MY main crop of potatoes were a disappointment last year as they were covered in lots of tiny scabby bumps. They peeled off and the crop was still useable with good flavour but what went wrong and what can I do to stop it happening again this year? The variety was Desiree, bought from a reliable source.

C JARMAN

REG SAYS: The problem here is common scab and it is more likely to be encountered during dry years or on very well drained soils with alkaline conditions also favouring the disease.

It is also worth remembering that some varieties are particularly prone to developing the condition including, among maincrop varieties, Maris Piper, Majestic and Desiree, particularly on newly dug plots which were previously grassland. So from this selection I’d guess that you had a susceptible variety, grown in an alkaline soil during a dry early summer, so the recipe was in place for common scab to appear.

It is a particularly interesting organism as it has some of the attributes of a bacterium, coupled with the mycelium (white threads) of a fungus and there is no chemical control available.

Control measures to combat this problem includes never applying lime before planting potatoes.

Dig in some green organic matter, such as lawn mowings, at the rate of about three bucketfuls per sq. m, making sure that they are well incorporated into the soil at a good depth just prior to planting.

Remember not to use lawn mowings within one month of putting on a lawn weed killer.

In dry years watering the crop as the plants begin to flower, as this shows that tubers are developing, can also reduce instances of common scab.

However, the best thing to do is to choose a resistant variety and here is a selection in various categories: First Early: Colleen, Pentland Javelin, Accord, Arran Pilot, Home Guard, Swift. Second Early: Carlingford, Nadine, Maxine, Kondor, Cosmos. Early Maincrop: Wilja, Picasso, Pentland Crown. Maincrop: Sante, Golden Wonder, Valor, Cara, Verity.

Salad Potato: Nicola.

Can the common scab organism persist in the soil? Well, yes it can but it already exists in most soils anyway as part of the natural micro-life teeming there, only becoming a problem when all the right conditions are present at once. For this reason the planting of affected ‘seed’ tubers usually provides very little risk of spreading additional infection, unless the eyes are infected and this is very rare.