POTATOES 115 



shows nothing but a slight darkening of the skin over the 

 affected area. Soon, however, this area becomes somewhat 

 sunken and of a leaden colour. When cut open the diseased 

 tissue shows a rusty or foxy-red appearance which is at 

 first confined to the skin region, but which gradually pene- 

 trates deeper and deeper until the whole tuber is destroyed. 

 When a tuber is destroyed by Phytophthora alone the decay 

 is of the nature of a dry rot ; but when (as is much more 

 often the case) other minute organisms follow, wet rot 

 often sets in. 



" If slightly diseased tubers are planted in spring, the 

 mycelium continues its development and the majority rot 

 completely in the ground without producing any plants. 

 The remaining few grow and give rise to plants of varying 

 degrees of vigour, and, unless they are infected independently 

 later on, most of these remain quite healthy. On the other 

 hand a very small proportion of these may produce above 

 ground one or more diseased shoots bearing the fungus in its 

 fruiting condition. Such primarily diseased shoots bearing 

 the aerial spores of the fungus and derived from affected 

 tubers have been shown to be centres from which the new 

 crop may become infected. Great care should, therefore, 

 be taken to plant sound ' seed.' Infection of the crop may 

 also occur from spores borne on diseased sprouts or shoots 

 of diseased tubers which have been discarded during winter 

 or spring from pits, clamps, or other stores, and have passed 

 the winter anywhere in the open without having become 

 completely killed. Care should, therefore, be taken to 

 destroy or bury deeply all such useless tubers." * 



The only way to prevent this disease is to spray the 

 haulms before the plants become affected. This is done 

 with a very poisonous solution known as Burgundy 

 mixture, which may be purchased in small quantities at 



* Board of Agriculture, Leaflet 23. 



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