POTATO BLIGHT
Dig for Victory Leaflet No. 17
Potato Blight on leaves
BLIGHT attacks potatoes when the weather is warm, wet and muggy, and may kill the tops as early as the end of July or the beginning of August. If this happens, not only is the yield reduced, but the tubers may become infected and rot in the ground or in store.
Spraying will protect the tops from blight, if it is properly done; and care at lifting time will protect the tubers from infection. At the present time, when maximum production is essential, every allotment holder and gardener (except those in smoky industrial districts) should consider spraying his main-crop potatoes as an insurance against blight.
And all growers, without exception, should take precautions to avoid infection of the tubers at digging time. These instructions will help growers to get good disease-free crops.
What to Spray
Potato cut in half to show signs of disease
Early varieties do not usually need spraying, as they are generally lifted before blight appears. Mid-season and late varieties, however, should be sprayed, especially the varieties British Queen, King Edward and Up-to-Date, which are very susceptible to the disease.
Spraying should not be done within 10 or 12 miles of large industrial centres, for acid fumes in the smoke may react with the spray and cause injury to the plants.
If in doubt, ask the secretary of your allotment society to obtain information, or enquire at your local Council Office.
The Spray to Use
There are three kinds of spray from which to choose :
READY-MADE SPRAY MIXTURES.
There are numbers of these on the market. Some are sold as Bordeaux powders or pastes. Others are given special names. They have only to be mixed with water according to the instructions given on the tins or packets. For this reason they are more convenient to prepare than the home-made mixtures, and there are a number of officially approved brands on the market.
HOME-MADE BORDEAUX MIXTURE.
This is made from bluestone (sulphate of copper) hydrated lime and water. The blue-stone should be 98 per cent pure, and it is best to buy either the powdered or granular form, as these dissolve much more readily in water. The hydrated lime must be fresh ; it can be purchased in small quantities and is more convenient to use than quick lime, as well as being free from grit. The quantities to use are :
- bluestone . . 4 oz.
- hydrated lime . . 5 oz.
- Water . . 2½ gal.
This amount of spray is sufficient to treat the potatoes on about 2 rods of ground, or rather less when the haulm is big.
Measure out 2½ gallons of water into a bucket or other container. A wooden, earthenware or enamelled vessel is best, but a galvanised bucket may be used if it is well washed afterwards.
Pour off about a quart of the water into an earthenware or enamelled jug and stir into this the 4 oz. of blue-stone. (Bluestone must never be put into metal vessels unprotected by enamel.) While it is dissolving, shake the 5 oz. of hydrated lime into the water remaining in the bucket and stir well.
Finally, when all the copper sulphate has dissolved, slowly pour the blue solution from the jug into the milk of lime in the bucket, stirring well all the time.
The sky-blue colour of Bordeaux Mixture appears at once, and the spray is then ready to apply. It should always be used the day it is made. It is best to pour it into the sprayer through a fine gauge strainer or a piece of muslin, to prevent any grit or other impurities from clogging the nozzle.
Immediately after use rinse out with clean water all vessels which have held the spray mixture.
HOME-MADE BURGUNDY MIXTURE.
This is made in exactly the same way as Bordeaux Mixture, except that 5 oz. of washing soda is used instead of the hydrated lime. The soda should be fresh and in the form of crystals; if it is old and broken down to a white powder, the soda solution made from it will be too strong.
How and When to Operate
The fine misty type of spray is best, for it will settle on the under sides of the leaves as well as on the upper sides. If no spraying apparatus or syringe is available, the mixture may be sprinkled over the plants from a watering-can with a fine rose.
The object should always be to get a covering of spray on all the leaves.
Spraying should be done on a fine day so that the spray has a chance to dry before rain comes; it will then stick better to the leaves.
Secret of Success
The secret of success is to get the spray on before blight spots appear on the leaves. Failing that, the spray should be applied as soon as the first blight spots are seen. The best time, in most parts of the country, is just before the plants meet in the rows, which is usually in the latter half of June or early July.
The second spray should be given about three weeks later. In a year when blight is bad a third spraying in August may be necessary.
Allotment holders should make a practice of having all the maincrop potatoes in their allotment group sprayed at the same time, particularly if they can find someone with a knapsack sprayer or bucket pump to do the work.
Dusting : an Alternative
Dusting plants with a copper-lime or Bordeaux dust is sometimes done instead of spraying, especially if water is not easily available. It is less efficient than spraying, so that more applications are necessary.
The dusts must be bought ready-made, and four or five applications should be given at fortnightly intervals, beginning before the plants meet in the rows. It is best if the dust is applied early in the morning while there is still dew on the leaves. Various types of small hand-dusting machines are available for applying the dust.
Care at Digging Time
Tubers should never be lifted when there are blight spots on the leaves, as they may then become contaminated with the disease and later rot in storage. Do not dig the crop, therefore, until about a fortnight after the haulm has died down completely. If the tops remain green very late in the season, cut them off a fortnight before you intend to dig.
If possible, dig only in dry weather, and see that the tubers are dry before storing. Never use the haulm to cover potato tubers, even for a short time. There is no objection to putting it on the compost heap.
One other precaution is good earthing-up. This helps to protect the tubers while growing. Use plenty of soil and make the sides of the ridges compact.
This transcribed from the March 1947 printing of the leaflet which was on better quality paper than earlier versions. It also included the photographs.
The 1943 leaflet had the same content but was slightly smaller and on poorer quality paper that wouldn’t have reproduced photographs well. It did, however, have a list of other Dig for Victory leaflets that was omitted from the 1947 reprint.
‘DIG FOR VICTORY’ LEAFLETS
1. VEGETABLE PRODUCTION (WITH CROPPING PLAN).
2. ONIONS, LEEKS, SHALLOTS, GARLIC.
3. STORING VEGETABLES FOR WINTER USE.
4. PEAS AND BEANS.
5. CABBAGES AND RELATED CROPS.
6. ROOT VEGETABLES.
8. TOMATO GROWING—IN A GREENHOUSE OR IN THE OPEN.
9. BORDEAUX AND BURGUNDY MIXTURES.
10. JAM AND JELLY MAKING.
11. BOTTLING AND CANNING.
12. SEED POTATOES.
13. STORING POTATOES FOR FOOD AND SEED.
14. DRYING : SALTING : PICKLES : CHUTNEY.
15. POTATO GROWING.
16. PESTS AND HOW TO DEAL WITH THEM.
18. BETTER FRUIT: DISEASE CONTROL.
19. HOW TO SOW SEEDS.
20. HOW TO DIG.
