84 PROTECTION OF PLANTS, 1921-22 



measure 23^ microns in average diameter. In the spring the resting spor- 

 angia may germinate and the released zoospores are motile in the ^oil water 

 for a time. Eventually they become amoeboid by the withdrawal of the single 

 cilium and in this stage they enter the young tissues of stolon or buds. After 

 the infected tissues have been depleted of food the fungus in each cell rounds 

 up, develops a protective wall and becomes a summer-resting sporangium. 

 This may germinate, the resulting zoospores giving rise to secondary 

 infections. Toward the close of the growing season winter-resting sporangia 

 are formed in a similar manner. Another type of summer sporangium is that 

 in which thin-walled structures, sometimes in groups, are developed in a 

 sorus. Each thin-walled sporangium contains numerous zoospores which are 

 smaller than those from the ordinary type of resting sporangium. 



Other Hosts 



To date the tomato is the only other host known to be susceptible in 

 America. In England Solanum nigrum and S. dulcamara are slightly suscep- 

 tible. 



Varietal Susceptibility 



The work of Weiss and Orton shows that Green Mountain, Cobbler and 

 Burbank are immune while Rural New^ Yorker, Early Ohio, Early Rose^ 

 Triumph, Pearl and Up-to-Date types are susceptible. 



Control 



This disease does not, so far as in known, occur in Canada and samples of 

 any doubtful tubers should be sent immediately to a reputable authority for 

 determination. Where the disease does occur the onlj' control is the planting 

 of immune varieties. 



(b) Leak of Potatoes 



With the development of distant markets storage and transit diseases are of 

 increasing economic importance. ''Leak" is one of the most serious of these 

 diseases. It has been ascertained by Link that "Leak" is practically co-exten- 

 sive with the potato crop of the United States. So far as the writer knows, it 

 has not been reported from Canadian markets. 



Symptoms 



The disease is first apparent as a small brown discoloration around a wound 

 such as might be caused during digging. The causal organism lives in soil and 

 can enter the tuber only through wounds exposing the inner tissues. The fungus 

 grows through the tissues causing the tuber to become brown over the entire 



