Symptoms: One of the commonest potato diseases. Dark brown blotches that are often surrounded by a yellow halo that quickly spreads to rot the whole leaf. Leaves quickly become wet and rotten.
Prevention and/or cure: Often spread by infected tubers being left to sprout on compost heaps. Warm, wet and still weather causes a rapid spread. Dry weather can halt the disease, so it’s worth removing the first few infected plants. Cut off the potato tops (haulms) and burn them. Don’t harvest any tubers for 3 weeks; this allows the skin to set. If you notice potatoes almost at the soil surface, mulch the rows with leaves or straw, or even cover with more soil, to prevent the tubers going green – which renders them inedible. Where blight is a regular problem, you can reduce its effects by growing more resistant varieties, using deep ridges to allow the blight to be washed off by rain, or by growing early varieties which should give a decent crop before blight strikes.
See here for more information on potato blight.
What is it?: Scab
Symptoms: Unsightly brown marks on the tubers, but potatoes can be peeled and eaten.
Prevention and/or cure: More common on dry soil and those that are more alkaline. Grow resistant varieties, use certified seed, improve the water holding capacity of the soil and never add lime before a potato crop.
Credit: Garden Organic – https://www.gardenorganic.org.uk/pests-and-diseases#potato
NB: See SE Essex Seed Potato Day – http://seeog.org.uk/2017/10/se-essex-seed-potato-day-saturday-24-february-2018-1000-to-1400/