Potato Problems: Blight, Scab and Green Potatoes

The threats change as the crop develops

Potato problems are almost fixed by growth stage. Insects at emergence, disease when rain arrives around flowering, and soil and light at harvest. Knowing the order means you only have to look at the right thing at the right time.

Read blight from temperature and rain

Two or more days of rain at 59 to 68F (15 to 20C) gives blight the conditions it needs. Dark green, water-soaked patches appear on the lower leaves, and white mold shows on the undersides. Once it moves it can kill a plant in days, and the tubers stop growing there and then.

Prevention has three parts: do not crowd the plants, earth up so the ridge drains, and keep the lower leaves off the soil. Spraying after it has started rarely stops it, so the practical approach is to act before a wet spell arrives.

Do not dig blighted foliage back into the ground. Take it off the plot, or the organism carries on to the tubers and into your store.

Aphids carry virus

The sap the aphids take matters far less than the virus they bring with them. A plant with crumpled or mottled leaves will not recover. Pull it out and dispose of it so it does not spread.

Silver or reflective mulch cuts the number arriving considerably. If you save your own tubers for seed, the virus load builds quietly year on year. Buying certified seed every year is, in the end, the most reliable protection there is.

1. Reduce the arrivals Lay silver or reflective mulch around the plants. The reflected light makes it harder for winged aphids to settle.
2. When you find them Clusters on shoot tips or leaf undersides can be brushed off or the leaf removed. While the numbers are small, hands are enough.
3. Plants showing symptoms Crumpled or mottled leaves mean the plant will not recover. Pull it and take it off the plot before it spreads.

Pick leaf beetles off in the morning

Leaf-eating beetles graze the leaf surface into a comb-like pattern, and left alone the foliage goes pale and the plant loses vigor. Grown next to aubergines or tomatoes, they move between the two and multiply.

Both adults and larvae are slow moving, so on a cool morning you can knock them into a container by hand. Yellow egg clusters on the undersides of leaves can be removed with the leaf, which takes out the next generation in one go.

1. Go out early A cool morning is best. In the heat of the day they move fast and drop into the foliage the moment you touch them.
2. Deal with the eggs first Yellow egg clusters on the leaf undersides come off with the leaf. That removes far more insects than chasing adults.
3. Think about what is next door Grown beside aubergines or tomatoes they move between crops and build up. Simply separating the beds changes how many you get.

Scab comes down to soil pH and history

Scab makes rough, scabby patches on the skin. The potato underneath is perfectly edible, but you peel it thickly and it does not make a good gift. It is caused by an organism in the soil, and unlike most diseases it gets worse as the soil moves towards neutral or alkaline.

So the whole of the answer is to keep the pH between 5.0 and 6.0. Do not lime, avoid fresh manure and poultry manure, and once you have had it in a bed, leave three or four years before growing potatoes there again.

Very dry soil while the tubers are bulking also increases it, so not letting the bed dry out around flowering helps indirectly.

The organism stays in ground where it has appeared. Move the crop next year, and if you genuinely have nowhere else, choose a variety with some scab resistance to limit the damage.

Greening is not a disease, but do not eat it

A potato exposed to light turns green on the surface. The color itself is chlorophyll, but solanine builds up alongside it, which is bitter and can make you unwell. Peel green areas away thickly, and if the greening is extensive, do not eat the potato.

Greening happens in two places: in the ground during growth, and in storage afterwards. Earth up twice while growing, and block the light completely once lifted, and you prevent almost all of it.

Very small immature tubers and the tissue around the eyes carry more of the same compound. If small children are eating them, discard the tiniest potatoes and cut generously around the eyes.

Sorting out an outbreak

The first thing to establish is whether you are looking at disease or a disorder. Check whether there is mold on the leaf undersides, and whether the whole plant went at once or only part of it, and you will know whether spraying is even relevant.

Then separate saving this year's crop from not carrying the problem into next year. Lift what you can, and get the infected foliage and that particular bed out of the rotation. That is the realistic compromise in a home garden.

1. Decide to lift earlier Once more than half the foliage has died the tubers have stopped growing. In a bad disease year, lifting earlier than usual means less damage.
2. Take the debris off the plot Dead foliage and damaged tubers leave the plot. Dug back in, the organism stays in the soil and starts next year off.

A disease that appears two years running in the same bed will not be stopped by management. Moving the crop is more effective than any other remedy.