176 NEBRASKA STATE HORTICULTURAL SOCIETY. 



Considering the minuteness of many of the disease producing 

 organisms, you can readily see that this task of making and 

 keeping pure cultures has been no easy one. There are two 

 things that have very greatly facilitated the operation: first,, 

 the discovery that it w^as impossible for even the minutest or- 

 ganisms to get through a plug of cotton. It seems strange that 

 such a simple discovery was not made until the middle of the 

 nineteenth century. Second, the plate method, of growing 

 bacteria and fungi, and the principle of intermittent steriliza- 

 tion, has made the isolation of these minute forms possible. It 

 is as easy at present for the pathologist to grow bacteria and 

 fungi in pure cultures in test-tubes, Petri-dishes, and damp- 

 chambers as it is for the gardner to grow his plants in flower- 

 pots, flats and benches, and with "needle" and "loop" and 

 "colony lifter" he transplants these minute organisms with no 

 less difficulty than the gardner experiences in the use of trowel 

 and dibble. 



The advantages of the pure cultures are many. The disease 

 organism can be subjected to a variety of conditions: treated 

 to any food medium imaginable; given acids or alkali; its 

 growth tested at high temperatures or low temperatures; the 

 action of poisonous substances on its growth observed; the 

 products of its growth determined; its life history eludicated, 

 and inoculations made into the tissues of the host plant. 



It may be opportune that I review in brief some of the things 

 that the working pathologist must determine in case of the ap- 

 pearance of a new disease. The first question that he must ask 

 and settle if possible is the cause. Is the disease due to a par- 

 asite or does it belong to the category of inorganic or constitu- 

 tional diseases? The exact cause is often difficult to determine, 

 as may be noted in the case of several well known examples as 

 "peach yellows", "peach rosette", and "little peach". The 

 cause of these is but little better understood than when they 

 were first observed, yet much valuable information has been 

 obtained as a result of their study. In order to demonstrate 

 beyond a question that a given disease is due to a certain para- 

 site, four steps involving a considerable amount of labor and 

 time in laboratory and green-house, and field are required: 



1. The demonstration of the constant association of the 

 organism with the deseased condition of the plant. 



