44 DISEASES OF TROPICAL PLANTS 



CH. 



parasitic and saprophytic species, and is widely dis- 

 tributed. The two most important families are the 

 Erysiphaceae and the Perisporiaceae. The most char- 

 acteristic types of the former are the powdery mildews, 

 which form delicate white mycelium over the surface of 

 the leaves and sometimes of the young stems and fruits. 

 The white appearance of the plant is very noticeable, 

 but the mycelium is frequently mistaken for a layer of 

 dust. The mycelial threads send short projections 

 (haustoria) into the leaves and absorb a considerable 

 amount of food. The covering also interferes with the 

 action of the sunlight on the chlorophyll of the plant. 

 The mycelia produce upright branches, which become 

 constricted, forming numerous chains of conidial spores. 

 These spores are carried by the wind, and when they fall 

 upon the proper host plant, and under suitable condi- 

 tions, grow and thus cause the spread of the disease. 

 The perithecia (ascogonia) are small spherical bodies, 

 which are at first white but finally become black, and 

 are clearly visible to the naked eye. Upon examination, 

 with the microscope they are found to have rather 

 complicated systems of appendages which are character- 

 istic of the different genera. These appendages aid in 

 the distribution of the perithecia, since they catch and 

 hold on other plants. The appendages vary in the 

 different genera, and present definite characters which 

 are very important in the determination of the species. 

 Within the perithecia are small transparent sacs which 

 contain the spores. The perithecial stage is usually 

 not so common in tropical countries as in the colder 

 parts of the world. The powdery mildews are very 

 abundant in the tropics, and include Erysiphe communis, 

 (Walls) Lev., on tobacco, and many other species on 

 beans, peas, and other plants ; and Phyllactinia corylea, 

 (Pers.) Karst, on mulberry (page 246). Thielaviopsis 

 ethacetica, Went., which is the cause of the pine-apple 

 disease of the sugar-cane (page 84), and Thielavia basi- 

 cola, (B. & Brl.) Zopf, which causes the root disease 

 of the tobacco (page 150), belong to this order. 



