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Potatoes – October 2020

1 October 2020

Potato groundkeepers and crop health in 2021

Volunteer potatoes are a significant threat to crop health principally because they can act as a reservoir of disease inoculum (e.g. blight, Rhizoctonia, black dot, powdery scab, aphid-transmitted and spraing viruses) and pests (e.g. potato cyst nematode, aphids, free living nematodes/spraing). Also, the presence of volunteer potatoes above a threshold population in seed potato crops can result in the crop being downgraded at inspection.

Volunteer potato plants can grow from overwintering tubers. If these tubers were infected with blight then some resulting plants can act as a primary source of blight the following season spreading inoculum early in the season. If the volunteers arise from either non-infected overwintering tubers, or from true seed from potato berries, then the blight pathogen can still produce large numbers of spores on their unprotected foliage, though this will probably occur later in the growing season.

There is no single, highly effective method of controlling volunteers and an integrated control strategy is required. Integrated control measures that can be utilised at this time of year are:

  • Timely and thorough haulm desiccation to ensure stolon detachment at harvest. Inadequate stolon detachment will result in tubers being returned from the harvester to the ground along with expelled haulm.
  •  Ensuring that harvesters are set up ideally to minimise the crop fraction that is not lifted (e.g. appropriate web size and also harvester settings to avoid cutting tubers)
  • If at all possible, establish the following crop without ploughing so that unharvested tubers are closer to the soil surface and therefore more likely to be killed by frost, or eaten by wildlife. Tubers can be killed when subjected to frost (minimum of -2 to -3 degrees Centigrade). At these temperatures, tubers need to be exposed for approximately 2 days. Tubers on, or close to, the surface generally get frosted, but tubers can be at depths of up to 30 cm. Frosts severe enough to penetrate to this depth are rare.
  • The following crop should be very competitive, e.g. wheat or barley or grass, to smother growth of volunteer potatoes

Integrated control measures for other times of the year are:

  • A high level of agronomy to minimise the production of small tuber sizes that can’t be harvested
  • Selective herbicides applied to the following crop
  • Glyphosate applied pre-harvest to other crops
  • Glyphosate applied to fallow land or to stubble

Forward planning for 2021 – sampling for nematodes

It is never too early to sample soils for nematodes, particularly as sampling fields when the soil is moist gives the most accurate estimate of nematode populations (especially free-living nematodes). The populations of potato cyst nematode (PCN) being found in ware land in particular have increased dramatically over the last 10 years and in some cases are unmanageable, requiring a 15 year break from potatoes to reduce populations to manageable levels. So it is wise to know what population and species of PCN that you have.

Potato cyst nematode and free-living nematodes (that transmit tobacco rattle virus that leads to spraing and can cause significant feeding damage and effects on yield) can be sampled as soon as any fields planned for next seasons’ potatoes have been harvested. Sampling for PCN can be done at any time, but for free-living nematodes (FLN) ideally sample a few days after there has been some rainfall. Knowing the nematode populations present now will allow you to decide whether you are going to have to do something about nematode management for next season.

Results from the trials carried out by SRUC have shown that feeding by free-living nematodes can have a significant impact on yield, so knowing the population of nematodes in the soil and the spraing virus status can help in planning whether a nematicide treatment is required.

Fields should be tested for the presence of PCN, free-living nematodes and nematode transmitted tobacco rattle virus, the cause of spraing in potato tubers, and growers are recommended to sample sooner rather than later for these nematodes.

Full details on soil sampling can be found here

Sampling for diseases

The SRUC Aberdeen Crop Clinic offers diagnostic tests on soil to assess the risk of blackdot and powdery scab. Avoiding fields with high levels of powdery scab sporeballs is desirable. SRUC offer a PCR based test for powdery scab sporeballs in the soil that will help indicate fields with a particularly high risk so they can either be avoided or a resistant variety can be used.

To obtain an accurate assessment of the risk from disease fields greater than 4ha in size will need to be divided into smaller sub-samples. Around 10g of soil should be taken from 100 points using a PCN augur (1kg soil in total). Sampling should take the form of a W pattern. The distance between sampling points will need to be adjusted according to field shape and size (especially if under 4ha). Samples should be placed in a strong plastic bag, sealed to prevents soil leakage and labelled with the field name, section or part of field (if applicable), name of sampler, date of sampling and previous cropping history (5 years).

Samples should be kept cool and dispatched as soon as possible to SRUC Crop Clinic, Ferguson Building, Craibstone Estate, Bucksburn, Aberdeen, AB21 9YA.  Tel. 01224 711206 or 01224 711279.


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