1921.] 



Mosaic Disease of Potatoes. 



337 



this takes place there is a marked reduction in the yield. In 

 extreme cases the growth of the plants may be completely 

 stunted, and when this stage has been reached the crop is 

 reduced to practically nil. 



An important point to be noted in recognising the disease is 

 that, though mottled foliage is one of the principal diagnostic 

 characters and is usually very marked and conspicuous, this 

 is not invariably the case. The mottling appears to be modi- 

 fied materially by climatic conditions. In cooler and damper 

 regions, typically mottled foliage may be found throughout the 

 entire season, but in the hotter and drier parts of the country 

 this feature, though quite apparent earlier in the season, may 

 become much less marked later and may even disappear 

 entirely. The crinkling of the foliage, however, remains.* 

 This is comparable with observations made in the drier States 

 of America, where, though the yield is very much reduced, the 

 mottling symptoms are completely suppressed. It has been 

 proved experimentally that this is a climatic effect. Plants of 

 the same stock of seed were planted in INIaine and Colorado. 

 Mottling occurred in Maine but none developed in Colorado. 

 That the stock did not lose the disease but was still infected was 

 ehown bv the fact that when the southern-grown crop was 

 returned to its northern station, the mottling reappeared. It 

 would also seem from certain experiments that, though 

 typical mottling is more conspicuous in the north, the effect 

 of the disease may be more serious in the hotter parts of the 

 country. 



Intensity of the Disease and Effect on Yield. — As will be 



gathered from the above description of symptoms, the intensity 

 of the attack varies greatly. In general the effects of the 

 Mosaic disease are more severely felt in the drier and warmer 

 parts of the country, and, as indicated in the preceding para- 

 graph, climatic conditions appear to be the main (though 

 perhaps not the only) factors governing the degree of 

 intensity exhibited. Owing to Mosaic disease having only 

 recently been recognised in England, few precise records exist 

 as to its effect on the yield. It is probable that in most parts 

 of Scotland and in the cooler and damper parts of England 

 and Wales the diminished yield due to the disease is relatively 

 slight. In the warmer and drier parts of England, however, 



"^The mottling symptoms are more clearly seen on a dull day or when a 

 shade is thrown over the plant. A white sheet of pajior held under tlio h^af 

 a\Ho assists in throwing tiie mottling into relief. 



D 



