Apr. i, 1921 
Lea}roll , Net-Necrosis, and Spindling-Sprout 
75 
varieties respecting either disease. The Irish Cobbler is the only one 
which in commercial fields often shows more than a trace of leafroll in 
northeastern Maine, and it seldom shows more than a trace of mosaic, 
while the Green Mountain and Bliss Triumph varieties are seldom found 
with only a trace of mosaic. 
RELATION OF LEAFROLL TO CLIMATE 
The Bliss Triumph variety contracts leafroll readily on Long Island 
and in Bermuda, in contrast to its behavior in northeastern Maine (24). 
This and similar differences between other geographical regions in regard 
to the rapidity of the spread of leafroll have been attributed to climatic 
differences. The question also arises whether or not greater prevalence 
of sources of infection, the importance of which is suggested by Murphy 
and Wortley (14), may be the chief or only cause of more rapid increase 
of leafroll in some regions within any given variety. As far as is now 
known, the greater prevalence of sources of infection on Long Island or 
Bermuda may now be the more important factor in the greater spread of 
leafroll there, with greater abundance of aphids helping both now and in 
the past to increase sources of infection. Climate, in turn, may differ 
enough to influence the abundance of aphids. In Virginia they are 
abundant in the winter ( 21 ) when entirely absent in Maine, except in the 
egg stage; and it seems probable that in sections intermediate as to cli¬ 
mate their season begins earlier than in Maine and that they have greater 
opportunity to become numerous upon potatoes and to spread leafroll. 
Whether or not the relative abundance of aphids is an important factor 
can be tested best by experiments in which parts of the same stock are 
grown at two or more places with surrounding conditions similar in 
regard to leafroll. The prevalence-of-source explanation is not necessa¬ 
rily supported by the less rapid spread of leafroll in Maine, as compared 
with that of mosaic, from diseased plants to those near by. It is possible 
that more severe inoculation by aphids is required to transmit leafroll 
than to transmit mosaic, and that these insects are abundant enough on 
Long Island and Bermuda to spread both diseases readily, but usually 
not in Maine. Inoculation of bruised leaves with virulent juice has 
proved far easier with mosaic than with leafroll. Aphids when intro¬ 
duced artificially have transmitted leafroll within the Green Mountain 
variety about as readily as they have mosaic (p. 59 and Table III). On 
the other hand, natural field inoculation by aphids in northeastern Maine 
evidently would be greater, even for mosaic, if there were more aphids. 
Hill selections made for freedom from mosaic in partly diseased fields in 
the valley of the St. John River in Maine ordinarily have given more 
satisfactory results than those made in the same manner in fields near 
Presque Isle, in correlation with less prevalence of aphids as indicated by 
counts made on leaves taken at random at the same time of year. Also, 
the number of aphids in Maine ordinarily is small enough for interseasonal 
