Late Blight is a fungal disease which attacks potatoes and tomatoes. Sometimes it is found on related plants; I have seen it on kangaroo apple and wild solanum, a common weed. The disease is usually caused by the blight organism over-wintering in infected potato tubers. It can produce vast numbers of spores when conditions are wet and warm. Sources of infection are infected seed potatoes, self-set potatoes, decomposing ptoatoes in compost heaps, old potato or tomato haulms, and infected tomato plants.
Anyone growing potatoes or tomatoes needs to be able to identify late blight so that he or she does not unwittingly become a source of infection.
Once established, blight can destroy a crop remarkable quickly. I have seen a healthy bed of potatoes destroyed (in early July) two days after first infection. It can be quicker than this.
TO REDUCE THE CHANCES OF GETTING BLIGHT
1.Do not use own-seed potatoes unless you have inspected the seed and know it to be blight-free.
2.Destroy unplanted tubers at the end of the season.
3.Pull up and destroy any self-set potatoes.
4.Do not compost old tubers; throw them away.
Resistant varieties will slow down the spread of blight but will probably not prevent it. Very resistant potatoes in the UK are the Sarpo varieties and Setanta. Fairly resistant varieties are Verity, Cosmos, Orla and Golden Wonder.
Many of these are based on copper compounds. The choice for the home gardener is much more restricted than for the commercial grower. A dilute solution of Bordeaux or Burgundy mixture (copper sulphate / lime or copper sulphate / washing soda) is economical and effective. However it has to be applied as a preventative; it is less useful as a treatment.
REMOVAL OF DISEASED MATERIAL
If using containers, infected plants need segregating straight away, and spraying. Plants should be checked regularly every 2-3 days, and more often if the weather is wet. In a bed, diseased haulms need removing straight away and the rest of the plot should be sprayed. Thinning or shortening the haulms can also help. Once the haulms collapse (e.g. after a heavy storm) it is very difficult to prevent blight if the weather turns warm.
Nigel Deacon / Diversity website
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