70 



DISEASES OF ECONOMIC PLANTS 



circular, rotten spot. This spot under favorable conditions 

 rapidly increases in diameter, the rate varying from 1-10 

 mm. each day. The spots may be either single upon the 

 fruit or very numerous, depending upon the abundance of 

 the infecting spores. When a spot becomes quite large, it 

 is usually somewhat depressed, with a shriveled, wrinkled 



surface, due to loss of 

 turgor of the under- 

 lying tissue caused by 

 evaporation. As the 

 spot ages minute spor- 

 ing pustules of the 

 causal fungus, barely 

 visible to the naked 

 eye, appear, first at or 

 near to its center. The 

 tissue of the fruit is 

 affected to some depth, 

 although the progress 

 of the disease is not so 

 rapid downward as lat- 

 erally. The pulp of the 

 apple at the margin of the rotten region is usually bitter, 

 thus giving rise to one of the common names of the 

 disease. 



Fruit of any age after its formation is liable to infection 

 if suitable climatic conditions and infective material obtain, 

 though the disease is most noticeable and does most dam- 

 age as the fruit approaches maturity. A series of hot, 

 wet days favors a destructive attack, while cold checks 

 it. The fruit as soon as it is badly rotted usually falls 



22. — Apple in advanced stage of bitter roi 

 (glomerellose). After Scott. 



