164 



SCIENTIFIC AGRICULTURE. 



January, 1922. 



size. The s'em-end is frequently first af- 

 fected but as the tuber matures pustules 

 may be found anywhere on the surface. 



The first sign is a tiny, brownish-purple 

 pimple on the surface surrounded by a 

 more or less translucent zone, the whole 

 not exceeding one-twelfth of an inch in 

 diameter. This spot increases in size and 

 the skin is gradually separated from the 

 tissues below, wliich are stimulated to ab- 

 normal growth so that the protuberance 

 is formed. Usualh- the epideraiis is rup- 

 tured by this growth just about the time 

 the organism is ready to go into the spore 

 stage. The turned-back epidennis is typ- 

 ical of the pustules of pow^dery scab in 

 uninjured specimens. (See Fig. 6). As 

 the parasite matures the host cells are in- 

 vaded, broken down and killed so that only 

 remnants are found. At this time the para- 

 site ceases its vegetative life and forms 

 spores which are aggregated into balls. It 

 is tlie spore balls together wuth remnants 

 of cellular tissues which form the powdery 

 mass found in tlie pustules at maturity 



Figure 6. — Green Mountain tuber with typical 

 spots. Note the turned-back epidermis leaving 

 the masses of powdery material exposed for 

 dissemination. (After Melhus et al, Jour. Agr. 

 Res. 7: 1916.) 



and which account for the common name 

 "Powdery Scab". If one rubs out the 

 powdery mass the tissue below is seen to 

 resemble the epidermis of tlie tuber to a 

 certain extent and in fact there is present 

 a layer of cells functioning as a cork layer, 

 which fact accounts for the other common 

 name for the disease — ' ' Corky Scab ' '. 



In Europe, Great Britain and Ireland, 

 a more advanced stage of the disease is 

 found where the tuber as a whole is mal- 

 formed and tissues severely disorganized. 

 This is known as the canker stage. That 

 it does not occur in this country is prob- 

 ably due to the shorter growing season. 



Life History. 

 Spongosporu suhtertanea requires an 

 abundant .supply of moisture for germi- 

 nation, as well as warmth. Hence it is 

 that the disea.se is more serious in poorly 

 drained soils or during highly liumid sea- 

 sons. The spores in the spore ball germ- 

 inate giving rise eventually to naked mas- 

 ses of pro'cplasm known as "plasmodia". 

 A Plasmodium moves slowly over a surface 

 Avhich is moist by means of psendopodia 

 which are merely pr.strusions of the naked 



Kig. 7. — Roots of potato with galls caused by 

 SpongoEpora subterranca. 



p:otopla.sm. in this way, by moving in 

 soil water among the interstices of the 

 soil, the Plasmodia reach roots, stolons and 

 tubers.' Whichever tlie part attacked 'he 

 l)lasm()diiim i)enetrates the epidermis and 

 gradually jiasscs down to the cells below. 

 Those aiT stimulated tc excessive growth 

 so that "giant cells" are formed which may 

 also divide, thus giving an abnormal num- 

 ber of cells in the tissue and accounting 

 for the gall formation on loot and stolon 



