96 Minnesota Plant Diseases. 



cases death. Likewise, when trees grown in the protection of 

 forest shade are suddenly transplanted to a prairie or isolated 

 by the cutting down of surrounding trees, they may fall a prey 

 to sun scorch. 



External causes may be fertile in many other predisposi- 

 tions: Wounds by pruning, root injuries, insect boring, hail- 

 stone wounds and injuries by lightning strokes, frost cracks and 

 sun scalds, etc. 



It has already been explained that disease is never inher- 

 ited. On the other hand, it is a fact that natural conditions, 

 e. g., of form or of habit, which are, in reference to certain dis- 

 eases, causes of predisposition or immunity, may be inherited. 

 Such accidental conditions as wounds are of course not capable 

 of transmission. In other words, only natural or so-called 

 normal predispositions are inheritable. 



Variation and predisposition. The selection of varieties in 

 agriculture and horticulture is very greatly concerned with this 

 phase of the subject. Variation in structure, form and habit 

 give to plants different degrees of resistance toward certain dis- 

 eases, some greater and some less. Of course' there is likewise 

 variation in respect of other conditions and one may select vari- 

 eties for those conditions. For instance, wheat varieties may 

 be selected for their fitness for milling, size of grains, crop 

 yield and other characters. Fruit tree varieties are selected 

 for size of fruit, keeping qualities, yield and so on. Now one 

 can also select varieties of agricultural plants for the resistance 

 which they exhibit towards a given disease. For instance, cer- 

 tain varieties of strawberries will resist the strawberry spot fun- 

 gus more successfully than others, and where this disease is 

 prevalent might be very desirable. Again, some apples or crabs 

 are more susceptible to apple scab than others, and the selec- 

 tion of these varieties may be a distinct advantage. Of course 

 such varieties might not be the most desirable in other respects. 

 In other words, the intelligent grower of plants has before him 

 a very complex problem. The object to be gained is the best 

 crop under the existing conditions. These conditions he must 

 know thoroughly before he can solve the problem, and the 

 varieties must be selected accordingly. It should be empha- 

 sized that the conditions must be thoroughly understood and 



