July 14, 1923 
Degeneration Diseases of Irish Potatoes 
113 
SUMMARY 
(1) Degeneration diseases of potatoes, in the absence of known identi¬ 
fying causes, are symptom complexes whose elementary unit symptoms 
can and should be determined in the same standard variety or varieties 
and in the same environment. 
(2) Research with these diseases is developing a somewhat distinct 
technic and terminology. 
(3) In the Green Mountain variety, several degeneration diseases 
have been distinguished and transmitted—namely, mild mosaic, leaf¬ 
rolling mosaic, rugose mosaic, streak, leaf roll, spindling-tuber disease, 
and unmottled curly dwarf. 
(4) In Green Mountains, mild mosaic was not transmitted by contact 
except in stem and tuber grafts. 
(5) A leaf-mutilation method of inoculation has certain advantages 
over other methods. In Green Mountains this method transmitted mild 
mosaic, with the effectiveness increased by insect-cage or greenhouse 
conditions as compared to open-field conditions, by inclosure within 
a damp chamber, and by repetition, 
(6) In Green Mountains aphids (Macrosiphum solanifolii Ashmead) 
transmitted mild mosaic, both alone and in combination with leaf roll 
and with spindling tuber, while negative results were secured with flea 
beetles (Epitrix cucumeris Harris) and Colorado Potato beetles (Leptino- 
tarsa decemlineata Say.). 
(7) In Green Mountains, aphids from plants with a *‘curly-dwarf” 
combination apparently consisting of leaf-rolling mosaic and spindling 
tuber together, transmitted the curly-dwarf combination to part of a 
hill and spindling tuber alone to the other part, distinction being made 
between different tuber units of the second generation. 
(8) Leaf-mutilation inoculation transmitted both rugose mosaic and 
streak readily in Green Mountains. 
(9) Leaf roll was transmitted neither by contact except in grafts, nor 
by leaf-mutilation inoculation. 
(10) Spindling-tuber disease is characterized, and proofs given of its 
being a degeneration disease, spreading in the field, being perpetuated 
by the tubers (shown in part by mechanical measurements), and being 
transmitted by tuber grafts, stem grafts, leaf mutilation, and aphids. 
(11) Unmottled curly dwarf was transmitted by leaf-mutilation inocu¬ 
lation and by aphids. 
(12) Combinations of symptoms exist that include more than one 
degeneration disease in the same plant. Aphids sometimes transmit 
only one disease from such a plant, but more often transmit the 
combination. 
(13) In Irish Cobblers, leaf roll was transmitted by grafting and by 
aphids but not by leaf-mutilation inoculation, which is successful with 
all other degeneration diseases tested. 
(14) In New White Hebrons in 1921, leaf-roll and net-necrosis per¬ 
centages increased with the average weight of the tubers. 
(15) Leaf-mutilation inoculation of Green Mountains in 1920 effected 
intervarietal transmission of rugose mosaic, a combination of leaf-rolling 
mosaic and spindling tuber, and unmottled curly dwarf, but was in¬ 
effective with mild mosaic and leaf roll. Comparison inoculations with 
aphids with four of the sources of inoculum gave similar results, trans¬ 
mitting rugose mosaic to Green Mountains from curly-dwarf plants in 
the Rural group. 
