Not All Nematode Suppressing Cover Crops are Created Equal

Cover crops provide many benefits to soil including the addition of organic matter, erosion control, and may act as catch crops for nutrients and as living mulches that suppress weeds. Some cover crops also suppress soil populations of plant parasitic nematodes. However, cover crops vary in their ability to suppress nematodes as a function of crop or variety type and the types of plant parasitic nematodes that are present. The various nematode suppressing cover crops also differ in the way they suppress nematode populations. Knowing what type of nematode is causing problems in the field and understanding how different nematode suppressing cover crops work will help in the selection of the proper cover crop and the management required for successful control.

Figure 1. Marigold cv. CrackerJack provides very good plant parasitic nematode control

Marigolds are a popular cover crop for suppressing nematodes; however, only certain varieties of the French dwarf (Tagetes patula), the African (T. erecta) and South American (T. minuta) marigolds reduce numbers of root lesion and root-knot nematodes (Figure 1). In Ontario, research has shown that root lesion nematodes were effectively suppressed by the French dwarf marigold variety 'Creole' and the African marigold variety 'CrackerJack'. Marigolds produce polythienyls and thiophenes, compounds which are toxic to nematodes. Only certain varieties of marigolds produce a high enough concentration of polythienyls and thiophenes to suppress nematodes. Since these nematicidal compounds are exuded from roots, the marigold crop should be left in the ground for at least 2 months at a high enough density to produce a concentration that is lethal to the nematodes.

Some studies on marigolds indicate that the nematicidal compounds are released into the soil from the decomposing tissue after incorporating marigolds as a green manure. Recent research has shown that certain bacterial associated with marigold roots are also suppressive to root lesion and other nematodes, suggesting that nematode suppression is a result of multiple properties of this cover crop.

Brassicas crops such as rapeseed, mustard and oilseed radish have also been shown to suppress root knot and root lesion nematode populations. When the leaves and other aerial parts of brassica crops break down, they release glucosinolates which are further broken down to form several nematicidal compounds such as isothiocyanates and nitriles that are toxic or that interfere with the nematode reproductive cycle. In fact, the glucosinolate breakdown products are similar to the chemical fumigant metam sodium, which degrades in soil to methyl isothiocyanate. Glucosinolate compounds are also responsible for the pungent flavours and odours associated with mustards and horseradish. Although most brassica crops produce some glucosinolates, certain types of brassica crops such as mustard, rapeseed and oilseed radish produce more glucosinolates than others. Varieties of oriental mustards such as 'Cutlass' and 'Forge' and oilseed radish such as 'Adagio' and 'Kernel' produce more glucosinolates than others. Not much if any glucosinolates are produced in roots and therefore the brassica crops must be ploughed under as a green manure to obtain nematode suppression. Incorporation of mustard as a green manure is best done prior to the plants setting seed, which will eliminate the potential of this crop becoming a weed problem in subsequent crops.

Figure 2. Oriental mustard cv. Cutlass produces glucosinolates that break down into nematode suppressing by products.

Certain varieties of Pearl millet such as Canadian Forage Pearl Millet 101 are not good hosts for root lesion nematodes. The nematodes will invade pearl millet roots, but reluctantly. Those root lesion nematodes that do invade pearl millet roots do not reproduce well, resulting in a population decline of the pest. Some reports indicate very good control of root lesion nematode with certain varieties of pearl millet. Resent research results indicate that incorporating the pearl millet as green manure will increase the carbon:nitrogen ratio in the soil, which further suppresses some plant parasitic nematode species. Some pearl millet varieties are actually good hosts of root lesion nematodes. Choosing the correct variety is critical for nematode suppression.

Figure 3. Canadian Forage Pearl Millet 101 suppresses plant parasitic nematodes.

| Top of Page |