Nematodes aren’t usually a serious pest in lucerne, but can reduce stands and thus lower production. Also lesions caused by nematodes allow diseases, like bacterial wilt or fusarium, to attack the lucerne plants. There are three groups of the most occurring nematodes in lucerne: root-knot nematodes, stem nematodes and root-lesion nematodes.
Nematodes or roundworms are a large group of very common worms. There are an estimated 10 million existing species. The nematode species that feed on plants are commonly called eelworms. They are so tiny that you need a microscope to see them. Some nematodes feed on the outer surfaces of a plant while others burrow into the tissue. Soil-dwelling nematodes are the most common culprits, but some species can damage plant roots, stems, foliage, and flowers.
No matter where they feed, these tiny worms can seriously damage your crops with their sharply pointed mouths by puncturing cell walls. The real damage occurs when a nematode injects saliva into a cell from its mouth and then sucks out the cell contents. The plant responds to the parasitic worms with swelling, distorted growth, and dead areas. Nematodes can also carry viruses and bacterial diseases inject them into plants. The feeding wounds they make also provide an easy entrance point for bacteria and fungi.
Meloidogyne hapla (and Meloidogyne chitwoodi to less extend) can live as parasites in the root tissue of lucerne. They develop small oval galls and excessive branching occurs on the roots. The galls may look like nitrogen fixing nodules, but can be distinguished as healthy nodules are usually pink coloured.
Heavy infection with root-knot nematodes can lead to stunted growth. Moreover, infected plants are more vulnerable to disease infections.
As nematodes can come along with seeds, the best way of prevention is to use clean, nematode-free seeds. The best way to control root-knot nematodes is to use resistant varieties. Highly resistant varieties show almost no susceptibility. On crop rotation it is recommended to rotate with cereals if known there are root-knot nematodes in the field. As many weeds are host to Meloidogyne hapla, weed control is an important aspect too.
Ditylenchus dipsaci can be a destructive pest, especially in areas with higher rainfall during cool weather. Infection with stem nematode causes the stem base to become swollen and roughened. Swollen nodes and shortened internodes are very visible symptoms. Infected stems may blacken and are easily broken off. With severe infection a symptom called “white flagging” may occur: regrowth after cutting lacks all green colour and completely white leaves appear on several plants through a field.
The use of resistant varieties are the most practical means to control stem nematodes. There are enormous differences between alfalfa varieties in their infection rate. Highly resistant varieties have more than 75% resistance, whereas susceptible varieties may show only 20% resistance.
Pratylenchus penetrans is a parasite which moves in the root system and damages plant cells. High numbers can reduce growth in alfalfa. Damage is difficult to assess because no clear above-ground symptoms can be found, only side effects like stunted growth or nutrient deficiency. Infection with fusarium wilt or decreased cold tolerance might be an indication of infection with root-lesion nematodes. Usually a soil and root sample has to be analysed by a laboratory to prove infection.
Very little is known about the difference in resistance between lucerne varieties. Therefore, a long-term crop rotation and application of the right catch crops for nematode control are recommended if lucerne is grown on farms with root-lesion nematode occurrence.
Typical nematode infection can be recognized by spots in the fields with infected or dead plants. The infection spreads from the borders of the spot towards the whole field. Control in an early stage might be effective, by plant burning or soil fumigation.
In lucerne, the sensitivity to nematodes is largely genetically determined. That is why all our lucerne varieties are tested for their sensitivity to nematodes at an early stage in our breeding programs. This has resulted in many varieties with excellent nematode resistance. The nematode resistance of the Barenbrug varieties is also recognized by official studies in Northern France.
Examples of nematode resistant varieties are: Artemis, Alexis, Barmelie and Soraya.
Several diseases can affect lucerne. Leaf spot, wilt diseases, crown and stem rot are the most common diseases. Find out how you can prevent them.
