SO.ME DISEASES OF THE POTATO. 



139 



SOME DISEASES OF THE POTATO. 

 By George Massee, F.L.S., V.M.H. 



Phytophthora [npestans, De Bary. 



This disease is so well known that, unfortunately, little can at present 

 be added to our knowledge of it. Extended experiments, such as might 

 be carried out under the auspices of the Royal Horticultural Society, are 

 still needed to demonstrate clearly the sources of infection. Opinions 

 differ as to whether young Potatos can be infected directly by spores 

 carried into the soil by rain, &c. Some authorities do not admit the 

 usually accepted statement that the mycelium spawn passes up from 

 diseased seed Potatos into the foliage. Experiments conducted at Kew 

 prove that in some instances this does occur, even when the Potatos are 

 only very slightly diseased. 



It is, perhaps, not stating too much to say that a very large per- 

 centage of disease is due to two specific causes, both of which could be 

 prevented. Unfortunately, the means of prevention do not generally 

 commend themselves to the majority of Potato-growers. I have 

 observed the important fact that, when diseased Potatos are planted, after 

 the crop has been lifted, the remains of the old seed Potatos, when 

 brought to the surface of the ground, will produce a crop of the fungus 

 bearing myriads of spores. If such old seed Potatos are kept buried in 

 soil until the following year, and then exposed to light under favourable 

 conditions, fungus fruit is still produced, and continues to grow so long 

 as a scrap of the old Potato remains. I have now in the laboratory at 

 Kew Gardens scraps of last year's seed Potatos covered with the fungus, 

 and with the spores thus produced have successfully inoculated the leaves 

 of young Potato plants. 



It is not rare to see in horticultural periodicals statements to the 

 effect that ten acres of badly diseased Potatos were ploughed in, not 

 being considered worth lifting. Now in face of the above statement as 

 to the copious growth of the fungus on diseased Potatos when exposed to 

 the air, it is not difficult to understand where the germs that first infest 

 a crop come from, and, with the well-known necessary conditions of 

 moisture and warmth, an .epidemic breaks out at once. Lacking such 

 necessary conditions, the fungus, although present, cannot attack the 

 Potato-leaves ; hence the absence of disease does not necessarily prove 

 the absence of the fungus, but only the absence of those conditions 

 necessary to enable the fungus to attack its host. In all probability the 

 fungus is always present in land where Potatos are grown at short 

 intervals. 



I can realise the thoughts of the practical man on being told that it 

 is as important to collect the old " sets," or the whole of a crop of 

 diseased Potatos, as it is to gather the sound ones. Nevertheless such is 

 the fact ; and although at first sight such work might appear not to pay, 

 it would eventually do so. 



