Just when you were looking forward to harvesting a fantastic crop of potatoes; the blight fungus hits. Phytophthora infestans wipes out the leafy tops (haulms) and your dreams of delicious potatoes. Infamous for being the cause of the tragic Irish famine years ago, it may not cause famine for gardeners but it certainly infuriates. The precise time it hits really depends on the weather, but it is about now that is likely to hit, so be prepared!
What does potato blight look like?
The first symptoms usually develop on the foliage; dark brown patches, each several millimetres across. In humid or damp weather a white, slightly fluffy fungal growth may be visible on these patches. The infected leaves shrivel and die and the infection moves in to the stems causing dark brown to black blotches. Within a few days the plants collapse and the rows of potatoes start to look as if they’ve been hit by a huge blowtorch. If the weather conditions change shortly after infection first occurs then the symptoms may progress only extremely slowly and the haulms remain upright. But underground, the infected tubers develop gingery brown discoloured patches within the flesh and brown, slightly sunken patches on the surface. If kept, they often develop secondary infections, which rapidly turn the dry discoloured tubers in to slimy and often smelly masses. It renders the whole crop useless.
Blight infected tomato fruits develop dark brown or gingery brown discolouration on the skin, followed by internal discolouration, and like infected potato tubers, although this rot is itself a ‘dry’ rot, the fruits may collapse due to a secondary soft rot infection. The stems and leaves develop dark brown patches, which increase in size, making the plant appear wilted and flopping, and ultimately causing it to die.
Kill or cure
The fungus responsible, Phytophthora infestans, is the most devastating infection of potatoes and tomatoes. It is largely a problem of outdoor tomatoes as those grown in greenhouses are generally sheltered from the fungal spores by which the infection is spread. The spores are carried by rain splash or on air currents and so pretty well any susceptible plant can be attacked. I find that early crops can often be lifted before the infection is around, whereas main crop potatoes are far more likely to suffer from blight infections. With tomatoes, the best solution is to try to grow your plants in a greenhouse, frame or similar structure.
The blight fungus is very weather-dependant and for the infection to occur there needs to be at least two consecutive 24hr periods each of which have a minimum temperature of 10C (50F) and where there is a relative humidity of at least 89% for a minimum of eleven hours. Sadly these conditions are surprisingly easy to achieve, especially towards the middle to back end of summer when blight usually hits. I’m a great fan of the BBC weather app – it acts as my early warning system!
So what can you do?
- With tomatoes if you do not have a greenhouse then there are numerous relatively inexpensive growing-bag covers and mini-greenhouses that would help to dramatically reduce the risk of infection.
- It is certainly worth investigating growing resistant or tolerant varieties that are available, I find cherry tomato types fairly resistant but the best of all is Tomato ‘Lizzano’.
- With potatoes there is a lot to be said for growing those showing resistance such as ‘Sarpo Mira’ and ‘Sarpo Axona’, or those which show a very useful level of resistance e.g. ‘Estima’, ‘Kondor’, ‘Romano’, ‘Lady Balfour’, ‘Pentland Crown’, and ‘Valor’.
- If blight does hit, remove the infected potato haulms immediately: this should prevent the spores from being washed down in to the soil, and so prevent the tubers being infected.
- Prompt removal of the haulms, or of the tomato plants will also reduce the risk of the soil becoming contaminated. Any infected material should be bagged up and binned or burned, not composted.
- With crops like potatoes, and infections like potato blight, crop rotation is especially important – if you clean up thoroughly after an attack of blight and rotate your crops then if any spores did manage to get in to the soil, you should still be in with a good chance the following year….weather permitting!