302 



FUNGOUS DISEASES OF PLANTS 



encouraged by certain weakening influences, such as the age of 

 the leaf, the presence of flea-beetle injuries, etc. When large 

 spots near the margins of the leaves become confluent, such ex- 

 tensive areas are affected that there may result a rolling up of the 

 edge, which might be mistaken for the tip burn, a disease gener- 

 ally due to climatic conditions. 



The injury from the early blight results, therefore, in an early 

 death of the leaves, as a result of which the vines dry up and the 

 losses to the growing crop are often very considerable, amounting 



to as much as 50 per cent. The 

 disease is said to be more likely 

 to begin at the time of flowering 

 and while the work of the plant is 

 directed toward the development of 

 tubers. This fungus produces no 

 rot directly. 



This Macrosporium is found not 

 only upon the potato but also upon 

 tomatoes and upon the jimson weed, 

 {Datura Stramonium). There is 

 also a very considerable difference 

 in the susceptibility of the different 

 varieties of potato, but at present no 

 wholly resistant sorts are known, 

 although the general question of 

 the resistance of potatoes to diseases 

 is receiving special attention in the chief potato-growing regions 

 of the world. 



The fungus. Within the tissues the mycelium is light brown to 

 olivaceous, and the conidiophores arise through stomates or push up 

 between the collapsed epidermal cells as erect or assurgent fruiting 

 hyphae 50-90 x 8-9 /*. They are septate, slightly curved, and, as 

 is characteristic of this genus, the conidia are produced singly, so 

 far as observed, upon the host. The conidia have been described 

 as "obclavate, brown, 145-370 x 16-18/*, terminating in a very 

 long, hyaline, septate beak (apical cells) equalling fully one-half the 

 length of the spore (often exceeding this) ; body of spore with 5 to 

 10 transverse septa, longitudinal septa few or lacking " (Fig. 137). 



Fig. 



36. Early Blight of the 

 Potato 



