Causal agent(s) and transmission
Potato cyst nematodes (PCN) are sedentary endoparasites that have a survival stage, the cyst, being able to survive in the soil for many years (up to 10 or even 20 years, depending on the climate) and this is obviously an efficient means of dispersal.
Adult forms are visible to the naked eye in the form of small balls becoming brown (cysts) (0.5 to 0.9 mm) attached to the plant roots. The colour of these cysts is initially pale yellow, then golden yellow in the case of Globodera rostochiensis, and white for G. pallida before turning brown.
Though none of the life stages in either species are naturally motile (except the male), cysts are very easily transported by any soil movement on farm tools, tractor wheels and by any other items likely to carry soil: e.g. boots, runoff water etc. and any plant organs used for planting (tuber, bulb, corm).
These nematodes complete one life cycle synchronised with the potato growing cycle, with a very high multiplication rate of 1: 400-500.
Both species of the potato cyst nematodes are present in most potato production areas where they are considered as major potato pests. Because eradication is impossible, their quarantine status and mandatory control measures, such as soil testing especially reinforced on seed material, help to maintain cyst nematode-free areas for as long as possible.
Symptoms on foliage
During the growing period, the presence of potato cyst nematodes in a field may be noticeable by the presence of patches (roughly circular foci) of poor vegetation (photo 1). Yield loss is rather proportional to the level of soil infestation and in sandy soil, it can reach 50%. However, very frequently adequate fertilizing regimes may often conceal the problem.
Symptoms on tubers
Fine examination of the root system of lifted plants at tuber formation (photo 2) may reveal the presence of white or yellow globes, i.e. females (photos 3 and 4) and brown cysts (photo 5).
In the case of very high potato cyst nematode densities, direct damage resulting from bites of nematodes can be observed on the tubers, in the form of small superficial tiny brown necrotic spots (photo 6).
When the presence of potato cyst nematodes is suspected, the analysis of soil samples is the most reliable form of diagnosis providing the soil sampling method is done thoroughly.
Once introduced into a field via cyst-infested soil on farm equipment or plants for planting, Globodera populations will establish almost anywhere where the potato crop is able to grow.
Intensive cultural practices like absence of rotation, poor control of groundkeepers and use of non-certified seed tubers may be favourable conditions for potato cyst nematodes development. Natural decrease of cyst viability in the soil is favoured by irregular temperatures and low soil-moisture.
Due to the species specificity of the genetic resistance introduced into potato cultivars, improper soil diagnosis for Globodera spp. populations and repeated use of inadequate resistant cultivars may lead to a shift within mixed populations from a prevalence of G. rostochiensis (for which host resistance is available) to pure G. pallida populations (for which host resistance is still scarce).
Once Globodera spp. cysts are introduced in a field, there is no efficient curative control method. Thus preventive methods must be used:
- planting in fields found to be free from potato cyst nematodes after soil analysis; o use of certified seed tubers;
- long rotations (not less than 4 years);
- control of groundkeepers;
- cultivar choice: numerous cultivars are resistant to G. rostochiensis. The type of resistance to G. rostochiensis involved is considered as “total” since there are no newly formed cysts after a growing season. For G. pallida, similar type of resistance has been very difficult to find so far; when present in a cultivar, resistance to G. pallida is referred to as “partial” because, after growing such cultivars in G. pallida-infested fields, there are still some newly formed cysts though fewer than on susceptible cultivars;
- soil treatments using nematicides lessen the impact of potato cyst nematodes on yield but do not completely prevent the development of newly formed populations;
- cultural practices like early potato production (as opposed to main crop) may limit newly formed populations by disruption of the Globodera life cycle thus limiting the formation of new cysts;
- other alternative methods have shown some positive results. Solarization – thermal destruction of the soilborne cysts – can be considered in favourable climatic areas. Catch (or bait) crops, involving planting the potato host – preferably a resistant cultivar – and destroying the crop before cyst formation may considerably reduce population densities if monitored by a nematologist expert.
A combination of these methods may help to keep the potato cyst nematode populations below the economic threshold levels.