272 DISEASES OF TROPICAL PLANTS OH. 



for many fungi. This is especially true in tropical 

 countries. 



Drainage. Poor drainage frequently results in weak 

 plants which are especially susceptible to disease, and 

 at the same time makes conditions especially favourable 

 for the growth of the organisms of disease. Improve- 

 ment in the drainage system where the character of the 

 soil will permit will usually give the desired results in 

 such cases. 



Fertilizers. The use of fertilizers presents a much 

 more complicated problem than the mere supply of 

 plant food to the agricultural crop. The proper use of 

 fertilizers depends upon the experience and good judg- 

 ment of the farmer, and no definite rules can be given 

 for all countries, all soils, all crops, and all conditions. 

 In general we believe that strong rapid-growing plants 

 are more vigorous, and therefore more resistant to 

 diseases. However, in some cases a rapid growth means 

 a soft plant which is more susceptible to the diseases 

 than if the same plant were grown more slowly. Over- 

 fertilization, or rather the improper use of fertilizers, 

 may also result in soft fruits and vegetables which are 

 especially susceptible to rots and very unsuitable for 

 long shipments. Fertilizers may also act upon the 

 soil in such manner as to make it unfavourable for the 

 growth and development of certain organisms of disease. 

 Lime is detrimental to many soil fungi, and is used as 

 a remedy for Plasmodiophora brassicae, Wor., the 

 cause of club root on cabbage and related plants. Lime 

 has also been recommended for use in trenches around 

 trees suffering from root diseases, to prevent the spread 

 of the organisms through the soil ; but its value has been 

 questioned by some persons with whom the writer has 

 corresponded. 



Spraying. Spraying is medicine. When people are 

 sick they want medicine, although the proper applica- 

 tion of the laws of sanitation and health will frequently 

 give much better results. So when the crops are 

 diseased the grower usually wants to spray, although 



