By Innes Jessiman

Senior potato consultant, SAC Consulting

It has become apparent that some potato varieties are proving very difficult to store long-term because of the build up of high levels of black dot on tubers during the storage period.

This is particularly important to the potato ware producer as any infection of black dot on ware potatoes will increase the risk of rejection by the processor and packer. Pre-planting is a good time to plan the best control programme possible to limit black dot development on susceptible varieties

What actions and options are available to the ware grower to reduce the risk of contamination?

Levels of black dot in soil naturally decline over the course of a rotation. However, experience has shown that a one in five rotation is not sufficient for soil contamination to fall significantly enough to reduce the risk to the next crop especially where potato volunteers persist.

Growing a black dot resistant variety in the rotation does not constitute a control measure, because the fungus still multiplies on roots, stems and stolons.

Black dot tends to be worse on lighter soils, therefore one option for control is to grow susceptible varieties in heavier fields. This may not be ideal during a wet season but it might ensure harvest is planned early.

The SRUC Crop Clinic, at Craibstone, offers a DNA soil test which can give the grower an assessment of the level of black dot soil contamination within a particular field. Results can be used to reduce the risk of black dot infection by allowing the grower to select a more resistant variety for a contaminated field.

This is particularly important where the history of field is unknown, for example on rented fields. The soil test can provide a category of risk. Where the risk of black dot in the soil is assessed as moderate or high an application of a soil fungicide as a control measure should give reasonable control.

Although seed inoculum is of considerable less importance than soil inoculum, it makes sense to minimise the significance of seed inoculum. Obviously, healthy seed can be planted and will not be a source of the disease to the subsequent crop.

Washing a sample will check on the health of a seed stock. However, if seed is infected with black dot, apply a seed treatment and this needs to be carried out before dormancy break and thus needs planning well ahead of planting.

Crops that are placed under any stress during production are often prone to greater infection. In practice, this means that, for example, fertiliser levels should be optimal and soil structure should not impede root development.

There is a clear link between the level of black dot on harvested tubers and the duration of a crop in the ground, and so early harvest reduces risk. Planning harvest to start as soon as skin is set is a priority.

Reducing the time in the ground by mini-chitting and, in high risk situations, by earlier haulm destruction (thereby sacrificing some yield) should also be considered. We have often maintained that as September proceeds, solar radiation falls dramatically and it takes a long time for tuber bulking to occur.

To minimise development of black dot post harvest, pulling store temperature down by 0.5°C a day as soon as a store is loaded will help. Many growers store at 3°C, however holding crops slightly below this temperature should reduce development of black dot symptoms.

Growers should also appreciate that dust and debris do harbour black dot and a lot of other disease spores. We always recommend a thorough cleaning of the potato store at end of the season, it is good practice to remove as much dust and dirt as possible and the use of a disinfectant for both the store and the handling equipment is recommended.

* For more information on the soil test for black dot and the sampling procedure, contact the SRUC Crop Clinic, Craibstone or your local SAC Consulting Office.