180 Maine Agricultural Experiment Station. 1920. 



eastern Maine. Also, in the latter region hot weather some- 

 times has been observed as apparently causing mosaic plants to 

 lose their mottling, which again was plain after the return of 

 cool weather. 



The effect of differences in locality upon the symptoms 

 shown by plants already diseased of course has no real effect 

 upon the spread of mosaic. Undoubtedly one condition alone — ■ 

 the abundance of aphids, if no other — may vary enough in dif- 

 ferent regions or localities to affect the spread of the disease. 

 Such problems are yet to be worked out. 



VARIETAL RESISTANCE AND IMMUNITY. 



The Irish Cobbler variety is practically free from mosaic 

 in northern Maine, in marked contrast to the Green Mountains 

 and Bliss Triumphs. Other varieties resemble the Cobblers. 

 The- extent and possible causes of immunity and resistance 

 among the varieties are being determined. 



FERTILIZER VARIATION. 



Variations in the constitution of commercial fertilizers and 

 the addition of special substances to the soil have not had any 

 great or important effect upon mosaic already acquired. How 

 much they might influence the resistance of susceptible varieties 

 to mosaic infection, is being studied. 



SPRAY METHODS. 



Direct effect of any kind of spray upon mosaic has not been 

 evident and could hardly be expected in view of the difference 

 between the location of the spray on the outside of the leaves 

 and that of the infectious substance in the juice. However, cer- 

 tain spraying methods may help to affect the mosaic problem 

 through the control of plant lice. 



Methods of Control. 



hill selection. 



For several years many groups of healthy hills, numbering 

 many hundreds of plants altogether, have been selected in the 



