THE SPREAD OF PLANT DISEASES. 133 



powdery mildew, club-root of crucif ers, potato scab, wheat and oat 

 smut, downy mildew of grape, black rot of grape, pear and quince 

 leaf blight, pear and apple scab, melanose of orange, and damjj- 

 off fungi. 



(4) Indirect Methods — I.e., by furnishing the host plant with 

 the best possible conditions for groAvth and the parasites with 

 the poorest. To apply these measures properly requires a very 

 considerable body of knowledge, both of the needs of the crop 

 grown and of the life history of the parasite. Here is just 

 where many growers fail, and this is Avhy so many hothouses 

 seems to have been erected as a special banqueting chamber for 

 bugs and fungi of all sorts. The first thing that the owner of 

 such a house needs to do is to learn the habits of the parasites 

 and study the requirements of his plants until he can properly 

 repress the one and cater to the other. Many houses are badly 

 adapted to the requirements of the crops grown in them. 



Some of the diseases commonly observed in houses, and due 

 almost wholly to mismanagement, may be noted very briefly : 



The powdery mildew of the rose, cucumber, etc., is brought 

 on very largely by chill due to improper ventilation. Cucumber 

 anthracnose and the timber rot are fa.vored by excess of water 

 and insufficient ventilation. Some houses are so constructed that 

 they cannot be properly ventilated. Timber rot is found specially 

 in damp corners, where the water remains in the angles of stems 

 and leaves for hours together. The lettuce rots due to Botrytis 

 and to the downy mildew are often brought on by excess of 

 water and irregular heating. If time permitted, man}' other 

 instances might be cited. 



Sometimes these troubles are due to the attempt to grow too 

 many kinds of plants in the same house. In such cases the 

 grower generally tries to follow a medium course in the matter 

 of heat, ventilation, and water-supply, and in doing so furnishes 

 normal condition for none of his plants. 



Certain field diseases may also be restricted by stimulating the 

 plants to make the best possible growth, or in case of the onion 

 smut, as Dr. Thaxter and Dr. Sturgis have shown, by growing 

 the plants in soil free from the fungus and not setting them out 

 into the infected earth until they are beyond the receptive stage. 



Concerning care in the use of manure, in the selection of seeds 

 and cuttings, and in the buying of trees and other plants, 

 I need not saj' more than I have already done. 



