A M E R I C A N P O T A T O J O U R N A L 13

breathing pores of the surface of the tuber with ease. I f the soil becomes very dry dur ing this period the pota to may come th rough relat ively clean, or even escape infection. An ex- t remely wet soil during this period would also tend to check the growth of the fungus, and allow the potatoes to escape in- fection. Possibly soil conditions during the remainder of the season will not have much effect on the amount of scab. F rom this it m a y happen tha t scab is more severe in one par t of the field than another, depending on differences in growth, soil mois ture or amount of the organism.

In view of the informat ion given let us now consider the l imita t ions of seed t rea tment . Obviously we mus t conclude tha t the masses of rhizoctonia on the planted set should be killed, because of the active growth of this fungus , and the destruct ive possibi l i ty dur ing the same season. Bu t we must not fo rge t t ha t the p lants m a y be severely a t tacked by the rhizoctonia fungus carried in the soil. Indeed, the in ju ry f rom the rhizoctonia in the soil m a y f a r exceed tha t caused by any borne on the set.

With scab, the si tuat ion is quite different f rom the stand- point of the importance of scab on the set, because of the very restr icted growth of the scab fungus during the crop season. Consequent ly the amount of scab appear ing on a new crop must depend on the amount of the scab fungus normally in the soil. Hence, it would appea r tha t a clean or scabby crop has prac t ica l ly no relat ion to the p lant ing of scabby or clean sets t ha t season as the case may be. One mus t also bear in mind t h a t the amount of ~cab int roduced by the set might be insignificant when compared to the amount of scab fungus a l ready in the soil.

This note is s imply to ment ion the na tura l l imi ta t ions to effective tuber t r ea tmen t agains t scab and rhizoctonia, and is not in tended as an a rgument against the common practice in the . s e of ei ther fo rma ldehyde or corrosive subl imate solution.

Virus Diseases of Potatoes in British Columbia WILLIAM NEWTON, Laboratory of P l a n t Pathology, Saan ich ton , B. C.

Long before virus diseases were accurate ly diagnozed and their infect ious na ture established, the f a rmers of Bri t ish Columbia recognized tha t pota to s trains degenerated. I t has now been definitely proven t h a t potatoes do not degenerate,

14 A M E R I C A N P O T A T O J O U R N A L

do not run out, unless they are infected with virus diseases. I need not discuss the symptoms of mosaic, streak, leaf roll, spindle tuber and many other forms of virus diseases t ha t exist in the province, for we have recently discovered tha t a potato p lan t may be infected by several forms of virus disease s imultaneously. This condition m a y be expressed by disease symptoms quite different f rom the common virus diseases.

The f a rmer s ' problem is to el iminate as f a r as possible all virus diseases in his crop. Ample evidence has been obtained tha t yields are subs tant ia l ly lowered by mild mosaic and many other less conspicuous forms of virus diseases. Many of these forms are so difficult to diagnose, possibly because they are the expression of two or more forms at tacking the plant simul- taneously, t ha t we believe it is bet ter for the f a r m e r to focus his a t ten t ion upon the normal r a the r than upon the diseased plant. When once the f a r m e r is fami l ia r with the normal hea l thy type of the variety t ha t he is growing, it is possible for him to keep down the amoun t of virus disease to a very low level by removing the "off - type" p lants in his seed plot. There is only one ge'neral symptom of a virus infected potato plant . I t is an "off- type" plant . As soon as we discover t ha t the "off - type" charac ter can be t r ansmi t t ed by aphids or by other means to normal plants, we brand the p l an t as virus infected. Such p lants are a potential danger to the potato industry . The f a rmer has not the time to determine whether the "off - type" p lants in his seed plot are capable of infect ing normal plants . His safe ty lies in the destruct ion of these " rogues ' a s soon as they appear . A delay might prove costly.

Do Virus Infec ted Pota toes Recover?

There still persists the view t h a t " run out" potatoes recover if grown in certain districts. All experimental work up to the present disproves this view. Al though certain types of virus diseases are masked in certain distr icts and appa ren t l y have but sl ight effect upon yields, the disease is a lways present in jus t as active a form as in the districts where it can readily be detected. There are certain distr icts in the Eas t Kootenav where virus diseases are difficult to detect, but even in these districts a l though the yields are not conspicuously reduced there is no doubt t ha t if virus infected and virus free seed were grown side by side, a difference in favor of virus disease free seed could be noted. A very extensive investigation by the Min- nesota pathologis ts proved tha t 100% infected potato seed wa~ in la ter generat ions a lways 100 % infected regardless of where the seed was grown. Our own investigations confirm these results. I t does not a lways happen tha t 25% virus infected tubers remain 25% infected in la ter generations. Sometimes the disease increases and sometimes it decreases.

A M E R I C A N P O T A T O J O U R N A L 15

I t is easy enough to explain why it increases, but it is not so appa ren t why the amoun t of virus infected tubers should diminish when the potatoes are grown in certain districts. In distr icts where there are no aphids or other insects tha t t rans- mi t virus diseases f rom p lan t to plant , the proport ion of dis- ease free tubers tends to increase f rom year to year, fo r the virus infected plants yield fewer tubers than the normal plant. This process of improvement is so slow and uncer ta in t h a t it is h a r d l y wor th mentioning, even if we were cer ta in t ha t dis- t r ic ts exist where virus diseases do not spread f rom p lan t to plant . The modern f a r m e r can rely only upon "certified seed potatoes." In this connection we conducted an experi- men t using small and large commercial seed, not certified. We found tha t the percentage of virus disease was much grea ter in the crop f rom the small seed than in the crop f rom the large seed. This confirmed our view tha t virus infected p lan ts are usua l ly un th r i f ty , producing a larger proport ion of small seed than the normal plant. When certified seed was used, there was no difference in the amoun t of disease in large compared with small seed.

Digging Date and Rhizoctonia Control of Potatoes

R. R. HURST, Laboratory of P l an t Pathology, Charlot tetown, P. E. I.

Rhizoctonia is one of the most troublesome diseases of pota- toes, and while its control is usually sought by t rea t ing the seed potatoes, nevertheless, additional importance is at tached to the dates upon which the crop is harvested. Invest igat ions conducted by the Division of Botany, have demons t ra ted the influence of digging dates upon the occurrence of Lhe rhizoc- tonia sclerotia which indicates the presence of disease on potato tubers. These sclerotia are s imply the winter or rest ing stage of the rhizoctonia fungus and their development depends, to a considerable extent, upon the conditions prevai l ing at digging time, for it is general ly agreed t h a t moisture and low tempera- tures s t imula te the fungus th reads to fo rm up into the char- acterist ic knots or sclerotia. I t is apparent , therefore, tha t any th ing which will divert these na tura l influences, will, like-