DISEASES CAUSED BY FUNGI g 



There are many other parasites, however, against which this 

 type of resistance fails. To them a better-developed plant may 

 mean merely an increase in available food material, while the 

 effect of manuring and good weather may be to develop a softer 

 type of growth which is the easier to infest. In the case of 

 Rosellinia disease of limes, the conditions which best suit the 

 tree best suit the fungus also, with the result that the finest 

 trees are most liable to attack and are most frequently killed. 

 Cacao canker and pod-rot are most prevalent in the well-enclosed 

 humid situations which favour vegetative growth, and in dealing 

 with diseases of this type a point has to be sought to which the 

 humidity can be reduced so as to check the fungus without unduly 

 hindering the development of the trees. In the growing of cotton 

 a similar situation in respect of boll diseases is met by planting 

 so as to take advantage of the wetter months for growth and of 

 the dry season for crop production. 



Immunity by Evasion. 



The case of cotton just referred to is an example of the way 

 in which diseases may be " resisted " by evasion. It has 

 frequently been found with short-term crops that the best avail- 

 able protection against a disease or pest lies in growing early 

 maturing varieties, or in planting to escape a season when a 

 particular disease is prevalent. The seasonal distribution of 

 diseases as experienced on early and late plantings of the same 

 crop is frequently quite marked. It may be determined according 

 to the particular case by the effects of climate on either the host 

 or the parasite. 



The highly important effect on disease of the practice of 

 crop rotation may be put under this heading, since it mainly 

 depends on the temporary freedom from infestation to be secured 

 by planting disease-free material in uninfected soil. 



Resistant Species or Varieties. 



Resistance in the stricter sense of the word, and its complete 

 development immunity, depend on inherent properties of the 

 plants concerned, and may be transmitted to their descendants. 

 Such resistance may or may not be independent of variation in 

 the conditions under which the plants are grown ; sometimes 

 a degree of resistance which has proved constant under one set 

 of conditions is not developed when the circumstances are 

 changed. 



The characters on which resistance depends are almost always 

 obscure. Attempts to connect its existence with special morpho- 

 logical features are seldom successful, though it may exist in 

 correlation with some difference of form. It appears to be in 

 most cases a property of the protoplasm. In the case of certain 

 obligate parasites resistance has been shown to arise from 



