T21 
further extending the application of measures for checking the 
spread of potato disease: S. . | 
In very few cases does it appear to be possible to entirely 
exterminate the deep seated mycelium of a parasitic fungus by 
merely cutting out the diseased areas of the host plant. Where 
as is the case with certain Exoasez the perennial mycelium grows 
forward with the host plant, pruning back beyond the point of 
infection frequently gives satisfactory results. The same, 
though to a lesser extent, is true of the apple canker Necétria 
ditissima. If, however, one attempts to get rid of the mycelium 
of such a fungus as Polyporus hispidus erowing in the trunk 
of an apple tree by excising the diseased area one rarely suc- 
ceeds. In such cases the attacks are not noticed until the 
fungus has penetrated throughout the tree trunk, when it begins 
to form its sporophores. Should these be removed the 
mycelium soon gives rise to a fresh crop. ae 
Setting aside for the time prophylactic treatment—in itself 
always more or less troublesome and often costly—we may turn 
to consider fresh aspects of the problem of treating plant 
diseases. The hope of many is that the near future may bring 
us races of plants not susceptible to the particular diseases they 
now suffer from. Evidence for the possibility of this is gradu- 
ally accumulating. We know for instance that if a large num- 
ber of the varieties of any plant are cultivated under the same 
conditions so that all are freely exposed to the same chances of 
infection that the incidence of the disease is not the same in all 
cases. Some varieties are found to be especially susceptible to 
it, whilst others remain more or less free from disease. A repe- 
tition of the experiment season by season gives the same result, 
so that we may divide the varieties up into susceptible and 
immune forms, between the extremes on either side, many 1n- 
termediates being met with. 
Other things being equal the varieties least susceptible to 
disease should be cultivated. Unfortunately there are many 
points to be considered in choosing varieties for general use and 
we cannot guarantee that the disease-resisting type shall be 
satisfactory in all respects. This is the case for instance with 
the potato known as Evergood. Its introduction would have 
solved the problem of preventing potato disease to a great ex- 
tent had it not been for the fact that combined with its un- 
doubted disease resistance were bad eating qualities. The 
question which then arises is whether it is possible to do away 
with these bad qualities and still retain the power of resisting 
the attacks of fungi. That this is within the scope of the plant 
breeder is I think possible, though many difficulties are still to 
be surmounted. Granting the possibilities of this the difficulty 
has to be faced that the parasite may gradually adapt itself to 



