Potato Cyst Nematodes (PCN)

Potato Cyst Nematodes in Kenya

The Golden potato cyst nematode (PCN), Globodera rostochiensis, was first formally identified in Kenya at the end of 2014. A researcher, looking at nematodes that affect cabbages for his MSc, found the cysts in soil samples taken in Nyandarua County. Subsequent sampling located more PCN cysts in soils from Ndaragwa, Ol Joro Rock, Ndundori, Ol Kalou, North and South Kinangop.

Origin

Potato cyst nematodes (PCN) originated from the South American Andes where they coevolved with their preferred host, the potato. They have now spread to many regions of the world infesting approximately 65 countries. There are two main species, the White PCN, Globodera pallida, and the Golden PCN, Globodera rostochiensis. Both look very similar and are difficult to visibly distinguish from one another.

Symptoms and effect

PCN can be very debilitating to potato production and costly to manage. If left uncontrolled infestations by PCN can reduce yields by up to 80%. In England & Wales where over 60% of agricultural soil is infected with PCN, it costs the UK potato industry an estimated annual KES 7,000,000,000 (billion) to manage.

Potato plants with roots infected with PCN are much stunted, darker, yield less, and wilt more easily in dry spells. They have much smaller roots which are less efficient in taking up water and fertilisers and have more stem cankers. In the field you will see patches in the rows where the plants don’t meet. PCN infected plants are much more susceptible to disease such as Verticillium Wilt and Rhizoctonia solanii. At harvest you get smaller tubers and less of them. PCN reduce the sugar content and quality of the tubers, leading to darker flesh, a lower fry rating and shorter shelf life.

PCN infecting a field in Germany (credit Heinicke)

PCN cysts on plant roots (credit Heinicke)

PCN Life Cycle

1) The life cycle starts with the cyst; the old, body of the female containing approx. 200-500 eggs and measuring 0.3mm in diameter. The tanned cuticle protects the eggs from chemicals and desiccation and they can stay dormant in the soil for up to 20+ years in the absence of a host plant.
2) Every year a very small portion of the eggs hatch spontaneously. Most, however, will only hatch from host plant root exudates. Juveniles emerge that swim towards the roots. At this time they are vulnerable to treatment by nematicides and biological control.
3) The juveniles penetrate the roots just behind the growing tips, and migrate to the centre of the root where they set up feeding sites, called syncytium. The nematodes feed on the root sap, debilitating the plants. When mature, females rupture through the root cortex, at this stage they can be seen attached to the roots. The mature males migrate out of the roots and towards the females to fertilize them. Fertilized females fill with eggs, before dying. Their outer skin (cuticle) tans and hardens to form a protective coat. As the roots die the cysts drop off into the soil then drops off the root and into the soil. The cysts are very difficult to see in the soil and are easily spread around with movement of soil. Cysts can also lodge in the eyes of the tubers, or in the soil attached to the tubers and can easily be spread with infected tubers.

PCN Management Options

Have your soil tested for PCN in our laboratory before you plant your potatoes. We will test for presence/absence and the number of cysts per gram of soil. A high number of viable cysts means it will not be economical to plant potatoes in that soil, because of severe yield loses.

Management of PCN is difficult and costly, and may not be practical. A farmer’s primary aim should be avoidance and an attempt to keep fields PCN free.

Long rotations leaving at least 6 years between potato crops, to allow the egg populations in the cysts to decline are highly recommended. Make sure that there are no host crops or ‘ volunteer’ potato crops in the field. Hosts include plants from the Solanum family, night shade, tomato, capsicum and egg plants.

Cysts are spread in the soil – so make sure that no soil is moved from an infected field on tractor tyres, tools, shoes, dust, plants and especially potatoes. Make sure everything is clean and soil free between fields. This will also help prevent movement of diseases!

Cysts float and move around easily with runoff water, protect your land from water erosion moving on or off your fields. This is good agricultural practice and will benefit you in many ways! Using borehole water will help restrict potential infection from outside.

Always buy potato seed from a reputable source, and make sure the seed is clean of soil. If not wash it well away from production fields and rivers.

Biological control for example treating the potato tubers at planting with Paecilomyces lilacanus (against nematodes) and Trichoderma sp (against diseases), will increase your yields and benefit your soils, although research continues to look for effective varieties.

Planting PCN resistant potato varieties can reduce yield loss due to PCN and reduce the number of cysts in the soil. Planting PCN tolerant varieties can reduce the yield loss due to PCN BUT will increase the number of cysts in the soil. Again, PCN is new in Kenya, and this needs more research.

Planting trap crops that stimulate the eggs to hatch, but do not host the nematodes has been known to substantially reduce PCN cysts in the soil AND can increase your soil fertility. In Holland a great deal of research has been done on Solanum sisymbriifolium as an effective trap crop.

Use PCPB approved nematicides to kill the hatching juveniles just after planting and throughout hatching. The White PCN, G. pallida, hatches for up to 12 weeks, and the Golden PCN, G rostochiensis, hatches for up to 6 weeks. Nematicides are toxic to humans and the environment and use should be approached with extreme care. Most pesticides previously used to treat PCN have been withdrawn due to their toxicity and danger to health.

A list of biological control agents and nematicides can be found on www.inputs4ag.com , the one stop farm inputs directory for Kenyan farmers.