On the Potato Disease 



199 



the tubers. The mycelium may also pass down the diseased stems 

 and thus infect the tubers. 



Mr. Massee remarks {Journal of the Board of Agricidture, July, 

 1906) "that mycelium can be easily demonstrated in diseased tubers. 

 This, and the fact that a field of potatoes, apparently healthy and 

 vigorous, may within twenty-four hours, under suitable climatic con- 

 ditions, be reduced to a blackened foetid state, suggested the following" 

 experiments (see page 196), which tend to show that vigorous mycelium 

 may just as well be the cause of the epidemic as the rapid diffusion of 

 spores." 



Mr. Massee observes that "in every fungus epidemic proved to be 

 due to the diffusion of spores, the disease always originates from 

 one or more primary centres of infection, and gradually extends ; 

 whereas, in the case of the potato disease the appearance of the 

 epidemic is often simultaneous over a considerable area." In reply 

 to this it may be plausibly suggested that there are probably many 

 primary centres in the case of the potato field. Nothing is more 

 probable. 



The influence of warmth and moisture is shown chiefly in favouring 

 the germination of the zoospores. 



It may be that hot, dry weather favours the diffusion of zoospores 

 by wind. It is possible that insects may carry the zoospores from 

 plant to plant. Perhaps it is by this means that the corolla of flowers 

 is sometimes infected. 



There is a strong impression amongst potato-growers that certain 

 hybrids will resist the disease for a time, but after a few generations 

 suffer as much as their predecessors. What constitutes resistance ? 

 In the case of tuber infection it is probable that a thick skin (epi- 

 dermis) will be very useful in rendering difficult the entrance of the 

 .spores. So also in respect to the leaves and petals, it may be that 

 in some plants these are less easily pervious than in others. It is, 

 however, not improbable that the transitory immunity so frequently 

 observed in hybrids is explained by the fact that none of the tubers 

 planted contained mycelium, they having all been carefully selected. 

 By degrees, as the cultivation of the hybrid became more and more 

 extensive, there would be increasing risk of infected seed tubers. 



An all-important problem as regards the prevention of the disease 

 is that which concerns the wintering of its germs. Is this usually 

 effected in the tubers kept for planting, or in the soil, or in refuse of 

 the last year's crop left about in the field ? As regards the question 

 of early removal of the haulm when disease is shown, it is clear that 

 it must be answered differently according to the opinion entertained 

 as to the mode in which the parasite has gained access. If it has 

 grown upwards from the tuber it can obviously do nothing as pro- 

 tecting the new crop. It may, however, still be very beneficial in 

 protecting the adjacent plants and the soil. If, on the other hand, 



