Blight: How to protect your summer tomatoes and potatoes

How to grow healthy, blight-free tomatoes. Photo / 123rf

By: Claire Mummery

After spending three months in the UK during a wet and humid summer, I am delighted to arrive home in time for a Kiwi summer. Yet, having seen infestations of blight wiping out many people's crops in Britain, I am wondering if we may be plagued by the same weather conditions here, resulting in a season ahead full of garden disease.

Traditionally - in a time before the impacts of climate change were apparent - Kiwis have planted on Labour weekend or even earlier; fooled into believing summer had arrived with early high temperatures. Although blight can strike at any time, it is more likely if you plant your tomatoes or potatoes in the ground early.

Blight can be caused by dampness and high humidity, overhead watering including sky water, wind, soil splash and lack of crop rotation. It is also more likely if your garden hygiene wasn't optimal last summer.

Blight comes in two forms of fungi; early blight and late blight, which appear on your plants in two different ways.

Early blight attacks the stems, leaves and fruits, first creating dark brown lesions on the stems and leaves in a circular pattern. As the plant matures, a yellow ring appears around the brown lesions and eventually forms a hole in the leaf. Early blight can spread from previous infections, as it can overwinter in the soil or compost. This is why hygiene and crop rotation are imperative, carefully removing all tainted plants and keeping them away from the compost.

Late blight is a fungus (known as phytophthora) that can affect your plants throughout the growing season, usually from flowering onwards. However, this does not spread from the previous season, as it is a different fungus and generally originates from spores on infected seeds, tubers or transplants. It can also spread by wind dispersal when temperatures are above 25C and humidity is high.

Late blight appears on older leaves and the oldest part of the stem, and it severely affects the fruit. Beginning as grey spots, they get darker as the disease matures and a white fungal growth forms on the underside of the leaves and along the stem. This fungus can spread very quickly and you can lose your whole crop overnight. Late blight was the cause of the potato famine in Ireland in 1845-1849.

Late blight on a tomato plant. Photo / 123rf

Four best methods prevent blight

Always buy healthy heirloom plants and thoroughly inspect your plant before buying it. Buy seed potatoes from a reputable source or plant your own seeds.

Take your time before planting into the ground - wait for drier weather and consistently warm temperatures above 22C. Continue to plant tomatoes and potatoes until January to ensure a good healthy crop, as the weather warms.

Implement good garden hygiene by carefully removing diseased plants and disposing of them carefully to prevent further spread.

Make sure your plants are well fed throughout the season, especially feeding regularly with kelp to support them through stressful weather conditions.

Happy gardening!