We take for granted the simple potato, and if you have been buying them from the local market or grocery store for your entire life, you might think they arrive in bushels from a magic place (Denmark? Ireland?) and never start off as chunks of seed potatoes with potential to rot. In fact, rot and bugs and under the ground catastrophe never come into your mind as you smell the earthy goodness of a fresh potato or as you liberally salt and butter the delicious potato on your plate.

In comparison with some other vegetables, there are so many things that can go wrong with a simple potato.

Blight

The most famous of potato diseases, and there are two types, early and late. Early blight is caused by the fungus Alternaria solani, primarily affects leaves and stems, and is not quite as bad as its friend, late blight. You certainly don’t want either!  Symptoms of early blight are dark blotches on the leaves that seem to spread out in concentric rings. The plants look thin and yellow.

Late blight is caused by a different fungus, Phytophthora infestans., and can destroy your crop completely, starting with the foliage and finally destroying all tubers which the spores have come in contact with. Late blight potatoes have a dark, shrivelled appearance. The spots are black, the potatoes are shrunken and rotted inside. Often this becomes visible only after they are picked, so as noted in my previous post, you must check your stored potatoes for damage before you lose the entire lot.

If you notice a small number of affected leaves with patches, signaling either early or late blight, you can try removing those and disposing of them. Burn if possible, as the spores responsible are airborne and will quickly find a new home on other potato plants or on their cousins, the tomato.

Any period of warm, humid weather, while beneficial for other plants, increases blight risk.

Virus diseases

The mosaic virus seems to be the biggest problem, especially, I have heard from friends, in South Africa. You can tell your plants are affected by looking for bright yellow to white mosaic patterns that sometimes cause large areas of leaf
tissue to looked bleached. Potato leaf-roll virus is discernable from, you guessed it, leaves that roll inward.

Aphids play a role in transmitting these viruses, and controlling aphids is tough. You may get lucky, as the potato aphid has numerous natural enemies. There are many predatory arthropods that feed on them including ladybird beetles, lace wing larvae, and syrphid fly larvae. If you are left to fight alone, try a spray of insecticidal soap and water from a spray bottle, or even a blast from the garden hose to knock the annoying beasts off their perch.

Scab

Such a nasty name. Common scab, caused by Streptomyces scabies, appears on the tuber skin as slightly raised patches, and as the plant grows the skin breaks, leaving the scabs with rough edges. The bad news is that this pathogen can also get to your beets, radishes, rutabagas, turnips, carrots and parsnips. The condition tends to be worse on heavily limed soils and under very dry conditions.

The good news is that this is usually just unsightly, and the potatoes CAN be eaten after you scrape off the patches.

To control scab, start off on the right foot by only planting unblemished seed potatoes and to practice crop rotation, avoiding the plants I’ve just mentioned.

Slugs and Wireworms

Keeled slugs, spotted by their yellowish colouring along the bottom of their back end, are the big problem here – they grow comfortably under the leaves and then tunnel into the flesh of the potato. What a treat for the hungry slugs – they will feast until the spud is hollowed out.

They often use pathways already dug by their friend the wireworm, or clickbeetle larvae. These are coffee-coloured and half- to an inch long,

The best solution here is to choose less insect-susceptible varieties of potato, like Romano, Pentland Dell or Golden Wonder, and avoid sowing your potatoes into a site bordering grass, compost heaps or piles of organic waste, which is where the wee beasties like to congregate.

There are wireworm and slug traps that you can build to control these pests organically. Save a rinsed can from baked beans, and punch holes in both sides and the bottom. Fill the can with potato peelings or chunks of potato, and bury the can upright in the garden border. You should get many visitors looking for a free meal! Empty the can weekly and reset your trap. Do this before you plant your next crop to increase your chances of having healthy potatoes survive the season.

Love the organic life you live,

Arabella