Are cover crops hosts for lesion nematodes?

Cover crops in orchards have many benefits, but could there also be risks? By Kyra-Kay Rensburg.

A Hortgro-funded project led by Dr Rinus Knoetze, senior researcher at the Agricultural Research Council, screened cover crops earmarked for trials in apple orchards to see whether they can host lesion nematodes that feed on the roots of South African apple trees.

Orchard cover crops have many uses, including improving soil structure by adding organic matter and preventing the build-up of soil pests. But, cautions Knoetze, using cover crops that are hosts of plant-parasitic nematodes can result in the multiplication of those nematodes in apple orchards.

Lesion nematodes in the genus Pratylenchus are specifically a problem on apple trees. They are endoparasites that enter and live inside roots. The nematodes cause wounds in the roots that often develop secondary bacterial and fungal infections. Affected trees decline, and yields suffer. Very high infestations can kill trees.

Testing nematode preferences

Large numbers of nematodes were needed for the study, so Knoetze bred them in Petri dishes. The process involves inoculating nematodes on surface-sterilised carrot discs. The nematodes multiply to produce thousands of offspring per carrot disc.

“It’s quite a finicky process because of contamination of fungi and bacteria, so we needed to do some industrious optimising work to be able to culture enough of the nematodes,” explains Knoetze.

To assess whether plants are good hosts, Knoetze included a known favourite host in each trial.

“Once we see the response of the good host plant, we can compare that with the cover crops that we are screening,” says Knoetze, “and we can determine if a plant under scrutiny is a good host or not.”

The primary lesion nematode occurring in South African apple orchards is a species called Pratylenchus hippeastri, according to Knoetze. “There aren’t a lot of these cover crops that we’ve tested that are good hosts for it, so that was good news for us.”

However, he found that another species of lesion nematode, Pratylenchus penetrans, which occurs less frequently in South African apple orchards, uses more cover crops as hosts and is better adapted to colder conditions than Pratylenchus hippeastri and Pratylenchus vulnus. Identifying the species of lesion nematode occurring in an orchard should help the grower choose the appropriate cover crop.

Implications for industry

The results of the project are summarised in Table 1. “We are not yet advising growers on which cover crop to use for specific purposes,” clarifies Knoetze, “but we can now say to them, if they want to use a specific cover crop, whether it’s a good idea with regards to the nematodes.”

Based on his trials, he concluded that Indian buckwheat and nasturtiums could potentially increase the numbers of all three species of lesion nematodes. Pink serradella, rye and subterranean clover are considered good hosts for Pratylenchus penetrans and can allow the reproduction of Pratylenchus hippeastri during spring and summer.

Table 1: Cover-crop host status of the three main Pratylenchus species known to occur in South African apple orchards.

Brassicas such as mustard and rapeseed have proven to be poor hosts for lesion nematodes. “Their potential use is for biofumigation,” says Knoetze. “After these cover crops are grown in orchards, they can be incorporated into the soil. Glucosinolates in the tissues will be hydrolysed to produce isothiocyanates during the breakdown of the plant material. This is thought to help to control nematodes in the soil.”

Marigolds (Tagetes patula), on the other hand, have an allelopathic effect. Apart from being a poor host, their roots exude a secondary metabolite believed to deter nematodes.

“Research on the use of specific cover crops is still ongoing,” says Knoetze. “However, we know which cover crops we can safely use to avoid nematode troubles.”

What are people saying about this research?

“Root lesion nematodes are very important pests on pome fruit. Regenerative agriculture and particularly orchard floor ecology rely on cover crops as one of the basic soil health principles. It is, therefore, vital that the host status of these cover crops with respect to root lesion nematode is established. Research which covers this aspect is key in our move to a more environmentally friendly approach to controlling pests.”

Sheila Storey. Director, Nemlab.