Nematode Management in Bedding Plants in the Landscape
Ornamental Disease Note No. 31
Tom Creswell, Extension Plant Pathologist

Nearly everyone who grows plants outdoors eventually has problems with nematodes. Many annual ornamentals are at risk if planted in soil heavily infested with nematodes.  They may develop serious root problems, grow poorly, and fail much sooner than they should.  

Nematode Symptoms and Diagnosis

Figure 1. Healthy impatiens on left, root-knot nematode infested impatiens on right. Note symptoms of wilt, chlorosis and stunting. Figure 2. Root galls formed by the root-knot nematode.

Above ground symptoms are similar to those resulting from many kinds of root injury (Figure 1). Foliage loses its luster and wilts; plant yellows and eventually loses leaves from prolonged root stress; new flushes of growth are weak, with fewer and smaller leaves than healthy plants; and damage is spotty since nematodes are rarely distributed evenly in the soil.

Root symptoms vary widely.  Some kinds of nematodes produce distorted growth of roots or stop growth completely; others kill the cells on which they feed, leaving patches of dead tissue as they move on.  Fungi and bacteria which cause root rots, wilts and other plant diseases often infect nematode-damaged roots earlier and more severely than uninjured roots.  Therefore, depending on the kind of nematodes involved, nematode damage may include galls, stunting and decay of roots; roots are often darker in color than healthy roots. 

Dozens of nematode species are associated with landscape ornamentals, but relatively few of these cause most of the serious problems.  The root-knot nematodes (Meloidogyne spp.) are by far the most important.  Their easily recognized galls on roots make their presence obvious (Figure 2).  Galls result from the growth of plant tissues around juvenile nematodes that feed near the center of the root and secrete plant growth hormones that stimulate cell growth.  Root-knot gall tissue is firm without a hollow center, and is an integral part of the root; removing the root-knot gall from the root tears root tissue. Nodules form on roots of many legumes because of beneficial Rhizobium spp. (nitrogen-fixing bacteria) and most other natural nodules or bumps are loosely attached to roots and have hollow centers.  Active Rhizobium nodules have a milky fluid in their centers.

Laboratory Soil Sample Analysis

Laboratory soil sample analysis is the only way to determine all of the kinds of nematodes associated with a problem and may be necessary to identify the most effective control measures.  Contact your local county extension agent or the NCDA for more information on nematode assays.

Managing Nematode Pests of Ornamental Plants in the Landscape

One of the first questions raised by home owners when told they have a nematode problem is “What do I spray?”  While several “cures” for nematode infected plants have been marketed over the years, none have been shown to completely eradicate nematodes from existing plants.  However, the care given to ornamentals can have a substantial effect on their performance. 

The main tools we have in reducing the damage from nematodes are outlined below.  Use of resistant plants and provision of optimal care are two of the most important. With proper care, it is possible to establish and maintain an attractive landscape, despite nematodes.   

1.  Prepare new planting sites properly. 

Give plants the best chance to become rapidly established.  Native soil in which annuals are to be planted should be prepared well, including removal of any old roots, debris, etc., from the site.  Water and nutrient-holding capacity of the soil and activity of natural enemies of nematodes are improved by incorporating organic soil amendments into the soil before planting. 

2.  Treat soil. 

If nematodes build up to high levels on the preceding plant(s), or if plants that especially favor nematodes were recently removed from the site, soil solarization may give the new plant(s) a better chance.  Soil solarization is a non-chemical way to reduce soil pest populations, but it takes a lot of work and the area must be left bare 6 to 8 weeks during the summer.  Clear polyethylene is used to cover moist soil that is well tilled and ready to be planted, so the heat generated my sunlight hitting the soil will be trapped in it and raise the soil temperature high enough to kill many nematodes, fungi, and weed seeds in the upper few inches of the bed.  Solarization works best in hot climates and sandy soils. 

3.  Replace infested (contaminated) soil. 

For small areas it may be simpler to remove all soil or planting mix from an annual bed and replace it with new nematode-free planting medium.  Nematodes eventually will invade the new medium, and bedding plant roots can grow out of the new medium into the infested native soil, but infection will be delayed.  Be sure to dispose of the removed soil in an area where that will not be used for future plantings. 

4.  Use nematode-free stock. 

No matter how perfect and pest-free the planting site, a nematode infection already started in roots of transplants is right where it must be to do the most damage.  Buy only top quality plants.  Reject any that have clear evidence of nematodes or other hard-to-control pests. 

5.  Select plants that are well adapted. 

Plant suitability to the location is important at all levels:  region (climate), soil type, shade, drainage, etc.  Plants that are “out of place” are more likely than well-adapted ones to suffer environmental stress.  Moreover, a plant species that is “well adapted” to an area probably has some degree of tolerance or resistance to locally common pests, such as nematodes. 

6.  Avoid nematode-susceptible plants. 

Do not use plants that are very susceptible to nematodes known to occur in a planting site.  Plants vary widely in their susceptibility to different nematodes. There are many attractive plants that could be planted into a particular site without serious damage or immediate losses due to nematodes.

Most references to “nematode susceptibility” in popular literature refer to one or more root-knot nematode species, unless they specify others.  Such lists are often incomplete, but it is quite likely that any plant identified as “nematode susceptible” should not be planted where root-knot nematodes are known to be serious.   

A University of Florida scientist named C. C. Goff recognized the value of knowing the relative susceptibilities of annual ornamentals nearly 60 years ago.  Goff conducted field trials to study reactions of many annuals to root-knot nematodes over four years. Results of his test are summarized in Table 1.  His results at one location do not guarantee the same success with every root-knot nematode population.  We now know that different varieties of many species of annuals vary greatly in their reactions to root-knot nematodes.  We also know that there are more kinds of root-knot nematodes than were known then, so Mr. Goff’s work with the root-knot nematode population at one site does not apply equally well to all other nematode populations and sites.  However, we believe that his was one of the most common species of root-knot nematodes, so he clearly has given us a good place to start. 

7.  Keep other pests under control. 

Stress from insects and disease can set plants up for nematode infection just as readily as over- or under-supply of nutrients or water.  Overuse of pesticides can also injure plants, so use judgment in their application. 

8.  Maintenance.

Give the plants optimum care from the start and for as long as you want them to perform well. “Optimum” does not mean “maximum”.  Fertilize as needed to maintain steady, healthy growth rather than excessive, succulent growth that invites attack by nematodes and other pests.  Water deeply to encourage development of a deep root system that can exploit a large volume of soil for water and nutrients.  Frequent shallow watering causes plants to develop shallow root systems that are less able to withstand nematode attack.  Sudden dry periods or pest outbreaks can weaken plants in an incredibly short time.  Even under normal conditions, erratic or inadequate watering can weaken a plant so that it can no longer tolerate a modest nematode population that had existed for years. Keep the plant root zone mulched to keep roots cool in hot weather and minimize evaporation of water from the soil surface.  Organic mulches also contribute organic matter to the soil, thus enhancing the capacity of the soil to retain water and nutrients.  Mulches reduce stress on the plant as a whole and the root system specifically, improving the plants chances to do well despite some nematode damage to roots.  Greater soil organic matter content also stimulates activity of natural enemies such as certain fungi, predatory nematodes, etc. that apparently help suppress nematode populations.

9.  No chemical controls.

Conspicuously lacking is any recommendation of a nematicide to treat nematode problems of plants after they are established in the landscape.  There is presently no effective nematicide that may be applied legally to ornamentals already planted in the landscape.

Table 1.  Susceptibility of some annual ornamental plants to root-knot nematodes in central Florida.  Ratings are the average all tests.  The rating scale ranges from 0 (no galls) to 100 (all roots heavily galled).

Common Name	Scientific Name	Rating
Not Infested, No Galls Found
Marigold, African	Tagetes sp.	0
Marigold, French	Tagetes sp.	0
Coreopsis	Coreopsis lanceolata	0
Argemone	Argemone sp.	0
Rudbeckia	Rudbeckia sp.	0
Ageratum	Ageratum sp.	0
Evening primrose	Oenothera lamarkiana	0
Gaillardia	Gaillardia sp.	0
Very Lightly Infested, with One or Few Scattered Galls
Michaelmas daisy	Aster tradescanti	1
Lupine	Lupine sp.	1
Calliopsis	Coreopsis tinctoris	4
Four-O’clock	Mirabilis jalapa	4
Cosmos	Cosmos bipinnatus	4
Zinnia, small	Zinnia elegans	5
Zinnia, giant	Zinnia elegans	11
Sweet alyssum	Lobularia maritima	7
Torenia, blue	Torenia fournieri	17
Torenia, white	Torenia sp.	17
Thunbergia	Thunbergia sp.	22
Blue sage	Salvia farinacea	10
Scarlet sage	Salvia splendens	20
Arctotis	Arctotis stoechadifolia	14
Phlox, big drummond	Phlox drummondi	19
Phlox, dwarf	Phlox nana compacta	31
Phlox, starred	Phlox drummondi stellaris	26
Statice	Limonium sinatum	18
Globe amaranth	Gomphrena globosa	25
Gerbera daisy	Gerbera jamesoni	24
Vinca, periwinkle	Vinca rosea	30
Stock	Matthoila sp.	31
Leptosyne	Coreopsis sp.	29
Lightly Infested, with a Number of Small Galls
Godetia	Godetia sp.	36
China aster	Callistephus chinensis	38
Pentstemon	Pentstemon sp.	38
Dianthus	Dianthus sp.	45
Portulaca	Portulaca sp.	40
Verbena	Verbena sp.	27
Lantern groundcherry	Physalis francheti	40
Perennial sweet pea	Lathyrus latifolius	42
Liatris	Liatris spicata	44
Clarkia	Clarkia sp.	23
Shasta daisy	Chrysanthemum maximum	48
Candytuft	Iberis umbellata	46
Mignonette	Reseda odorata	50
Cypress vine	Quamoclit pennata	50
Artemisia	Artemisia sacrorum viride	50
Petunia	Petunia hybrida	52
Moderately Infested, with Galls More Numerous or Larger
Acroclinium	Helipterum roseum	55
Linaria	Linaria sp.	56
Poppy	Papaver sp.	56
Moonflower	Calonyction sp.	58
Perennial chrysanthemum	Chrysanthemum sp.	59
Nicotiana	Nicotiana alata	59
Hunnemannia	Hunnemannia fumariaefolia	60
Annual chrysanthemum	Chrysanthemum coronarium	65
Dimorphotheca	Dimorphotheca aurantiaca	66
English daisy	Bellis perennis	67
Scarlet climber or cardinal climber	Quamoclit sloteri	71
California poppy	Eschscholtzia californica	71
Heavily Infested:  A Large Percentage of Roots with Large Galls
Coleus	Coleus sp.	71
Columbine	Aquilegia sp.	74
Sunflower	Helianthus annuus	73
Chinese forget-me-not	Cynoglossum sp.	73
Baby’s breath	Gypsophila sp.	77
Gilia	Gilia sp.	77
Matricaria	Matricaria sp.	80
Nasturtium	Tropaeolum sp.	85
Snapdragon	Antirrhinum majus	84
Hollyhock	Althea rosea	82
Salpiglossis	Salpiglossis sinuata	84
Pansy	Viola tricolor	87
Centaurea	Centaurea cyanus	90
Very Heavily Infested:  Practically All Roots with Many Large Galls
Butterfly flower	Schizanthus sp.	87
Morning-glory	Ipomoea sp.	91
Larkspur	Delphinium sp.	90
Lobella	Lobella erinus	94
Helichrysum	Helichrysum sp.	96
Amaranthus	Amaranthus sp.	93
Calendula	Calendula officinalis	93
Calendula, radio	Calendula officinalis	87
Balsam (Impatiens)	Impatiens balsamina	100
Blue lace flower (Didiscus)	Trachymene caerulea	73
Annual sweet pea	Lathyrus odoratus	96
Celosia	Celosia argentea	99
Dolichos	Dolichos sp.	100
Gourd	Cucurbita sp.	100
(Goff, C. C.  1936.  Relative Susceptibility of some annual ornamentals to root knot.  Univ. of Florida Agr. Expt. Stn. Bull. 291).

Other Links:
Plant Disease Information Notes Home Page
North Carolina Insect Notes

For assistance with a specific problem, contact your local NC Cooperative Extension Service

Recommendations of specific chemicals are based upon information on the manufacturer's label and performance in a limited number of trials. Because environmental conditions and methods of application by growers may vary widely, performance of the chemical will not always conform to the safety and pest control standards indicated by experimental data. All recommendations for pesticide use were legal at the time of publication, but the status of registration and use patterns are subject to change by actions of state and federal regulatory agencies.  05/95/150

Published by the North Carolina Cooperative Extension Service Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.

Revised October, 2000