2 18 
Journal of Agricultural Research 
Vol. XXV, No. S 
plants have grown to considerable size before inoculation is possible, in¬ 
fection is unlikely to occur, This being known, methods can be easily 
worked out for the insuring of propagation of rust-free nursery stock. 
TIP PLANTS INFECTED IN NATURE 
If it is possible to infect “tip plants” of Rubus occidentals experi¬ 
mentally with the systemic stage of the rust, what is the evidence that 
this is the method by which the disease is spread in nature? If one 
examines in early spring wild black raspberries in a region where infected 
plants are known to have existed in the past, he will find rusted tip plants 
arising from stolons which are connected with rust-free parents. Sections 
of such old stolons will show no hyphae. This originally suggested to the 
writer the probable method of primary infection. 
OLD PLANTS SUBJECT TO INFECTION 
There remains a question as to whether the rust is able to gain entrance 
into an old plant. The writer’s experiments along this line have not been 
as satisfactorily checked as is desirable. Eight plants were obtained 
from a region where rust was known to be present. They showed no rust 
in 1921 after they were planted in the greenhouse. Since they were very 
small and had new shoots starting out from the old crown at the time 
when the first teleutospores were available, they were placed under favor¬ 
able conditions for infection. Having been overwintered in the cold 
frame they were again brought to the greenhouse, where shoots bearing 
pycnia made their appearance on three of the plants within a few days. 
Apparently the inoculations had been fairly successful, but one can not 
be certain that the plants were not infected at the time they were trans¬ 
planted. 
In nature the new canes spring up from the base of old plants early in 
the season, so the gametophy tic stage of the long-cycled rust naturally at¬ 
tacks the tip plants, which are in the most susceptible condition from the 
latter part of July to September, and not the basal shoots from old canes 
which arise in the spring. It should not be forgotten that teleutospores 
sometimes mature on leaves of the old canes a very short time after 
aecidiospores are shed, no doubt early enough to lead to infection of the 
more tardily formed basal shoots. It is not an uncommon practice among 
growers to prune out all the old canes and some of the new ones after the 
crop is harvested. New shoots might, as a result, grow up from the base 
of the old plant following this treatment, so that the conditions would be 
favorable for infection by sporidia from telia which would be mature at 
this time. There is still another possibility. The teleutospores of Gym- 
noconia are known to be capable of germination as soon as mature. 
The writer’s experiments prove conclusively that some of these spores 
germinate in August and September if placed under suitable conditions. 
On the other hand no one has proved that the teleutospores may not also 
live over winter and be in condition for germination as the first new buds 
or shoots break through in early spring. 
It has been pointed out that rusted canes do not ordinarily become 
stolons and set tip plants, so the rust is not spread in this way to any 
great extent. New shoots arise from the base of the old plants in the 
spring, while the teleutospores mature and germinate from July to Sep¬ 
tember, the same time that the canes root at the tips. It must be con¬ 
cluded, therefore, that in nature the perennial stage of the long-cycled 
