February, 1922. 



SCIENTIFIC AGRICULTURK. 



205 



perimental inoculation Link found that at 

 86 deg. F. the fungus penetrated to a depth 

 of 4 cm. (11/2 in.s.) in 67 hours. 

 Varietal Susceptibility. 



Rurals and Burbanks dug during warm 

 weather ai'e especially susceptible and ino- 

 culation experiments b}' Link tend to show 

 that Triumph, Green Mountain, Early 

 Ohio, Rural New Yorker and Irish Cobbler 

 are susceptible. 



Control. . 



The disease can be controlled by care in 

 harvesting and handling potatoes and by 

 sorting out wounded tubers. 



(c) Late Blig^ht and Rot. 



This disease, caused by Fhytophthora 

 infestans (Mont.) De Bary, is too well 

 known to need emphasizing as to its eco- 

 nomic importance. It is not often that 

 a fungus can materially affect the policy 

 of a country but this is what Phytophtho- 

 ra infestans did in Great Britain. Late 

 blight was so serious in 1845 in England 

 and Ireland that the potato crop was a 

 failure. So much was this the case in Ire- 

 land that a famine occurred and many 

 thousands of Irishmen left Ireland for 

 America. To relieve the distress the Corn 

 Laws were repealed and in a sense this ini- 

 tiated a Free Trade policy. 



The disease is now controllable so that 

 epiphytotics are rare; nevertheless a 

 warm, wet summer is a season of worry to 

 the potato grower in the Maritime Pro- 

 vinces, Quebec, New England States and 

 New York. It is common in Europe from 

 east to west where the growing season is 

 moist and mild. 



Symptoms. 



Irregular spots at the margins or tips of 

 leaves are produced which are at first 

 water-soaked. The position of the lesions 

 is due to the drainage of the water on the 

 leaf surface in which the spores germinate. 

 If the weather becomes dry the lesions turn 

 brownish and dry out more or less. Under 

 humid conditions the mycelium in the leaf 

 tissues grows rapidly and sends out 

 through stomata in the lower surface bran- 

 ches which abstrict conidia in profusion. 

 The conidiophores are usually so numerous 

 under these conditions that a distinct pale 

 violet tinge is given to the affected lower 

 surface. If the disease is not checked the 

 leaves are rapidly destroyed and gradually 

 the stems are affected. 



On the tubers the first symptom is a 



slight darkening of the skin over an in- 

 fected area. Later this area becomes 

 slightly sunken and a dull reddish-brown 

 in color. Gradually the mycelium of the 

 fungus penetrates the tissues causing a 

 drs'-rot if no secondary organisms are 

 present. 



A general symptom in a seriously af- 

 fected area is the odor, which is difficult 

 to describe but is something like stale her- 

 ring-brine. 



Life History of the Fungus. 



Most Phycomycetes live over adverse 

 seasons as a sexual structure known as an 

 oospore. It was therefore natural to look 

 for oospores in Phytophthora infestans but 

 not until 1875 was any statement made 

 that oospores had been found. De Bary 

 had studied the disease previously and 

 concluded that the fungus lived over in 

 the tuber. In 1875 Worthington G. Smith 

 announced that he had found oospores of 

 the fungus. De Bary again studied the 

 case and again concluded that mycelium 

 lived over in the tuber. Since then, L. R. 

 Jones (1909), Clinton (1911) and Pethy- 

 bridge and Murphy (1913) have found 

 oospores in pure cultures of the fungus. 

 The work of Melhus (1915) shows that 

 mycelium living-over in the tuber can ini- 

 tiate an outbreak of the disease and that 

 oospores have not yet been shown to give 

 rise to the first outbreak. 



Regarding, therefore, mycelium in the 

 tuber as the over- wintering stage infec- 

 tion of a shoot just beginning to grow can 

 take place by the growth of mycelium from 

 a nearby lesion in the tuber. If the shoot 

 is attacked early in its growth dwarfing 

 will result so tha*^ when normal shoots are 

 8 inches tall the infected shoots may be 

 only just above ground. Sheltered by the 

 foliage of healthy stems and given satis- 

 factory temperature and moisture condi- 

 tions conidiophores will grow out through 

 stomata in the dwarfed shoot and conidia 

 will be abstricted. Not many are needed 

 to initiate the outbreak. A conidium germ- 

 inates in a short time in a thin layer of 

 water on the leaf giving rise to 8 zoospores. 

 These are motile for perhaps an hour, 

 then they come to rest, germinate by a germ 

 tube and infect the leaf. Under moist 

 summer conditions this is repeated ap- 

 proximately in 10 days, but the period 

 elapsing between successive sporulations in- 

 creases with decreasing humidity. 



