12 PROTECTION OF PLANTS — 1923-24 



rot because they have completed their development before the season of the 

 year that climatic conditions become such as to allow for the spread and deve- 

 lopment of the pathogene. Early varieties of oats frequently escape rust, appa- 

 rently for the same reason. On the College Farm at Guelph, during the past 

 two years, no bunt has developed in spring wheat, even when the seed was 

 covered with smut sportes before seeding. This is an example of escape and 

 not of resistance, as the variety of wheat sown was known to be subject to 

 bunt in AVestern Canada. The escape was probably due to the high tempera- 

 tures just after seeding, which were unfavorable to the germination of the 

 spores of the bunt fungus. In Northern Ontario leaf roll of potatoes does not 

 appear to spread. A variety grown on the same farm for years often remains 

 apparently immune to leaf roll, whicle the same variety when grown in Southern 

 Ontario for two or three years becomes badly affected with it. The apparent 

 immunit}^ to leaf roll of potatoes in Northern Ontario, is, in the opinion of the 

 writer, probably an instance of escape due to the comparative scarcity of 

 aphids and other insect carriers of the inoculum of leaf roll. Examples of 

 escape due to the absence of certain biologic strains of the pathogene are seen 

 in the cases of varieties of wheat, which, when grown in certain localities, 

 appear to be highl}' resistant to stem rust, but which, when grown in other 

 places, prove to be susceptible. Another illustration of escape of this nature 

 is Wells Red Kidne}" bean, which appeared to be resistant to the bean anthrac- 

 nose fungus in Now York, until the discovery of the gamma strain of the fungus 

 by Burkeholder. 



Tolerance of disease is again distinct from immunity and resistance. It 

 may be due to strong and vigorous growth. Tolerance is seen in the case of 

 the round-leaved mallow (Malva rotundifolia) attacked by Puccinia malva- 

 cearum. Frequently mallow plants are seen with the lower surface of the 

 leaves covered with telia, but the plants never appear to be seriously injured 

 by the fungus. On the other hand, hollyhock leaves affected by the same fungus 

 usually wither and die before the normal time. Too many of our weeds 

 appear to be tolerant of parasitic fungi. Tolerance is sometimes due to the 

 fact that the tissues of the host are able to react so as to lessen the injury. 

 Butler cites as an example of the type of tolerance, tea affected by red rust 

 (Cephaleuros mycoidea). The chief damage of the pea plant by this organism 

 is to the young twigs not protected by bark. The parasite works from the 

 surface towards the wood, it cannot penetrate cork. The tissues of the twigs 

 respond to the attack of the parasite by laying down corky tissues under the 

 point of infection, thus preventing further penetration and injury by the 

 parasite! It is "possible that we have another example of such tolerance among 

 varieties of potatoes infected by Spongospora subterranea. 



The causes for apparent resistance or immunity to disease and tolerance 

 are comparatively easily explained. But when we consider the causes for 



