[Vol. 1 



310 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



mostly in very moist valleys, where the wind has no free play. 

 The fruiting bodies of many Ascomycetae develop in dry air, 

 and it is not remarkable that that type of disease is found 

 in some parts of the West Indies, which have a drier climate. 



A group which has no representative in the tropics is that 

 of the powdery mildews (Erysiphaceae). These fungi occur 

 only in colder climates. The so-called false mildews or Perono- 

 sporaceae, on the other hand, are of considerable impor- 

 tance, these fungi seeming to thrive well under the moist and 

 hot weather conditions. We find the canker of rubber and 

 cacao (caused by Phytophthora Faberi) of far-reaching im- 

 portance. In both the rubber and the cacao the disease at- 

 tacks the bark and, in the case of the cacao, also the fruit. The 

 growth of this fungus depends upon a very moist air. This 

 is proved by the fact that when the trees are cut back severely 

 so that the trunk is exposed to sun and wind, the wounds often 



the disease is stopped. A plantation in which the 

 trees are planted far apart also suffers less. 



Another fundus — Phytophthora Nicotianae — belonging to 



heal and 



?5 



the Peronosporaceae is the cause of a dangerous tobacco 

 disease. The parasite kills the seedlings in the beds, the 

 plants "melt," and even the mature tobacco plants are at- 

 tacked. The fungus penetrates into the pith of the lower part 

 of the stem and the "tobacco-tree" falls. A third member 

 of this family destroys a large part of the Indian corn, so 

 widely grown by the natives. It is Peronospora Mayidis, 

 unknown, so far as I am aware, in the large corn areas of the 

 United States. The exceedingly moist climate, combined with 

 the excessive heat, evidently favors the attack by the fungus. 

 In the potato fields of the mountain districts of Java we find 

 a friend of our countries, Phytophthora infestans. Potatoes 

 are grown in the tropics between 1500 and 6000 feet altitude. 

 In the lower areas we find phytophthora-infected regions only 

 rarely, but the higher we ascend, the lower the temperature 

 (frosts may even occur in the nights) and the more destruc- 

 tive the phytophthora becomes. The spores of the fungus (it 

 has been proved) cannot germinate at a high temperature, 

 which explains the occurrence of the disease only in the higher 



