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 1His 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).


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Revised October, 2000