There are thousands of nematode (eelworm) species worldwide, a small proportion being plant parasites attacking roots, bulbs, corms, leaves (damage pictured), stems and buds.
Most problems in ornamentals are caused by Aphelenchoides ritzemabosi and A. fragariae (leaf and bud nematodes on a range of nursery stock and herbaceous species), Ditylenchus dipsaci (stem or bulb eelworm on bulbs and herbaceous plants) and Meloidogyne spp. (root-knot eelworm). In addition to damage caused by feeding, nematodes can also be virus vectors.
Potato cyst nematode (Globodera [syn. Heterodera] rostochiensis and G. pallida) is a notifiable pest that has implications for growers using rented farmland for open-ground production because it is easily spread by cultivation.
Cultural controls and nursery hygiene are essential. With limited chemical control methods available, the HDC initiated project CP104, which has identified ways of using currently available (and some new) products more effectively. Leaf and bud nematodes are readily spread by overhead irrigation or propagation mist systems, while the plant propagation chain is also a significant potential means of spread. Plant debris and weeds can also harbour infestations.
Soil sterilants remain effective against soil-dwelling species. Some nematodes are beneficial. Species of Heterorhabditis, Steinernema and Phasmarhabditis are the basis of biological controls against vine weevil larva, sciarid flies and slugs.
How to recognise them
Tiny worms 1-2mm long just visible against a dark background, but through a microscope resembling eels (hence "eelworm") with blunt heads and a mouth-spear in most plant parasitic species. Female cyst nematodes swell to form spherical brown survival cysts 0.5mm in diameter. Root-knot nematode cysts are pear-shaped.
Leaf eelworm infestations may be confirmed by breaking affected leaves into water in a glass. After 30 minutes, any live eelworms that are present will be in a wriggling mass at the bottom.
• Bud and leaf nematodes Aphelenchoides ritzemabosi and A. fragariae
Discoloured, angular blotches on leaves are characteristically contained within the patterns of the veins. Damage can be confused with downy mildew infections — care must be taken when checking vulnerable crops. Bud feeding leads to stunting and distortion of emerging leaves and flowers.
• Stem (bulb) eelworm Ditylenchus dipsaci
Infested bulbs are soft at the neck and when cut across show discoloured brown rings of dead tissue. Any resulting growth is malformed with distorted leaves and flowers.
• Root-knot eelworm Meloidogyne spp.
Galls of up to 2cm develop on roots, often irregular in shape. Attacks disrupt normal root function. Plants show nutrient deficiency, wilt and eventually die.
Treatment: biological control
Some green manure crops, the most effective from the brassica family, such as Caliente mustards (Tozer Seeds), can be sown prior to planting. The mustards contain high levels of natural glucosinolates that form MITC — the sterilant released by some chemical fumigants — when chopped and incorporated into soil.
Nemat, a cultivar of rocket (Eruca sativa) with high levels of glucosinolate in its roots, is a specific trap crop for root-knot nematodes (Meloidogyne spp.).
French marigold ‘Ground Control’ can be used to treat land between crops of field-grown ornamentals. Planted on land containing neither crop nor weeds where the marigold roots are the only nematode food source, the marigolds produce a powerful internal broad-spectrum biocide that will kill the feeding pests.
Treatment: cultural control
Routine weed control is critical to prevent establishment or spread of stem and bud eelworms.
Eelworm-infested stock plants of many herbaceous perennials can be cleaned up by hot-water treatment. Timing and temperature must be strictly controlled to avoid damaging plant tissues.
Small batches should be tested for any effects of treatment damage first. A suggested starting point is 45°C for 15 minutes. Bulb treatment times are longer — three hours at 45°C. Batch sizes should be about one volume of plant material to three volumes of water.
Storing bulbs at 30°C for three weeks prior to treatment can reduce flower damage risks.
Leaf and bud nematodes can be transferred via cuttings and spread on propagation tools, dead and dying plant material and poorly composted material. Avoid excessive moisture in propagation and space plants to avoid leaf contact. Disinfectants should be routinely used to clean tools, equipment and surfaces.
Treatment: chemical control
Active ingredient Dazomet
HRAC code Z
Formulation Various including Basamid (Certis)
Action(s) Granular soil fumigant for pre-planting use outdoors or under protection.
Active ingredient Metam-sodium
HRAC code Z
Formulations Various — all have a current end use date of 31 December 2017
Action(s) Partial sterilant for glasshouse, nursery and outdoor soils with some activity against nematodes.
Active ingredient Oxamyl
IRAC code 1A
Formulation Vydate 10G* (DuPont) — end use date of 31 December 2016
Action(s) Systemic, carbamate nematicide.
Active ingredient Spirotetramat
IRAC code 23
Formulation Movento* (Bayer)
Action(s) Contact and ingested insecticide. Research work overseas and in HDC project CP104 has shown control effect on bud and leaf nematodes.
Fully updated by Dove Associates
Use plant protection products safely. Always read the label and product information before use.
* Extension of Authorisation for Minor Use (EAMU) required for use in ornamental plant production outdoors and/or under protection.
Dove Associates shall in no event be liable for the loss or damage to any crops or biological control agents caused by the use of products mentioned.