Potatoes are basically easy and rewarding to grow. But it is also easy to lose a crop, if you don't look after them!
Blight
Blight can wipe out your potato harvest in a wet August; affected potatoes rot in store. Recent wet summers have been ideal blight weather, seriously hitting commercial potato growers, with all the chemicals at their disposal.
Small allotment plots have little chance of escaping, since blight is carried by a wind-blown spore, which settles first on the leaves. If your potato leaves have yellow or brown patches in July or August, look underneath the leaf - if you have blight, you will see a white mould fringe to each blight spot in damp weather.
How to deal with it?
- When the weather is warm and humid, spray with Bordeaux mixture (it is currently approved for organic use) to kill the spores before the rain washes them into and through the soil.
- If there is any more than a touch of blight, cut the haulms off at least three weeks before harvesting your potatoes. Any blight spores which have fallen on the soil will die BEFORE they get to your potato crop.
- Prevention
- Burn potato waste (potatoes and foliage) - do not compost it or leave it lying around, as you will simply be building up a stock of blight for next time around!
- Plant your earlies early - so they are out of the ground before the blight season starts.
- Crop rotation is essential.
- Choose resistant varieties
Worth noting, though, that there's a whole variety of other viral infections which may look similar to blight. They are more or less endemic in most allotment sites, simply because potatoes are such a popular crop - and can only really be controlled or avoided by being very careful about removing and burning potato haulms, and volunteer potatoes.
Potato Eelworms
These are nematodes, tiny vegetarian worms, also endemic in many allotment plots. Your plants will die back early, and with severe infestations you may get very poor yields. If you want to confirm whether you have eelworm, carefully expose the roots of a plant, and look for pinhead sized white, yellow or brown cysts on the roots (a magnifying glass will help!). Control is not easy - the little sods can live for 20 years!
- Rotation will reduce the population - it needs to be longer than 4 years if you have a serious problem.
- Kestrel and Sante have some resistance to both white and golden eelworms; other varieties are resistant only to the golden.
- If you have a serious eelworm infestation, your potato rotation should be longer than four years. Tagetes (French Marigolds) are said to discourage and kill vegetarian nematodes, such as eelworm. You would have to plant the whole potato bed the year before you grow your potatoes.
Common Scab
An unsightly but superficial bacterial infection, which doesn't affect the eating or storing qualities of the potato. You just don't feel like cooking them in their jackets! Impossible to treat, but it is avoidable;
- The condition is aggravated by lime in the soil, so I only lime beds the year after potatoes; by the time their turn comes around, the soil should be acid enough for them.
- Water regularly during the summer.
- When you plant the potatoes, put a layer of grass cuttings in the bottom of your trench. Result - scab-free potatoes, and scabby grass-cuttings!
Black Scurf
Another unsightly, but cosmetic, problem. When you lift your potatoes, does soil stick? Are there black raised spots (not warts), under the soil, on top of the skin? Are they hard to wash or scrub off? Could be Black Scurf, caused by a soil-borne fungus.
In my case, I'm pretty sure that it came in on seed potatoes - all the affected potatoes came from the same source! A warning to use reputable sources for your seed potatoes?
It's also a reminder to rotate - where I might risk the occasional short-cut in a bed I "know" is healthy, this one bed will be on a strict four-year cycle.
However, it is only cosmetic - it can be scraped off easily, and doesn't affect the eating or storage qualities.
Wireworm
A pest which likes grassland, and is the grub form of the Click Beetle. They frequently attack potatoes sown in new or renovated plots. The year before, the beetles laid their eggs; these hatched hungry grubs in the spring - and found nothing to eat except your potatoes!
The good news, is that once they metamorphose into beetles, they promptly fly off to a more "friendly" grass patch near by - and lay their eggs there! But the eggs can sit in the soil for four to five years.
- When you first clear your bed, move the grass turves off the bed (and so most of "your" eggs), and put them on the compost heap. This alone will substantially reduce your population.
- Grow potatoes which can be harvested before September in the year after clearing the bed; you will then avoid the main damage season.
- Digging exposes them to robins and other birds.
- I have read of making traps - burying slices of potato (attached to skewers) in the bed; check the slice regularly, and replace when the wireworms tunnel in.
If you have any other ideas, let me know please - use the Guest Book link above.