How to Prevent Blight in Your Garden

18 July 2008, written by Jeremy Dore

It’s around this time of year that gardeners start to look forward to a bumper crop of potatoes but for those who have grown them before there is always the worry that the harvest will be spoiled by blight. Probably the most common plant disease, blight can wreck whole crops in a matter of a few weeks, as it did so devastatingly during the Irish potato famine in the 1840s where 1 million people died and a further 1 million emigrated. Although it is commonly associated with potatoes, blight also affects some other members of the Solanaceae family of plants, the most common of which is tomatoes. So what causes it and what are the best ways to tackle it?

Blight is a fungal disease which spreads through spores blown by winds from one area to another, rapidly spreading the infection. The early signs can be hard to spot, although brown patches on the leaves and stems quickly appear (see above picture). There used to be only one strain of blight but in recent years a second strain has developed and the two types can mate, which is worrying since the resulting spores can over-winter, although this has rarely been seen in practice.

Infected plants left lying on vegetable patches, or plants growing from infected tubers can both be sources of infection. Blight only spreads under warm humid conditions and the spread of blight each year doesn’t usually begin until a ‘Smith Period’ has taken place – defined as two consecutive days of temperatures above 10 degrees Celsius (50 degrees Fahrenheit) and humidity above 90% for eleven hours or more. That’s why 2007 was such a bad year for blight in the UK as the unusually wet and humid conditions aided the rapid production of the spores, furthered by rain washing them into the soil from infected leaves.

Once a potato plant is infected it is important to quickly cut down all the plant which is above soil level, to prevent the disease travelling down into the potato tubers where it can make them inedible and rot in storage. For tomatoes there is usually little that can be done. If a plant is infected then it’s just a matter of damage limitation – getting what crop you can.

Prevention is the best option, since no cure exists. So what can be done to stop the problem occurring?

Fungicides: The classic approach to prevention involved spraying plants with copper-based fungicides such as Bordeaux Mixture, which used to be allowed in organic agriculture until its recent withdrawal. Although this works, to be properly effective the fungicides need to be applied before blight strikes and can easily be washed off by rain, resulting in many applications and residues being washed into the soil and watercourses – not great from an environmental perspective.
Growing Early Potatoes: Because blight doesn’t usually spread until mid-summer, growing early potato varieties can overcome the problem and this is the approach I take since I love early salad potatoes. Earlies don’t usually yield as much as maincrop varieties but they can be harvested before blight strikes so you don’t risk losing your crop.
Reistant Varieties: Recently a small number of resistant varieties have been developed such as Sarpo Mira and Sarpo Axona, showing the best resilience. I know many people who have benefited from growing these and there are apparently more varieties undergoing trials at the moment which is good, since blight has a history of constantly adapting!
Growing Under Glass: For tomatoes, growing plants in a greenhouse or under cloches often completely prevents the disease from spreading onto them, although it’s less easy if there are lots of potatoes growing nearby.
Good Hygiene: It’s important to destroy all blight-infected plant materials, dig up and dispose of any infected tubers and make sure that none of them are composted. This prevents your garden from being the source of next year’s local blight outbreak! Likewise, it’s always a good idea to buy in fresh certified blight-free tubers each year.