November, 1921 



SCIENTIFIC AGRICULTURE 



93 



Diseases of the Potato 



P>y B. T. DICKSON, Professor of Botany, 



Macdonald College. 



(Continued) 



(b) Mosaic and Mosaic Dwarf. 



W. A. Orton first described Mosaic as a 

 disease of potatoes from observations he 

 made in potato fields at Giessen wliile vis- 

 iting in (lermany, although Quanjer be- 

 lieves that the diesase lias been kuov/ji for 

 a long time in Europe. Orton on his re- 

 turn found the disease to be quite prevalent 

 in Maine in the Green Mountain variety. 

 This was in 1912 and since that time the 

 disease has been found in practically every 

 potato-growing area of t,he United States 

 and Canada. 



The cause of the disease is not yet deter- 

 mined, although it is certainly a systemic 

 disease, and it has been placed here in 

 Group 1 because of the established fact 

 that aphids are agents of inoculation. 



While the disease may be regarded as 

 new, as compared with Late Blight for 

 instance, it is hig^ily infectious and 'has 

 spread with alarming rapidity. In 1919 

 careful estimates made in Aroostook Coun- 

 ty, Maine, of 40 fields of Green Mountain 

 and the same number of Bliss Triumph 

 showed an average of 28 per cent of in- 

 fected hills of Green Mountain and 46 

 per cent in the case of Bliss Triumph. 

 In some cases the diseased plants amounted 

 to 100 per cent. It is therefore of para- 

 mount importance that efforts be contin- 

 ually made to check the spread of this 

 insidious disease. The difficulty involved 

 lies in the fact' that t^ie potato tuber, a 

 vegetative part of the plant, is used for 

 seed purposes and the causal principle may 

 be present in the tubers as a result of late 

 infection without there being distinct signs 

 of the disease in the foliage. 



Symptoms of the Disease. 



The disease may manifest itself in either 

 of two ways according to the variety or 

 according to the locality. The typical 

 symptom from w.hic^h the name arose is 

 the mottling of the foliage. Lighter green 

 areas occur in the leaves and these lighter 

 green areas may be few or numerous, they 



may be very small or reach t^ie size of a 

 quarter of an inch, and in shape are usually 

 angular. The leaflets may also be more 

 or less ruffled or wrinkled owing to the 

 modified growth, and where tliis is the 

 case t,iie wrinkles are likely to obscure tiie 

 mottling at first sight. I'he above symp- 

 toms are typical for Green Mountain po- 

 tatoes. It migfht be added for those readei-s 

 who have at their disposal a microscope 

 ihat a thin freehand section through a 

 light green area bordered with the dark 

 green will show, even under the low power, 

 t;iat in the light part, the palisade tissue 

 has not been able to develop normally. The 

 cells instead of being from four to six 

 times as long as wide are approximately 

 cubical or distinctly shorter. 



The Irish Cobbler, on the other hand, 

 exhibits different characteristics. Here 



Plate 3. — Plant A is healthy. Plant B is suf- 

 fei-ing with mosaic. Note at 1 the scars left by 

 fallen leaves and at 2 a leaf which has just 

 dropped. 



mottling is not usual but instead the leaves 

 of affected plants are extremely wrinkled 

 and dwarfed. The leaflets are smaller, 

 the petioles are reduced and even the 

 haulms are dwarfed. This gives rise to 

 a type known as Curly Dwarf in extreme 

 cases. The Rural varieties also shoAV this 

 group of symptoms. 



