Plant Doctor Archive

Potato blight

I HARVESTED two buckets full of lovely purple potatoes from six seed potatoes, but was deeply disappointed to find most of them were unusable because of a nasty brown blemish inside. Can you please explain the cause of this and how it might be avoided next season?

FROM the photo it looks like the problem is caused by one of the blight diseases that can affect potatoes. Other symptoms you may have noticed on the aboveground parts of the plant a few weeks before harvest include brown patches on the leaves and stems and some leaves shrivelling and dying.

This type of disease is worse in warm, wet weather and in poorly drained soil, and some potato varieties are more susceptible to it than others.

The fungal spores can remain viable in the soil for many years to affect not only future potato crops but also related plants like tomatoes. So to avoid the problem next season, plant new, disease-free tubers in a different part of the garden where no potatoes, tomatoes or their relatives have been grown for several years. Make sure the soil is free draining and be careful not to overdo watering. Also, try to choose a spot exposed to full sun so there's plenty of fresh air flowing around the plants. Plant the tubers on ridges and keep building the soil up as the foliage grows to keep the newly forming tubers higher and drier than the surrounding soil.

If you do see any symptoms appearing on leaves, you could spray with a fungicide, such as Bravo, Champion Copper or Fungus & Mildew Spray, according to the label recommendations. Also, remove any affected leaves and at harvest collect up all the plant remains and either burn them or dispose of them in the rubbish to get rid of a possible source of future infections.

It's also worth remembering that potatoes prefer slightly acid soil, so don't be tempted to apply lots of lime. They also need quite high levels of potassium for good tuber development, so it pays to apply a balanced fertiliser at planting time.

Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.