402 Diseases of Economic Plants 



on willow and poplar, the lesions occurring on the trunks, 

 limbs, and twigs, with reddish pycnidia on the dead areas 

 near the edge of the canker or over the entire dead surface. 

 Strict inspection should guard against further spread of 

 the disease. Resistant varieties should be used, the trees 

 given plenty of water and protected against mechanical 

 injury. 



Leaf -blight {Marssonina populi (Lib.) Sacc). ^ — Primary 

 infection occurs upon the leaves, resulting in small, 3-8 mm., 

 circular, black dead spots which are quite definitely bordered. 

 As the leaves die, the patches upon the stems also turn black 

 and die. Infection proceeds to the supporting twigs and may 

 result in dead patches of bark upon quite large shoots. 



Cutting and burning infected twigs is advised. 



Galls (Macrophoma tiimefaciens Sh.) occur on the branches. 



RUBBER '^^ 



Trees and shrul)s that produce rubber are subject to 

 numerous destructive diseases, among them Cankers due to 

 various species of Nectria, Corticium, Fusicladium, and Di- 

 plodia; Die-back {Thyridaria) which kills the young shoots; 

 seedling diseases due to Pestalozzia, Helminthosporium and 

 numerous leaf fungi, and Root-rot due to Fomes, Irpex, 

 Hymenochsete and Poria. 



SASSAFRAS 



Heart-rot {Fames ribis (Sch.) Fr.). — Spaulding in 1907 

 described this rot as a serious injury to sassafras in Missouri. 

 It is also found upon stems and roots of various shrubby 

 plants, as rose and currant. By means of a wound the attack 

 is made upon exposed heartwood, proceeding slowly into the 

 sapwood. The sassafravs is thus immune until it reaches an 

 age bearing heartwood. The decaying wood is abnormally 

 light in color and slightly reddish, and the affected region is 

 bounded by a narrow black zone. Trees sometimes die from 

 the attack. 



