110 



SPORE D1SSEMIXAT10\ 



11.5 spots per leaf on apple trees 1% miles away from infected 

 cedars and 0.32 spot per leaf on trees 3 miles distant. 



The foregoing evidence regarding dispersal of rusts bv air cur- 

 rents is representative for this group of fungi but is inadequate in 

 indicating the importance of this agencv for other groups of fungi. 

 Stakman and his coworkers (1923) identified other genera, such 

 as Alternaria, Helminthosporium, Cladosporium, Cephalothecium, 

 and Ustilago, on their spore traps. Heald et al, (1915) found that 



^& 



Fig. 26. Spore trap of weather-vane type. A. The dish whose inner surface 

 is coated with glvcerin remains directed toward the wind. B. Dishes may 



be stacked during transport. 



ascospores of Endothia parasitica may be entrapped in the air in 

 considerable numbers 300 to 400 ft. from diseased chestnuts, sub- 

 stantiating the observations of others. Many similar observations 

 have been recorded for Venturia hiaequalis. Burrill and Barrett 

 (1909) showed that Diplodia zeae is distributed bv winds; Wolf 

 (1916) made the same observation for Cercospora personata. 

 Peronospora tabacina is verv quickly dispersed from infected to- 

 bacco seed beds to healthy ones several miles distant [Wolf et al. 

 (1934)], and it is reasonable to assume that its introduction into 

 New England and Canada was the result of the carriage of spo- 

 rangia several hundred miles through the air. 



The aerial dissemination of plant pathogens is briefly treated 

 in a recent report by Craigie (1939). His studies show that the 

 funp-i causing stem rust of cereals, leaf rust of wheat, and crown 

 rust of oats are air-borne in western Canada, being carried several 



