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Quick reaction to blight is vital to stem the rot
As if the recent reports on manure problems were not enough, blight (Phytophtora infestans) remains the scourge of the potato- and tomato-grower in wet summers. Along with slugs, it brings the most cause for despair as it begins its unstoppable spread through even the early potato crop before laying waste to tomatoes, indoor and out, through the later summer. Worse than this is that in the south of England it is endemic which means that, to all intents and purposes, your plants will probably suffer from it at some point.
The problem stems from climatic conditions. If temperatures are above 10C and there is more than 90 per cent humidity (ie rain), conditions are perfect for the spread of blight. Add to this the ability of the fungus to move easily via air, water and soil and it becomes clear what a ruthless killer it can be. The third factor is the ability of the spores to survive winter and reactivate in spring.
The fungus remains alive in the darker months in infected potato tubers left lying around the garden, in compost heaps and in stored tubers to be used for seed. The first signs are hefty brown spots or welts that spread quickly over the leaves. Once spotted, quick action is vital.
Despite all this, you never quite know what the extent of the damage caused by the fungus will be. In hot, dry years (remember those?) we saw very few incidences of blight, and many crops of tomatoes as well as long-season, maincrop potatoes such as Pink Fir Apples (which don't mature until September) came to fruition.
The past couple of years have been entirely the opposite, with crop failures nationwide. However bad the bad years are, we still persevere with both tomatoes and potatoes so we must look for steps to safeguard against blight. And while one cannot allow for acts of God in the sense of a wet season, there are still measures that can be taken when they happen.
The first is to cut off all the stems and foliage of infected potato plants an inch or so above the soil surface. This will stop the spread of the fungus from foliage to tuber. It may seem drastic but it will help and the potatoes will continue to swell to a certain extent. Do not on any account add the infected foliage or any infected tubers to the compost heap. They must be destroyed. Burning is the best way because you will have a usable product at the end of the process (potash). Afterwards, wash the tools that you have used to dig the tubers.
Where possible use resistant varieties, of which there are many more potatoes than tomatoes (see below). Plant with ample space to allow free air movement (3ft between rows is about right); this will give you scope for ridging up the rows, which will also help in the prevention of blight by creating a deeper layer of protective soil around and over the tubers.
When the seed tubers arrive from the supplier, check them carefully for signs of blight. They will have been cold-stored so rotting will have been arrested but not stopped. So, if you find a soft patch with that telltale hideous smell (caused by a secondary attack of bacteria on the fungus), get rid of the lot, send them back and ask for fresh. Similarly, if you have saved your own seed, check the store room regularly.
Until 10 years ago, Bordeaux Mixture was used as a preventive rather than a cure. It is lime and copper sulphate, which was originally used to control fungal problems on vines in France. It turns everything powder-blue but, a possible concern, it puts copper into the system and its use is restricted.
The best tomato cultivars include: Ferline and Legend (Thompson & Morgan seeds) and Fantasio (Suttons). Those potato cultivars showing most resistance include: Cara, Kondor, Orla, Markies, Valor and Sarpo Mira.
This article originally appeared in the Telegraph on 23/10/2008. link