496 THE farmers' handbook. 



The spores are the agents for the production of two important results : — 



(1) Rapidly spreading the disease through a standing crop by infecting 



the leaves of healthy plants. 



(2) Infecting the tubers and thus providing the means of propagating 



the fungus through its mycelium (collection of threads or hyphse in 

 the tissues), which assumes a dormant condition in the tuber. 



A spore alighting on a healthy leaf, will, if weather conditions continue 

 favourable, soon bring about infection. Many spores will fall to the ground 

 and others will be washed down by dew and rain. These may reach the 

 tubers in the ground and produce infection. This raises the question as to 

 the best time to dig, when a field becomes badly blighted. If the tubers are 

 dug while the ground is covered with diseased tops, immediately after their 

 collapse, there is danger of exposing many to infection from contact with the 

 diseased plants, whereas if left in the ground for some days there is the other 

 possibility of infection by the spores washed down through the soil. It is 

 also known that the mycelium may pass down the stem, and cause infection 

 through the underground parts. Experiments have proved that there is 

 less loss if the digging is delayed a week or more after the death of the 

 tops, except in very wet weather and on low heavy soils, 'n which cases 

 early digging becomes essential. 



Infected tubers may show little or no change. Typically there are dark- 

 coloured areas on the surface, which become more or less sunken and crumpled 

 and easily stripped of peel. If the skin is removed from these areas brown 

 patches or streaks are to be found just beneath, extending varying distances 

 into the flesh. As the disease advances these brown patches extend further 

 inwards, and finally the potato shrivels and dries. This condition has given 

 rise to the name of Brown Rust. More frequently, however, the diseased parts 

 become softened and emit a peculiar foetid odour, or rapidly become a foul 

 smelling, soft, rotting mass. This condition is brought about by the entrance 

 of bacteria, which set up putrefaction. A sound potato infected with blight 

 alone will become brown and dry up. The soft rotten condition is the more 

 usual, as potatoes are very often dug just after an attack, during damp 

 weather. Without being cleaned or dried they are put into bags, and may 

 remain in them for days during transportation. They are thus exposed, in 

 the first instance, to every chance of infection, if not already infected, and 

 then placed under conditions most favourable for the attack to spread. The 

 bacteria follow in the wake of the fungus and complete the destruction, with 

 the result that the potatoes often reach market in a soft rotten condition — a 

 dead loss or even an expense to the grower. Unless the foul smelling rot is 

 present some people think the disease is not due to blight, while many 

 growers attribute the wet rot condition to weather alone. While the presence 

 of the fungus in the tubers may produce such changes, there may be, on the 

 other hand, no visible indications of its presence. The mycelium may pass 

 into a resting or dormant condition, and only resume its activity when the 

 tuber is planted. This is the most important way in which the fungus is 

 perpetuated and a new crop is infected. It is by this dormant mycelium 

 that the disease has been spread all over the world. 



When an infected tuber is planted the fungus may grow into the shoots 

 produced, finally developing its spores which commence the infection of the 

 new crop. Haulms and potatoes from a preceding diseased crop, if left in 

 the field, may also cause infection. The fungus may also spread from tuber 

 to tuber in the soil, more particularly when the soil remains wet 



