How to reduce the risk of common scab in potatoes with LimeX

Wed 12 February 2025

Conventional wisdom suggests avoiding applying lime before planting potatoes due to the belief that it causes common scab. But our trials suggest this isn’t the case and can, in fact, reduce the incidence of common scab.

Common scab causes unsightly blemishes on potato tubers, impacting marketability and causing economic loss. Susceptibility differs among varieties, with Maris Piper being widely recognised as particularly susceptible.

Caused mainly by the bacterium Streptomyces scabiei, common scab develops when tubers experience a dry surface during the critical stage 3-6 weeks following tuber initiation.

While correctly applied irrigation during tuber initiation provides effective control, increased restrictions on water use and more frequent extreme summer heatwaves provide reasons to adopt alternative approaches to control.

Historically, going back to the 1890s, researchers have documented that increasing soil pH increases the risk of infection and severity, leading to recommendations to avoid increasing soil pH by practices such as liming.

Those recommendations are still widely believed and used, even when more recent reviews have pointed to benefits from liming before potatoes for common scab reduction. A review of RB209 in 2016 by Allison and Sagoo even recommended removing the warning relating to liming before potato crops in that widely used document. However, perhaps due to the potato section not being updated after the levy payer’s negative vote on AHDB Potatoes future, that has never been implemented.

One reason for this historical association is the discovery that analysing potato skins near the infected areas revealed a higher calcium concentration. This led to the erroneous assumption that excessive calcium in the soil caused the potatoes to absorb too much calcium, resulting in scab.

In fact, one of the roles of calcium within plant cells is to strengthen cell walls, so when an area has been infected by Streptomyces scabiei, the plant will transport calcium to that infected area to prevent the infection from spreading. Therefore, supplying the soil with an abundance of available calcium will allow the tubers to uptake the nutrient and help the potato to better defend itself.

Another factor could be soil pH. In AHDB’s 2018 review of liming, pH and common scab risk in potatoes points to an upper optimum pH limit for Streptomyces scabiei of 7.5. Pushing soil pH using LimeX to that limit could help reduce the scab risk.

Trials with LimeX in the 2010s and again more recently have shown that applications of LimeX before potato crops reduce the incidence of potato scab.

In 2012, applications of three rates of LimeX (3, 5, and 7 t/ha) compared with an untreated control in a field-grown crop of Maris Piper reduced the incidence of scab to 29% compared with 42% in the control. The application rate didn’t have a significant impact.

In 2013, the trials were repeated on four sites growing Maris Piper on a range of soil types at 5-10t/ha rates. In each case, there was a reduction in the proportion of scab on three of the sites.

Non-replicated strip trials in 2015 also showed a large reduction in scabbed tubers from 68-90% infected where LimeX was not applied to 26-37% from an application of 7.5 t/ha of LimeX.

In these trials, a marketable increase in yield of around 10% or 6t/ha was found.

A more recent trial with a large potato grower last season found similar results, again with increases in marketable yields. LimeX was spread in strips at 7.5 t/ha and 10 t/ha in a field of Maris Piper. Each strip was separated by an untreated 12m buffer.

The results reinforced the previous findings. An apparent visual difference in scab symptoms (figure 1) matched a figure of a 50% reduction in scab incidence between treated plots and those receiving 10 t/ha of LimeX. 

Figure 1: Comparison of scab symptoms between treated and untreated potato samples in a 2024 farmer trial.

Additionally, there was a small yield uplift and an increase in the number of tubers graded as >45mm in the LimeX-treated plots.

LimeX application timing is flexible. Ideally, it would be post-ploughing, pre-planting and then incorporated, but it is better to apply when you have access to the field, rather than holding off for ideal conditions.

The fine particle size of LimeX means it will begin to work to raise soil pH and increase available calcium to the root zone in just four to six weeks, which is critical when considering tuber initiation can be five to six weeks after planting.

Beyond scab prevention, the additional nutrient value of LimeX can help offset the cost of scab control. 10 t/ha of LimeX to prevent scab, as outlined in our trial work, would deliver 90 kg/ha of phosphate. According to RB209, this would be 36 – 90% of the crop’s phosphate requirement, depending on the soil indices. It’s a similar picture for magnesium.