Cover crops for PCN and other plant parasitic nematodes; which species work?

Cover Crops for Nematodes

Cover crops perform a range of functions. From protecting vulnerable soils to promoting soil biology to providing much-needed food for birds and bees, these crops often complement permanent land features such as hedgerows and woodlands.

Increasingly, cover crops – trap and biofumigant types – are increasingly being promoted as a means of managing potato cyst nematodes and other nematode pests found in soil. New research, however, has begun to shine a light on the contrasting level of performance between crop species which serves to complicate the choice of cover crop to be sown.

“Growers need to understand that there are a range of species involved, each with distinct patterns of behaviour and host ranges,” says Dr Matt Back, a reader in nematology at Harper Adams University.

“When we talk about cover crops, we need to appreciate that some species of crop will have a suppressive effect on one species of nematode but may enhance another,” he adds.

When deciding between the various species of cover crop available the first criteria is to understand your priorities, says Eric Anderson, senior agronomist for Scottish Agronomy.

“What’s the objective, is it a pollinator mix of nectar-rich flowers, a wild bird seed mixture, a deep-rooting plant to maintain soil structure or a biofumigant or trap crop for nematodes? A cover crop that fits all sizes doesn’t exist, so you first need to understand what it is you’re trying to achieve,” says Mr Anderson.

Trap crops, especially Sticky nightshade (Solanum sisymbriifolium), gained attention in the late 1990s and early 2000s when farmers were allowed to establish these crops on out-of-production land such as set-aside. Interest waned after 2008 when set-aside was abandoned as a policy under the reforms implemented by then commissioner for agriculture, Mariann Fischer Boel.

“Solanum sisymbriifolium worked well with set-aside because it needs the summer months from the end of May through to October to establish. This is difficult to achieve within the confines of a typical arable rotation,” says Dr Back.

The area of cover crops sown specifically for nematode control has remained low since at between 200-300 hectares – typically 0.2-0.3% of the crop area in most seasons. The majority of this being grown on Jersey because the harvest interval of fosthiazate and oxamyl until its withdrawal at the end of 2020 preclude its use in salad crops.

So why the renewed interest in cover crops? It’s partly due to the development of other Solanum species with suitability to UK conditions and a growing appreciation among growers and their advisers that cultural methods outside the affected crop offer the most sustainable means of managing populations for the long-term, explains Dr Back.

In 2022, a consortium consisting of Produce Solutions, Harper Adams University, CHAP, Vegetable Consultancy Services and a number of farming businesses received government funding from Innovate UK to investigate the relative performance of three Solanum species against known populations of Globodera pallida and G. rostochiensis at sites in the west and east of England. The Solanum cover crops provided by Produce Solutions include: Solanum sisymbriifolium (DeCyst-Prickly), Solanum scabrum (DeCyst Broadleaf) and Solanum chenopodioides (DeCyst Podium).

DeCyst Prickly - Picture Credit Richard Griffith, Produce Solutions

“In Kenya, where PCN is an emerging problem, African nightshade (Solanum scabrum) has emerged as an effective trap crop reducing numbers of PCN and root-knot nematodes (Meloidogyne spp.) by as much as 85%,” says Dr Back.

“Tall nightshade (Solanum chenopodioides), a native plant of South America that has become naturalised elsewhere around the world including lowland England, also shows potential. Some of our trials conducted in 2022 suggested it is a hardy type given that it established well in dry conditions and shows promise against PCN,” he adds.

The relative success of these crops often depends on good crop establishment although this too needs to be put into context, says Dr Bill Watts of Produce Solutions.

“PCN can and do migrate up to about a metre, which is how areas of infestation become bigger over time. While good establishment is important a big crop, even with a few gaps here and there can still achieve high efficacy,” says Dr Watts.

Experiments conducted in 2022 have monitored crop density and biomass in relation to macronutrients (P and K), seed rate and establishment method. Previous work from the Netherlands highlights the importance of the relationship between above ground biomass and root length density: the greater the root length density the better the trapping effect.

“The work found that root length density peaks once foliage reaches roughly 700 grams of dry matter per metre squared (7t DM/ha). This is broadly equivalent to a fresh weight of about 50-60t/ha and correlates to an efficacy of up to 75%. With African nightshade, a similar amount of crop dry matter per metre squared, could cause reductions as high as 85%,” says Dr Back. The Innovate UK-funded trial will continue for a further two years to give robust data spanning several seasons.

Trap or biofumigants: which is better?

Trap crops, such as the Solanum species offered by Produce Solutions, and biofumigants such as oil radish and Indian mustard, are not mutually exclusive options with both having a place in an integrated pest management strategy, explains Dr Watts.

“I see biofumigants as akin to an all-out assault on the PCN population and trap crops as a fatal temptation. A young population will be in a robust state of dormancy, so a biofumigant is likely to have more effect. Move on a season or two and a trap crop is likely to perform better,” he says.

Understanding when to utilise them within the rotation is central to success, he believes, with the middle and latter half of the rotation considered to be the best points at which to target PCN.

For potato growers in Scotland, oil radish, especially the variety Bento, has become a favoured biofumigant for the reduction in PCN populations.

“With Bento more than other varieties of oil radish, we have seen population reductions of 50% across a range of seasons and situations. Importantly, it seems to establish well in Scottish conditions,” says Mr Anderson.

Biofumigants differ from catch crops in their mode of action. Trap crops release similar complex terpene analogues to those released by potato crops to stimulate a hatch and attract juveniles into the plant roots where they eventually die having been unable to complete their life cycle. In contrast, biofumigants release glucosinolates, these are then converted into volatile isothiocyanates, a form of mustard gas, that permeates through the soil killing the hatched juveniles in the process.

“The quality and quantity of the glucosinolates produced by different varieties and types of brassica crop varies considerably hence why we talk about Bento and not other varieties of oil radish. To identify those varieties that produce glucosinolates in high volume, we will be conducting a screening trial this season while trials in Scotland and Canada will involve root-lesion nematodes (Pratylenchus spp.),” says Mr Anderson.

As with Solanum species of trap crop, good establishment is fundamental to success with oil radish.

“You must aim for maximum biomass. This means sowing in June as a non-cash crop or perhaps after vining peas or AD rye. Once it reaches 45 centimetres (18 inches), flail the crop to prevent seed return and leave it to grow on for another 10 weeks. The flail action stimulates the release of glucosinolates which because they’re in the root, don’t require incorporation in the way that Indian mustard does,” says Mr Anderson.

Research shows cover crops give contrasting control of root-lesion nematodes

Harper Adams University PhD student, Vongai Chekanai is investigating the effect of popular cover crop species on populations of root-lesion nematodes (Pratylenchus spp.).

Her research, which has one more year of field trials to complete, focussed on utilisation of cover crops for nematode control in narcissus crops. Given the wide host range of root-lesion nematodes, however, the findings have implications for other crop species.

“The published literature lists several crop types as suitable for reducing nematode populations, but the work is incomplete as it doesn’t cover all nematode species and cover crop varieties,” explains Ms Chekanai.

Five crop species, phacelia (Phacelia tanacetifolia), black oats (Avena strigosa), French marigold (Tagetes patula), oilseed radish (Raphanus sativus) and Indian mustard (Brassica juncea), were included in the trial. The variety of oil radish was not Bento.

The first series of trials were performed in the controlled environment of a glasshouse before moving to a field in Scotland with a history of daffodil bulb production. The preliminary findings, based on a successful first year, already give a clear indication of the contrasting performance.

“Only French marigold gave a statistically significant reduction in root-lesion nematodes with a reduction as measured per kilo of soil of 57%. Oil radish did reduce numbers, by about 9%, but it wasn’t statistically significant. Indian mustard was an excellent host which increased numbers by 293%, while black oats and phacelia maintained the nematode populations. The field results for Oil radish, French marigold, black oats and phacelia reflect that seen in the greenhouse trials,” says Ms Chekanai.

“Indian mustard seems an ‘excellent host’ for Pratylenchus species. It is a good source of food and provides a suitable host for the nematode to easily complete its lifecycle,” she adds.

With little impact that the summer drought of 2022 affected either the cover crop or the control plots, Ms Chekanai is confident that the data collected is statistically robust and will be strong enough to support advice to growers.

“Next season is about confirming what we saw in 2022 although I will drop Indian mustard and Phacelia and add forage chicory (Cichorium intybus) and lucerne (Medicago sativa) while I also hope to generate a significant result for oil radish,” says Vongai.