GO 



THE DEPARTMENT OF AGRICULTURE. 



These fungi, which derive their nourishment from living plants, set up an irritation 

 in the plant which gives rise to fungous diseases. 



For convenience the various potato-diseases may be included under three head- 

 ings: (1) Diseases of the "vines" and tubers; (2) diseases of the vines only; 

 (3) diseases of the tubers only. 



DISEASES OF THE " VINES " AND TUBERS. 

 LATE BLIGHT (Phytophttiora infestans) (Figs. 56, 57). 



This is the most destructive disease of the Lower Fraser Valley, but does not 

 seem to be of much consequence elsewhere. It is absent from the Dry Belt. The 

 disease shows first on the leaves, usually not being noticed until late summer or fall. 

 Brown, water-soaked spots appear, which gradually extend (Fig. 58). In warm, 

 moist weather the disease may spread so rapidly that in two or three days an entire 

 field looks as if hit by severe frost. A characteristic unpleasant smell accompanies 

 this blighting of the vines. The premature killing of the tops checks the develop- 

 ment of the tubers, thus reducing the yield. More serious still is the danger of the 

 tuber becoming infected from the diseased tops. Such tubers may rot in the soil 



Fig. 58. Late blight as it appears on leaf 

 (original). 



Fig. 59. 'Late blight as seen in tuber 

 (original). 



before being dug or in storage. Lightly infected tubers, however, often keep per- 

 fectly well, and it is such tubers used as seed which start the disease each season. 

 On the tubers the disease shows as leaden-coloured, often sunken, spots (Fig. 59). 



On cutting, a brown discoloration will be found in the flesh corresponding to 

 these spots and extending perhaps for a considerable distance within. Infection of 

 the tuber takes place from the disease in the " vines." 



Control. Spraying with Bordeaux mixture prevents the disease from develop- 

 ing in the foliage, and hence protects the tubers also. As the disease rarely appears 

 until the middle of August, it was thought that it would be satisfactory to begin 

 spraying about that time. Experimental work, however, has shown the desirability 

 of spraying early in the season. In the first place if spraying is deferred until the 

 vines are full-grown it is very difficult to cover the lower leaves and the centre of 

 the plant ; and, secondly, early blight may have seriously damaged the crop if condi- 

 tions have been favourable to it The first spraying should be Bordeaux mixture 

 G-6-40; later spraying may be 4-4-40 (sec article on spray mixtures). At least three 

 sprayings should be given, but in wet seasons this may not be enough, and it may 

 be necessary to spray at intervals of eight to ten days. 



It 'has been shown repeatedly that, even if no blight appears, spraying pays for 

 itself in increased yield, the foliage being stimulated to greater and more extended 

 activity. 



Lime-sulphur, so much used for fruit-trees, is unsatisfactory for potatoes. 



