Can cover crops suppress nematodes?

The results of glasshouse trials suggest that marigolds and brassicas could contribute to controlling lesion nematodes in apple trees. By Anna Mouton.

Lesion nematodes (Pratylenchus species) feed inside plant roots and create entry wounds that are susceptible to secondary bacterial and fungal infections. The resulting root damage impedes water and nutrient uptake. Young apple trees are especially vulnerable and may suffer reduced growth and yields.

Dr Rinus Knoetze, nematologist at the ARC Infruitec-Nietvoorbij, believes Pratylenchus is an underrated problem. “Other apple-producing countries place far greater emphasis than South Africa on controlling it,” he says. “We know it’s widespread here, but it’s difficult to demonstrate how harmful it is when you have no uninfected orchards for comparison.”

Although soil fumigation before planting kills nematodes, our markets increasingly resist synthetic chemicals to control pests and diseases.

“Fumigation is also only effective until you plant a nursery tree that’s already infected,” says Knoetze. “One reason why Pratylenchus is such a big problem in our industry is that many of our layer beds are infected.”

Soil application of nematicides after planting suppresses but doesn’t eradicate nematodes. Treatments are costly and must be repeated. Nematicides are also subject to the same market pressures as fumigation.

To find effective and sustainable alternatives to fumigation and nematicides, Knoetze led a Hortgro-funded pilot project investigating the use of cover crops to suppress lesion nematodes. Louis Fourie was the MSc student on the project.

How do cover crops work?

Marigolds are popular garden ornamentals grown for their prolific yellow and orange flowers. Researchers have shown that African marigolds (Tagetes erecta) and French marigolds (Tagetes patula) can suppress Pratylenchus species. The plants’ common names don’t reflect their nationality — Tagetes species originate from Mexico and other Latin American countries.

Tagetes roots produce and release α-terthienyl, which has nematocidal activity. Marigolds could also act as trap crops by arresting the development of nematodes. Nematodes that infect them cannot reproduce, decreasing the overall nematode population.

“This means that marigolds must be actively growing near the nematode problem,” says Knoetze. “There is some research recommending that you incorporate the marigolds into the soil after the growth period, but we didn’t find any evidence that it makes a difference.”

Brassicas include important crops (for example, broccoli, cabbage, mustards, and canola). Brassica species produce sulphur-containing glucosinolates to fend off hungry insects, although some insects are specialised predators of glucosinolate-containing plants.

Glucosinolates are converted to nematocidal isothiocyanates when brassicas are incorporated into the soil — a process called biofumigation. Previous research has shown that chopped brassica leaves and brassica seed meals can reduce plant-pathogenic nematodes.

“Conditions must be right for biofumigation to work,” says Knoetze. “You must incorporate the plant at the right growth stage, and the soil pH, moisture, and temperature matter.”

Biofumigation can also be enhanced by solarisation, which involves covering the soil with plastic after incorporating the brassicas. The plastic covering raises soil temperature, promoting the decomposition of the incorporated brassicas while trapping the nematocidal isothiocyanates.

Two types of glasshouse trials

Cover crops are a potential alternative to pre-planting soil fumigation and post-planting nematicide treatments. Fourie and Knoetze conducted glasshouse trials to test cover crops for both applications. They focused on Pratylenchus hippeastri, the most common root-lesion nematode in South African apple orchards.

Marigolds and brassicas were grown in pots for the glasshouse trials. The study included three brassicas (B. juncea, B. carinata, and Sinapis alba) and two marigolds (T. erecta and T. patula). The study also looked at the nematodes’ response to a suitable host (sweet corn {Zea mays}).

Cover crops before orchard establishment

To determine whether cover crops can suppress nematodes before orchard establishment, they were sown into individual pots previously inoculated with Pratylenchus hippeastri, with an unsown pot acting as the fallow control.

The population of P. hippeastri in soil and roots from each pot was quantified 16 weeks after sowing the cover crops. The soil was then amended by incorporating the finely chopped aboveground cover-crop material.

The effect of solarisation was assessed by covering half the pots for each cover crop with transparent plastic after soil amendment.

Four weeks later, a tissue-cultured G.778 apple rootstock tree was planted in each pot, grown for 12 weeks, and examined for P. hippeastri infection.

Cover crops after orchard establishment

To see whether cover crops can suppress nematodes in existing orchards, G.778 apple rootstock trees were planted in pots previously inoculated with P. hippeastri. The cover crops were individually sown into the pots, with an unsown pot acting as a fallow control.

After 16 weeks, the apple trees were lifted, the cover-crop material was incorporated into the soil of their respective pots, and the apple trees were replanted in the pots. Nematode populations in the cover-crop and apple roots, and soil were quantified.

The apple trees were grown for a further 12 weeks, after which the nematodes in their roots and the soil were again quantified.

Cover crops for soil preparation

The trials comparing P. hippeastri populations in pots sown with different cover crops showed that the nematodes struggle without host plants. Less than 5% of the nematodes survived in the control pots containing soil but no plants.

Marigolds appeared to suppress nematodes. Pots sown with T. patula had similar nematode survival to the fallow control, and pots sown with T. erecta had significantly lower survival than either T. patula or the fallow control.

In contrast, brassicas significantly improved nematode survival relative to the fallow control. Of the three brassica treatments, nematode survival was greatest in B. carinata and least in S. alba.

Nematode numbers declined to less than 20% of the introduced populations over time in all the cover-crop treatments. In follow-up trials that included sweet corn, a good nematode host, the nematode populations in the sweet-corn treatment increased by up to 40%, whereas populations in the cover crops and fallow soil all shrunk to less than 10% of the introduced populations.

“We included sweet corn to show that planting a cover crop that’s a good nematode host — something like rye or clover — before establishing an orchard makes the problem worse,” says Knoetze.

After the cover crops had been incorporated into the soil and apple rootstocks planted in the pots, the apple trees in the sweet-corn pots developed significantly more severe nematode infestations after 12 weeks than those in any of the cover-crop treatments.

In the cover-crop treatments, nematode infestations in apple trees differed in solarised and non-solarised pots. Solarisation significantly reduced infestation of apple roots in the brassica and T. erecta treatments, compared with not covering the pots. The plastic covering didn’t reduce nematode infestations in apples in the fallow control pots.

Cover crops for existing orchards

To assess the ability of cover crops to suppress existing nematode populations, Fourie and Knoetze sowed cover crops in pots containing infected apple rootstocks to simulate planting cover crops in an established, infected orchard.

Analysis of the cover-crop roots showed they are poor hosts compared to sweet corn, a known good host.

In the first of two trials, the fallow control represented a situation where an infected apple tree is grown without cover crops. It resulted in a five-fold increase in nematodes. On the other hand, all the cover-crop treatments resulted in significantly lower nematode reproduction compared to the untreated control.

In the second trial, the nematode numbers in cover-crop and apple roots were determined. A good nematode host (sweet corn) was also included, and the results confirmed that it’s an excellent host. Nematode numbers were significantly lower in the cover crops than in sweet corn, with B. carinata again being the best host and T. patula the poorest.

However, when the apple roots were analysed, there was no significant difference in nematode infestations between apples in the sweet-corn treatment and any of the cover-crop treatments, except for T. patula. Nematode reproduction was significantly less in the T. patula treatment than in the sweet-corn control.

Brassicas primarily suppress nematodes through biofumigation, which requires soil temperatures above 20 °C. During Knoetze’s pot trials, temperatures weren’t optimal for biofumigation, highlighting the importance of environmental conditions for successful biofumigation.

“I don’t think brassicas for nematode control are ever going to be a good application in existing orchards,” adds Knoetze. “You probably couldn’t grow and incorporate enough brassicas without compromising the growth of the apple trees.”

Field trials

As marigolds hold more promise for controlling nematodes in existing orchards, Fourie and Knoetze conducted a pilot study with T. patula and T. erecta in a five-year-old commercial Golden Delicious orchard on MM.109 in Grabouw. They assessed each species as a monoculture.

The marigolds were sown on the ridges in September, and nematodes were quantified in April and September of the following year. The orchard has micro-sprinkler irrigation.

There was no significant difference in nematode populations between the treatment sites at the start of the study or in April of the following year. From April to September, nematode numbers fell significantly in the treatments and control. However, the final evaluation showed that the T. erecta treatment had significantly fewer nematodes than the T. patula treatment and the control.

Although both marigold species established well, T. erecta is a larger plant and dominated the weeds on the ridges. Knoetze speculates that the bigger T. erecta also has a bigger root system, which probably produces more nematode-repelling compounds than T. patula’s smaller root system.

“The follow-up of this project is to test these cover crops in field trials,” says Knoetze. “Glasshouse trials can’t entirely simulate the survival of nematodes when an old orchard is removed and a new one planted.”

Meanwhile, the results suggest that growers might have better options than traditional cover crops. Fourie and Knoetze quantified the nematodes on an old apple site that had been grubbed and planted to triticale for two years.

“That site was full of lesion nematodes. We found it in the roots of the triticale and weeds,” says Knoetze. “Instead of triticale, you could rather have planted marigolds.”

Field trials continue this year.

What does industry say about this research?

“Lesion nematodes are problematic in the EGVV, specifically in orchards with rootstocks that aren’t tolerant to lesion nematodes. Currently, we are mostly planting Geneva rootstocks that are tolerant to lesion nematodes, and we generally don’t see problems with lesion nematodes in these orchards.

“Cover crops interest most growers, but there isn’t much science behind it. I think trials like these, which cast more light on specific benefits of cover crops, are valuable.”

Technical adviser Dian Craven. Fruitways Agri Services.