Mar. 15, 1925 
Degeneration Diseases of Potatoes 
525 
Virginia lots ranged from 32 to 100 
per cent, the Long Island lots from 8 to 
76 per cent, the northeastern Maine 
lots from 6 to 96 per cent, and the 
southwestern Maine lots from 0 to 34 
per cent, averaging, respectively, 79, 
48, 42, and 15 per cent. Evidently 
spindle tuber increased more in Virginia 
than elsewhere, and less in south¬ 
western Maine. A comparison of 
varieties as to susceptibility can not be 
made because the Green Mountains 
were the only one of the “healthy” 
stocks to be originally free from 
spindle tuber. 
The total disease percentages are 
higher for Virginia in every variety 
whether from between diseased Green 
Mountain rows or from between dis¬ 
eased Irish Cobbler rows. For all 
varieties the total for Virginia is about 
double that of northeastern Maine, 
about triple that of southwestern 
Maine, and about half again greater 
than for Long Island. The total for 
Virginia probably should be larger 
because of mild mosaic being masked by 
rugose mosaic. It may be pointed out 
here that the test in 1922 in north¬ 
eastern Maine was carried out in a field 
containing many plots with high per¬ 
centages of disease of various types; in 
southwestern Maine in an isolated plot 
with the climatic conditions unusually 
unfavorable for growth; and on Long 
Island and in Virginia in plots sur¬ 
rounded by commercial varieties. 
A few definite conclusions can be 
drawn from the preceding comparisons. 
Proximity to mild mosaic rows in 1922 
helped to increase the amount of mild 
mosaic contracted by Green Mountains, 
more in Virginia and on Long Island 
than in Maine. Rugose mosaic was 
contracted markedly in Virginia but 
not elsewhere. While the “healthy” 
stocks, exclusive of the Green Moun¬ 
tains, were originally more or less 
infected with the milder types of 
mosaic and with spindle tuber, they 
were free from leaf-roll. The spread of 
this disease was greater farther south, 
and in Virginia and on Long Island was 
not restricted to spread from leaf-roll 
rows to adjoining rows. Spindle tuber 
increased more in Virginia than else¬ 
where. The total disease percentage 
increased most in Virginia and least in 
Maine. 
One practical application of these 
conclusions is that the frequent state¬ 
ment to the effect that a hot climate 
causes rapid degeneration of potatoes, 
need not assume a direct effect, since an 
indirect effect by increasing the spread 
of degeneration diseases is now known 
to exist. It is also now more explicable 
why commercial stocks in northeastern 
Maine that are found to contain con¬ 
siderable leaf-roll usually can be traced 
back to a more southern source. It is 
plainly probable that the mosaic 
familiar to southern buyers of potato 
seed is not of the same type as that most 
often found in Maine. Emphasis is here 
given to the claim previously made 22 
that the degeneration problem “be¬ 
cause of its complexity may vary 
greatly from one locality to another” 
so that “control measures must be 
worked out for different sets of condi¬ 
tions, following research based initially 
on the general principles now fairly 
well understood.” 
A satisfactory explanation of these 
results would require more detailed 
study than has been made of each 
region relative to the presence of the 
various degeneration diseases in weeds 
and in neighboring commercial stocks, 
to the kinds, numbers, and develop¬ 
ment of infestations of transmitting 
insects, and to the effects of the dis¬ 
eases upon the development and 
maturing of the different varieties. 
Improvement should be made upon the 
methods used in making this test, 
especially in regard to original freedom 
from disease except for the one or two 
desired in any diseased stock. The 
requisite control measures to accom¬ 
plish this are not yet fully understood. 
SUMMARY 
(1) Previously reported results with 
at least seven distinct degeneration dis¬ 
eases of potatoes suggest several new 
problems. A review of the symptoms 
includes several new ones, namely, 
spindling sprouts as an occasional 
symptom of leaf-roll even when unac¬ 
companied by net necrosis, streaks 
and spots on corollas as symptoms of 
streak, and tuber cracking as a current- 
season symptom of unmottled curly 
dwarf inoculation. A fourth type of 
mosaic, “crinkle mosaic,” is also dis¬ 
tinguished tentatively; this is unrelated 
to “crinkle.” 
(2) Leaf-mutilation inoculations 
made in the field within the Green 
Mountain variety caused infection 
with mild mosaic, leaf-rolling mosaic, 
rugose mosaic, spindle tuber, un¬ 
mottled curly dwarf, and streak, and 
also with various combinations of these 
diseases. This was followed by their 
natural spread from inoculated hills to 
adjacent hills, especially by mild 
mosaic. 
2 2 Schultz, E. S., and Folsom, D. Op. cit., p,112. 
19975—25t-3 
