TRADITIONALLY, the application of lime before planting potatoes was believed to cause common scab and growers were advised to avoid it.
However new data suggests this isn’t the case and it can actually reduce the incidence of common scab.
Caused mainly by the bacterium Streptomyces scabiei, common scab develops when tubers experience a dry surface during the critical three to six-week stage following tuber initiation. Maris Piper is widely recognised as particularly susceptible.
Emily Whitmarsh, Technical Sales Manager for LimeX, an agricultural liming product for the correction of soil pH, said it may now be time to rethink applications.
“Historically, going back to the 1890s, researchers have documented that increasing soil pH increases the risk of infection and severity,” she said. “This led 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 even recommended removing the warning relating to liming before potato crops.
“Perhaps owing to the potato section not being updated after the levy payer’s vote on AHDB Potatoes’ future, that has never been implemented,” said Emily.
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.
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, says Emily. Therefore, supplying the soil with an abundance of available calcium will allow the tubers to uptake the nutrient and help the potato to defend itself better.
“We conducted trials with LimeX in the 2010s and again more recently which have shown that applications of LimeX before potato crops reduce the incidence of potato scab,” said Emily.
“In 2012, applications of three rates of LimeX 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.
“Then in 2013, the trials were repeated on four sites growing Maris Piper on a range of soil types at 5-10 t/ha rates. In each case, there was a reduction in the proportion of scab on three of the sites. 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. Emily says an apparent visual difference in scab symptoms matched a figure of a 50% reduction in scab incidence between treated plots and those receiving 10 t/ha of LimeX.
“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,” she said.
