204 



SCIENTIFIC AGRICULTURE. 



February, 1922. 



remain viable for several years and consti- 

 tute a source of infection if potatoes of a 

 susceptible variety are planted. Cases are 

 on record in which the disease has reap- 

 peared after an interval of ten years in 

 England. 



Life History of the Fungus. 



The resting sporangia which average 

 about 52 microns in diameter contain many 

 round zoospores which measure 21/2 mi- 

 crons in average diameter. In the spring 

 the resting sporangia may germinate and 

 the released zoospores are motile in the soil 

 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 niay 

 germinate, the resulting zoospores giving 

 rise to secondary infections. Toward the 

 close of the growing season winter rest- 

 ing sporangia are formed in a similar man- 

 ner. Another type of summer sporangium 

 is that in which thin-walled structures, 

 sometimes in groups, are developed in a 

 sorus. Each thin-walled sporangium con- 

 tains numerous zoospores which are smaller 

 than those from the ordinary type of rest- 

 ing sporangium. 



Other Hosts. 



To date the tomato is the only other host 

 known to be susceptible in America. In 

 England Solanum nigrum and S. dulca- 

 mara are slightly susceptible. 



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 is known, 

 occur in Canada and samples of any doubt- 

 ful tubers should be sent immediately to 

 a reputable authority for determina- 

 tion. Where the disease does occur the 



only control is the planting of immune 

 varieties, 



(b) Leak of Potatoes. 



With the development of distant mar- 

 kets storage and transit diseases are of in- 

 creasing economic importance. "Leak" is 

 one of t/ie most serious of these diseases. 

 It has been ascertained by Link that 

 "Leak" is practically co-exteusive with the 

 potato crop of the United States. So far 

 as the writer knows it has not been re- 

 ported 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 

 casual organism lives in soil and can enter 

 the tuber only through wounds exposing 

 the inner tissues. The fungus grows 

 through the tissues causing ti.c* tuber to 

 become brown over the entire surface. In 

 this condition it is soft, easily crushed and 

 pressure causes the exudation of a brown- 

 ish watery liquid. In advanced stages the 

 symptoms might frequently be mistaken 

 for those of tuber rots caused by Pusarium 

 species. 



Life History of the Fungus. 



Orton has shown that a similar rot may 

 be produced by Rhizopus nigricans Eh- 

 renb. but the work of Link demonstrates 

 that "Leak" is usually caused by Pythium 

 deharyanum Hesse. 



This fungus is aggressively parasitic if 

 humidity and temperature conditions are 

 satisfactory and is the common cause of 

 "damping off" in greenhouses and nur- 

 series. The mj'Celium is eoenocytie, except 

 with age, irregularly branched and rather 

 coarse. Conidia are produced terminally 

 on branches of the mycelium or they may 

 be intercalary. They are globose to ellip- 

 tic and average 22 microns in diameter. 

 They may germinate at once by one or 

 more germ tubes. Oospores are also pro- 

 duced which are smooth, spherical and 

 thick-walled and able to live over an ad- 

 verse period. Growtli is slow at 48 deg. F. 

 and practically ceases at 41 deg. F. while 

 it is best at about 86 deg. F. In an ex- 



