Common scab

About

Common scab makes rough, corky patches on the skins of potatoes and other root crops. The bacteria live naturally in many soils, so managing soil pH and moisture is the main defense.

Also called common scab, potato scab, scab

How to recognize it

Symptoms

Tubers and roots get tan to dark brown, rough, corky patches that may be shallow, raised or pitted; under severe stress pits can be deep.

Worse when

Neutral to alkaline soil, dry soil while tubers are forming, warm dry seasons, and light sandy or gravelly soils.

How it spreads

The bacteria live naturally in many soils and also arrive on infected seed potatoes.

When

Develops while tubers and roots are forming; found at harvest.

What to do, gentlest first

Prevention works best; most sprays only protect healthy leaves and can't cure infected ones. Always read and follow the product label.

1. Clean seed potatoes and resistant varieties Prevent

   Plant certified scab-free seed potatoes and resistant varieties such as 'Norland', 'Russet Burbank' and 'Superior'.

   UMass notes no potato variety is immune.

2. Keep soil evenly moist Prevent

   Water steadily while tubers are forming and just after, and add organic matter to help soil hold water.

3. Do not raise soil pH Prevent

   Scab rarely develops below pH 5.2, so avoid lime, calcium or potassium nitrate and fresh manure in potato beds.

4. Rotate Prevent

   Grow potatoes and other root crops in a new spot for three to four years.

Plants it affects

Avocado, Beet, Carrot, Loquat, Parsnip, Potato, Radish, Turnip, White sapote