Nematodes are the silent enemy. Lets look at avoiding disasters by spotting Potato Cyst Nematodes (PCN) early in potatoes.
Potato Cyst Nematodes – know what you’re dealing with before you plant
Because of the devastating and persistent nature of Potato Cyst Nematodes (PCN), it is vital to know what populations you are dealing with in your soil and what sensible steps should be taken to manage them. But just because you are not seeing visual patches of damage or detecting them in the soil, should you be relaxed?
In many fields, we can assume that Potato Cyst Nematodes are present where potatoes are widely grown, and indeed identified by looking for Cysts. When we want to look the potential damage that they would cause in order to more effectively manage it, we need to work out the population size which means looking for eggs rather than cysts.
US research shows that quite often 3-4 years of continuous potato crops can be grown in a field before the likelihood of detecting Potato Cyst Nematodes. The problem is two-fold; the chance of finding eggs in a small sample of soil is low, and testing enough of the field to randomly encounter a patch of nematode eggs.
In severely infested fields, 20-30 eggs per gram of soil can be picked up provided enough cores are taken. 1,500mls of soil per hectare it what we should be testing give ourselves a fair chance of detecting eggs, made up of 20 individual sampling points per hectare and thoroughly mixed.
On more prone sites (non-irrigated, light soils) take more samples, particularly if a tolerant variety is being grown.
The graph above courtesy of UK’s Agriculture and Horticulture Development Board (AHDB) Potatoes, shows how long it can take to pick up eggs in an infested field – in a 3-year rotation for example it can take four crops before they are likely to be identified, reinforcing the point that in a close rotation, much more detailed sampling is required.
10 eggs per gram of soil is the target; above this level you will start to encounter problems. Tolerant varieties are fine, but remember that these still allow Potato Cyst Nematodes to multiply, causing problems in successive crops. At 20-30 eggs/gram you really need to be using a resistant variety and using granular nematicides such as Mocap and Nemathorin in-furrow.
So when is the right time to test?
I always advise growers with known Potato Cyst Nematodes problems to test immediately AFTER the crop is harvested. If you are sensible then you won’t return to that field for 6 years as an absolute minimum, but sample known patches in the field while they are fresh in your mind – then re-sample those areas to monitor the decline before you decide to replant in that field.
Knowing the species of Potato Cyst Nematode is crucial – Rostochiensis resistance is fairly easy to find in a marketable variety, but Pallida less so. Pallida also tends to have a much longer hatching period in my experience – meaning that longer rotations are less effective.
When deciding on control measures, also keep in mind the final population. Yield loss from 5-10 eggs/gram may be very low, but allowing populations to build up will be costly for those farms on a tight rotation. Using a nematicide in this situation can be money well spent even though there may not be an immediately yield benefit in that crop.
David Jones is the Broad Acre Specialist at Crop Nutrition Laboratory Services Ltd. (CROPNUTS). David has a keen interest in soils and no till farming systems where he has undertaken work looking into weed levels and changes in soil structure, and has extensive experience in field trials and in the development of precision farming techniques. In his spare time he enjoys playing rugby.