PLANT DISEASES. 7I 



studied separately. Great difficulty is experienced in isolating 

 some of the types and growing them on for further study, and 

 laboratory methods to overcome this trouble are being devised. 

 It is also difficult to estimate accurately the relative prevalence 

 ot the various types, and here again methods are being worked 

 out. One important aspect which is kept prominently in view 

 is the part which algae and fungi respectively play in connection 

 with the accumulation of nitrogen in the soil. The question of 

 disease is also of the greatest importance. So far as is at present 

 known, no green soil algae cause disease, and probably some 

 of the types are very useful in the soil. Many soil fungi, however, 

 are known to cause destructive diseases, and an attempt is 

 being made to find out which types are harmful and which are 

 helpful, and what are the conditions under which the former 

 can operate. This will lead up to methods of encouraging those 

 which are helpful and eliminating the harmful. 



Possibility of Change in Fungi and Bacteria. — A second line of 

 investigation arose out of a study of one of the commonest and 

 most destructive fungi, Botrytis cinerea, in which it was found in 

 one instance that instead of the reproductive bodies being of 

 the usual black colour, part of them were black and part white. 

 This is the only known instance of white reproductive bodies 

 having been produced by this fungus, and it opens up a consider- 

 able field for enquiry; for if fungi have the power, so to speak, 

 of changing their colour, their capacity of attacking plants may 

 also be changed. In other words, a fungus which is now able 

 to attack, say, the potato, and cause a disease, might become 

 changed in nature so that it could attack not only the potato 

 but certain other plants. The change observed in Botrytis may 

 prove to be an entirely isolated case, but the question is in urgent 

 need of further enquiry. Certain destructive species of bacteria 

 and fungi are therefore being grown and studied; in each case, 

 some colonies are kept under constant conditions and others are 

 subjected to continually changing conditions, so that every 

 opportunity is offered to the fungi or bacteria to alter their 

 character. If any such alteration takes place, its nature will be 

 studied, to see whether it extends to the power of the organism to 

 attack its normal host plant, or gives to it the power of producing 

 disease in crop plants previously immune. Just how far its 

 powers may thus be modified is an important question. 



The Basis of Spraying. — Another investigation deals with the 

 fundamental nature of spraying. At present a farmer or fruit- 

 grower often sprays in order to kiU fungi; but no one knows 

 exactly how or why the fungi are killed, and until this information 



