Nematodes are microscopic, eel-like roundworms. The most troublesome species are those that live and feed within plant roots and those living freely in the soil and feed on plant roots. There are many different species of root-feeding nematodes, the most damaging are the rootknot nematode, Meloidogyne spp. Root knot nematodes attack a wide range of plants, including many common vegetables, fruit trees, and ornamentals.
Symptoms & Life Cycle
Plants with root-knot nematode infections show poor growth; they are stunted with yellowing leaves, and wilt easily. Root systems show characteristic knots or galls, which prevent the uptake of water and nutrients. Plants with root-knot nematodes are more susceptible to invasion by disease-causing fungi and bacteria. A microscope is needed to see root-knot nematodes; the active stage, the "juveniles", are about 0.5 mm long, and they are too small to be seen by the naked eye. The eggs are laid outside the root, and hatch to release the juveniles, which are mostly female.
The juveniles enter the root at the root tips. After traveling a short distance inside the root, they stop, and their presence causes the formation of giant cells, upon which they feed, and which form the characteristic galls. The juveniles in the root moult three times more, and when mature are pear shaped. The females stay inside the root, feeding and laying eggs. Each female lays more than 1000 eggs. The life cycle from egg laying to maturity takes about 25-30 days.
Spread over short distance occurs as the juveniles move in soil water in search of new roots to infect; over larger distances, spread occurs in the roots of seedlings, young plants or vegetative propagating materials. It is also possible for nematodes to spread in rainwater, and soil on tools, machinery or footwear.
There is increasing numbers of commercial hydroponic and outdoor ornamental growers becoming infested with plant parasitic nematodes, mostly by the root knot nematode Meloidogyne spp. Affected growers reported production losses up to 45%. To decrease the number of rose crops infested with root knot nematodes, more knowledge about the sources and spread of these plant parasitic nematodes is very necessary.
Detection & Inspection
Look for plants that are growing poorly and wilt readily even though they have adequate nutrition and water. Look at the roots and check the presence of root knots. Be careful when examining legumes that nitrogen-fixing bacterial root nodules are not confused for root galls caused by nematodes. Soil samples should be taken to determine whether nematodes are present and in sufficient numbers to cause damage.
Note: Consult Nematology specialists for methods of sampling and how to send samples to laboratory for analysis.
Biological Control The bacterium, Pasteuria penetrans, Pochonia chlamydosporia the fungi Purpureocillium lilanicus and Trichoderma viride have been long recognized as potential biocontrol agent for root-know nematodes.
Potting composts: in nurseries, use soil-less potting mix, or sterilised/pasteurised soil mixes.
Soil solarisation: kill root-knot nematodes (plus other pathogens and weeds) in the top 30 cm of (especially sandy) soil by using transparent polyethylene covers. Moisten soil and cover for 4-6 weeks.
Fallow land: keep soil bare for 4-6 months, but with frequent cultivation; make sure that weeds are controlled. The method exposes the nematodes to drying by the sun, and to starvation.
Rotation: growing non-host crops can reduce nematode populations.
Soilamendments: Add manures or composts to the soil, especially sandy soils, to increase ability of soil to hold water and nutrients.
Maintenance: Aim for optimum plant growth by frequent deep watering, and adding mulches and organic matter.
Collect the remains of the crop and debris and burn it.
Chemical Control A wide range of chemicals has been used for the control of root-knot nematodes, e.g., fumigants (methyl bromide, metham sodium, chloropiocrin), non-fumigants (organophosphates: fenamiphos, ethoprophos, thionazin, fensulphotion, carbamates: aldicarb, oxamyl, carbofuran, methoxyl, avermectins, milbemycins: Abamectin, Emamectin benzoate).