SYMPTOMS OF DISEASE 



17 



In addition to the somewhat general characters accom- 

 panying diseases, it is necessary for purposes of diagnosis to 

 note closely the detail characters of each sick part. 



In a spot the color of its margin and center; its border, 

 whether sharp or indefinite; whether the same upon both 

 upper and lower surfaces are important items to observe. 



In a rot these same 

 details, also the texture, 

 soft or hard, wet or dry, 

 the odor, and taste are 

 important. 



In the plant, on a 

 whole, it is well to note 

 whether the disease is 

 local or general, progres- 

 sive or stationary, spread- 

 ing regularly or otherwise. 



Certain minute struc- 

 tures should be looked for 

 upon rots, spots, cankers, 



or other diseased parts. Some of these, which may be 

 recognized with the naked eye or a good hand lens, are : — 



Py.cnidia or perithecia. — Small, nearly microscopic, 

 covered sporiferous pustules on the surface of the diseased 

 area. 



Acervuli. — Structures like pycnidia, but uncovered. 



Sori or spore clusters. 



Hyphae. — Erect, hair-Uke, very nearly microscopic 

 growths coming from the diseased area. Their presence 

 upon a white background may often be recognized by the 

 dirty ashen color which they lend to the spot. 



Fig. 7. 



- Sclerotia of lettuce sclerotiuiose, 

 natural size. Original. 



