July 14, 1923 
Degeneration Diseases of Irish Potatoes 
45 
leaf veins (Pl. 5, B) but sometimes also apparent on the upper leaf 
surface, stems, and petioles; or, it may appear as marginal necrosis or 
burning, beginning at the margin and progressing inward. Streaking 
may spread out into the parenchyma as spotting or burning. Leaf 
dropping concerns entire leaves, usually the lowest first, and then pro¬ 
gressively higher ones. It may begin with a collapse of the green petioles, 
followed by wilting and finally by necrosis of the petioles and blades. 
Premature death usually is preceded by leaf dropping and necrosis and 
finally involves the whole stem and shoot. 
The preceding is concerned with isolated unit symptoms. Certain 
frequently occurring groupings of these unit symptoms have been con¬ 
sidered as indicating the presence of certain viruses and have been desig¬ 
nated by various descriptive terms such as mosaic (32, 45), mosaic 
dwarf (21; 32 , fig . 29), curly dwarf (32, 38 ), streak (5, p. 50; 33), 
leaf roll ( 32 , 41) y and others. These symptom-complexes will be con¬ 
sidered later in this paper, except that it may be stated here that a de¬ 
generation disease is considered to be of the mosaic type if there is 
mottling and abnormal unevenness of the leaf surface. 
The writers have performed inoculation, or the introduction of material 
(inoculum) from’a diseased plant into a healthy one, by means of several 
methods. Tuber grafts have been made by keeping in contact the cut 
surfaces of split tubers. Stalk grafts have been made by splitting the 
healthy stalk and inserting the wedge-shaped base of a diseased scion 
and fastening with cord and water-impermeable tape. Leaf-mutilation 
inoculation consists of bruising the healthy leaves with the fingers or palms 
and applying to the spongy mass of mutilated leaf tissue juice that has 
been expressed from a diseased plant. This may be repeated at intervals 
of several days. Capillary-tube inoculation consists of inserting small 
glass tubes, containing the inoculum, into the stalk. Aphids have been 
transferred from the diseased to the healthy plant growing in separate 
cages or have been both transferred and allowed to disperse with the 
diseased and healthy plants in the same cage. In the latter case there was 
root contact, which has also been tried with aphids absent. Natural 
inoculation is effected by aphids. 
A control to an inoculated plant is reliable only when in the same 
tuber unit; that is, in the group of plants or hills growing from the 
separated parts (seed pieces or sets) of the same tuber. The members of 
a tuber unit are sometimes separated and planted in several places or 
cages. Current-season symptoms are those appearing in the same season 
in which inoculation occurs, in contrast to those following both inocula¬ 
tion and tuber perpetuation of the disease. These two classes of symp¬ 
toms, differing only as to the immediate origin of infection, have been 
designated respectively as primary and secondary by Quanjer (38, p. 36; 
39 , p. 130 ), and respectively as secondary and primary by Edgerton ( 12 , 
p . 7) following the nomenclature of sugar-cane mosaic. The incubation 
period is the length of time between inoculation and the appearance of 
current-season symptoms. If current-season symptoms are absent or if 
they appear in plants under cages or in other abnormal conditions, it is 
desirable to observe the second generation following tuber perpetuation. 
If they appear in normal conditions in a high percentage of the inoculated 
plants and not in the controls, uncontrolled natural transmission in the 
field, presumably by insects, will sometimes cause late-season infection 
of the controls so that observation after tuber perpetuation is of less value 
than that made before. The second-generation progeny of a hill will be 
