Plant Sanitation. 



260 



[March, 1909. 



bilities. We are able at the present 

 time in certain sections of the country 

 to advise growers of certain crops what 

 varieties to plant in order to insure a 

 crop relatively resistant to some 

 specific disease. 



At first thought there would seem 

 to be almost no limit to the practical 

 results obtainable. There are certain 

 valid objections, however, which stand 

 out prominently and which cannot be 

 overlooked. 



Difficulties and Objections. 



In view of present-day knowledge no 

 one will deny that all our common 

 plants vary, and every grower of plants 

 knows that cultivated varieties are 

 specially given to variation. How many 

 of us, however, have ever given thought 

 to the fact that the organisms causing 

 the diseases of our cultivated plants 

 may also vary? I say " fact " advisedly, 

 for there is an abundance of evidence to 

 prove that fungi and bacteria are 

 capable of as wide variation compara- 

 tively as we are familiar with in the 

 most variable of cultivated plants. 



That fungi vary in form when grow- 

 ing under different conditions, has been 

 repeatedly proven. I wish to point out 

 that fungi vary in their physiological 

 characteristics and in their degree of 

 parasitism. I wish that time would 

 permit me to go into a detailed dis- 

 cussion of this interesting subject, but 

 I will content myself with saying that 

 the breeder of resistant varieties has 

 this difficulty to contend with : The 

 organism which he has been endeavour- 

 ing to evade by the development of 

 resistant varieties may in time vary 

 in the direction of greater virulence, 

 and his carefully selected variety become 

 as susceptible as was the original stock. 

 In support of this objection, the history 

 of the Keiff6r pear may be cited. 

 When first introduced it was especially 

 resistant to the bacterial disease known 

 as fire blight, which in some sections 

 has prevented the cultivation of any 

 variety except the one under discussion. 

 There seems to be considerable evidence 

 to show that it is much less resistant 

 to the disease than formerly, particu- 

 larly in southern Delaware. A promi- 

 nent apple grower of Kent County, 

 Delaware, who has practised spraying 

 against apple scab for nearly twenty 

 years, recently expressed the opinion 

 that apple scab was much harder to 

 control now than when he began to 

 spray, and said that he believed it was 

 to variation on the part of the scab 

 organism in the direction of greater 

 virulence. The history of the coffee leaf 

 disease in Ceylon is also in support of the 

 objection. 



Another serious objection has fre- 

 quently been raised. Will resistant 

 varieties developed in a given locality 

 and in a praticularsoil retain their resist- 

 ance if transferred to another climate 

 and to soil of a very different character ? 

 Doubtless some of the varieties of 

 certain crops so far developed will retain 

 their resistance through a fairly wide 

 range of conditions, bat some may be 

 found wanting. The person who intro- 

 duces a disease-resistant variety should 

 test his production for several reasons, 

 and under a variety of conditions. 



The objection has been raised that 

 resistant sorts are inferior. This is 

 frequently the case. In the develop- 

 ment of immune varieties the market 

 requirements must be kept in mind. It 

 is, however, something gained to have 

 developed a variety showing high 

 resistance to some malady, even if the 

 variety is worthless commercially, for 

 this sort may then be used as one parent 

 in crossing with commercially desirable 

 varieties. 



Another difficulty in this work which 

 presents itself is that most of our culti- 

 vated plants have more than one 

 parasitic organism attacking them. 

 Some plants have a score or more 

 such diseases. A plant-breeder usually 

 develops his immune variety in some 

 particular section to the most serious 

 disease in that locality. When the 

 variety is transferred to another section 

 where another disease is the prevalent 

 one, he may find that the plant is not at 

 all resistant to the second trouble. 



Ward, Salmon, and others have shown 

 that there exist what have been termed 

 biological forms of certain species of 

 fungi, that is, a species of fungus may 

 include certain forms, one of which is 

 able to infect only a certain species or 

 group of species of host plants, while 

 another may infest other species not 

 infected by the first form, etc. These 

 forms are indistinguishable from each 

 other morphologically. In developing a 

 resistant variety of plant in one locality 

 we may be developing resistance to only 

 one biological form of the fungus. 

 When we transfer the variety we may 

 encounter another form of the same 

 fungus to which our plant will be 

 susceptible. 



Salmon also points out the possibility 

 of there being strains of the same fungus 

 having great virulence. 



Cause of Immunity. 



So far I have omitted any reference 

 to the cause of disease-resistance or 

 immunity. It is perhaps beyond the 

 scope of this talk to go into any detailed 



