056 (NG045), Entomology & Nematology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL. (https://edis.ifas.ufl.edu/ng045)
McSorley, R. 1994. Susceptibility of common bedding plants to root-knot nematodes. Proc. Fla. State Hort. Soc. 107: 430-432.
McSorley, R. Frederick, J.J. 1994. Response of some common annual bedding plants to three species of Meloidogyne. J. Nematol. 26: 773-777.
McSorley, R., Frederick, J.J. 2001. Host suitability of some vinca and salvia cultivars to two isolates of root-knot nematodes. Proc. Fla. State Hort. Soc. 114: 239-241.
Mendes, M.L., Dickson, D.W., Schoellhorn, R., Cetintas, R., Brito, J.A. 2007. Host status of petunia cultivars to root-knot nematodes. Nematol. Medit. 35: 91-94.
Om, N., McSorley, R., Frederick, J.J. 2008. Response of cut flowers and bedding plants to root-knot nematodes. Proc. Fla. State. Hort. Soc. 121: 370-373.
Stanley, J. Koklais-Burelle, N., Dickson, D. 2006. Host status of Meloidogyne floridensis on selected weeds and cover crops common to Florida. Nematropica 36:149.
Wang, K.-H., Dover, K., McSorley, R. 2004. Susceptibility of cultivars of Lisianthus, Eustoma grandiflorum to Meloidogyne incognita. Soil Crop Sci. Soc. Florida Proc. 64: 112-116.
Wang, K.-H., McSorley, R. 2005. Host status of several cut flower crops to the root-knot nematode, Meloidogyne incognita. Nematropica 35: 45-52.
Rating numbers and descriptions used to summarize data for Tables 2 and 3.
Bedding plant cultivars and their susceptibility to different species of root-knot nematodes.
Susceptibility of selected Petunia cultivars to M. incognita race 4 and M. mayaguensis.