THE 



POTATO 



C 



BLIGHT 



PROF. W. LOCHHEAD 



ONTARIO AGRICULTURAL COLLEGE, GUELPH, ONT. 



THE blight which attacked the leaves 

 and stalks of potatoes during the 

 latter half of August was felt over the 

 larger part of the Province. It is rather 

 early to give an estimate of the damage 

 done, for in many cases the tubers are still 

 healthy and moderately large, while in other 

 cases many have begun to rot. 



For some years the blight has not been 

 severe. This year, however, was an ab- 

 normal one as far as the weather is con- 

 cerned, and the moisture conditions were 

 favorable for the development of the fungus 

 which caused the blight. During July and 

 August there was much rain, with interven- 

 ing spells of warm, not lo say hot, weather. 

 As the fungus grows most rapidly in a 

 temperature of about 70° F. when the air is 

 humid, it will be readily seen that it was 

 possible for the disease to make a very rapid 

 headway this season. 



There is scarcely a year when there is not 

 a little blight present, but with its remarka- 

 ble powers of reproduction the fungus may 

 spread with amazing rapidity when the 

 conditions are favorable. 



The fungus blight is an internal parasite, 

 hence spraying with Bordeaux is of little 

 avail in preventing injury after the plant be- 

 comes infected. Spraying is valuable, how- 

 ever, in preventing the spread of the disease 

 to unaffected plants and should not be 

 neglected. Observations show that fully 

 99 per cent, of our potato-growers take no 

 precaution whatever to ward off fungus dis- 

 eases from their crops. Paris green is used 

 everywhere to kill the beetle, but Bordeaux 

 mixture is seldom or never applied to pre- 



vent blight. A spray composed of a mixture 

 of Paris green and Bordeaux would ward off 

 both the beetle and the blight. The fruit 

 grower has learned by experience the value 

 of this mixture in combating the codling 

 moth and the scab. Some years the scab is 

 not serious, but the fruit-grower always 

 takes the precaution to spray his trees 

 thoroughly, for he cannot forecast the sea- 

 son with any degree of accuracy. " Fore- 

 sprayed is fore-armed " is his motto, and it 

 ought also to be the motto of the potato- 

 grower. 



For the best method of preparing and ap- 

 plying the combination, Bordeaux and Paris 

 green, consult Spray Calendar, Bulletin 122, 

 published by the Department of Agriculture, 

 Toronto. For potatoes, however, use 8 oz. 

 Paris green instead of 4 oz. to the barrel. 

 Spraying should be done all through July 

 and August. 



The potato blight first shows itself as 

 small, brownish blotches on the leaves. 

 These rapidly enlarge so that in a few days 

 the entire field becomes blackened. During 

 the early stages a delicate, white, felt-like 

 covering may be seen at the margin of the 

 brown areas on the under side of the leaf. 

 This is composed of an immense number of 

 branching threads bearing spores. These 

 spores are readily dislodged from the threads 

 and are carried away by the wind to the sur- 

 faces of other leaves where they germinate 

 at once and send germ tubes into the leaf. 

 Once within the leaf they grow rapidly by 

 feeding on the juices, and soon set up the 

 diseased condition known as blight. If, 

 however, the leaves are covered with the 



