86 PROTECTION OF PLANTS, 1921-22 



The disease is now controllable so that epiphytotics are rare; nevertheless 

 a warm, wet summer is a season of worrj" to the potato grower in the Maritime 

 Provinces, Quebec, Xew 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 be- 

 comes dry the lesions turn brownish and dry out more or less. Under humid con- 

 ditions the mj'celium in the leaf tissues grows rapidly and sends out through 

 stomata in the lower surface branches 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 

 infected 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 dry-rot if no secondarj- organisms are present. 



A general sj-mptom in a seriously affected area is the odor, which is difficult 

 to describe but is something like stale herring-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 ease and again 

 concluded that mycelium lived over in the tuber. Since then, L. R. Jones 

 (1909), Clinton (1911) and Pethybridge and Murphy (1913) have found oospo- 

 res in pure cultures of the fungus. The work of Melhus (1915) shows that myce- 

 lium living-over in the tuber can initiate 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 in- 

 fection of a shoot just beginning to grow can take place by the growth of myce- 

 lium fi'om a nearby lesion in the tuber. If the shoot is attacked early in its 

 growth, dwarfing will result so that, when normal shoots are 8 inches tall, the 

 infected shoots ma}^ be only just above ground. Sheltered by the foliage of 

 health}" stems and given satisfactory temperature and moisture conditions, 

 conidiophores will grow out through stomata in the dwarfed shoot and conidia 



