62 
Journal of Agricultural Research 
Vol. XXV, No. 2 
evidence of such combinations has been obtained, consisting of the 
results of inoculations in which one or more of the combined diseases 
are transmitted separately to new hosts of the same variety. Such 
evidence easily discloses a more infectious disease in the combination 
but a less infectious disease will seldom be separated analytically from 
others not more infectious. Synthetic evidence, obtained by inoculat¬ 
ing in such a way that new diseases are introduced into diseased plants 
or so that diseases are combined in healthy plants, is difficult to secure 
except where transmitting insects are controlled, especially when dis¬ 
eases are involved with a long incubation period and a consequent infre¬ 
quency of current-season symptoms. 
There are many theoretically possible combinations of the several dis¬ 
eases that have been discussed. The number of actual combinations 
observed by the writers has been reduced from the theoretical number 
by several factors. In northeastern Maine, leaf roll is rare in native 
stocks and often decreases or disappears in infected stocks. Many com¬ 
binations quickly eliminate themselves by their excessive reduction of 
the yield and by causing premature death and thus reducing the chances 
for insect transmission. Varietal susceptibility also seems to be of influ¬ 
ence. Furthermore, without synthetic evidence it may be assumed that 
the results of some combinations may be a masking of some of the symp¬ 
toms of the separate diseases entering the combination. It is possible 
for the writers at present to show only the general significance of the 
principle that combinations of degeneration diseases may exist. 
The writers, even with the limitations just previously set forth, have 
seen symptom complexes that evidently were combinations of diseases. 
Double combinations are more easily diagnosed than triple combina¬ 
tions. Several combinations are given in Table IX as having been 
observed by the writers. Combinations are given by Murphy (29, p. 77), 
Ouanjer (39, p. 128 ), and Gilbert (17). 
Table IX.— Combinations of degeneration diseases thought to have been observed in 
Green Mountain potatoes 
Name of combination as 
formerly described. 
(In 
Medium mosaic. 
part-). 
Mosaic dwarf; bad 
mosaic. (In part.) 
Mottled curly dwarf. 
Mosaic dwarf; bad 
mosaic. (In part.) 
Medium plus mosaic. 
(In part.). 
Combined diseases. 
Mild 
mosaic. 
X 
X 
X 
X 
Leaf- 
rolling 
mosaic. 
Rugose 
mosaic. 
Streak. 
Leaf 
roll. 
Spindling 
tuber. 
Unmot¬ 
tled curly 
dwarf. 
a X 
a X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
X 
! 
X 
X 
»x 
X 
X 
X 
i 
o Probably first-season 5} mptcms of rugose mosaic. 
