40 BULLETIN 95*7, U. S. DEPARTMENT OF AGRICULTURE. 
Doran (28), in 1919, found that the seciospores germinate well in 
clistilled-water drop cultures for four weeks, when germination 
weakens. Germination ceased after six weeks. He does not give 
details of the conditions of storage of the spores. 
Pennington 22 compared the number of seciospores caught in spore 
traps with the number of infections on Ribes leaves and estimated 
that under the most favorable conditions 1 spore in every 25 which 
lodged upon a Ribes leaf produced infection there. An estimate 
for the season showed that not more than 1 in 100 produced infection. 
The Cronartium Stage on Ribes. 
THE INCUBATION PERIOD ON RIBES. 
The length of the incubation period between infection of Ribes 
by seciospores and urediniospores of Cronartium ribicola and the 
production of mature uredinia or telia varies greatly, according to 
the external conditions of temperature and moisture and the age 
and condition of the leaves infected. Examination of the records 
of 493 inoculation tests made in the greenhouse show that the shortest 
incubation period between infection and formation of mature uredinia 
is practically 7 days. These records show that 2.4 per cent of the 
cases fruited in 7 days, a like number in 8 days, 7.3 per cent in 9 
days, 10.4 per cent in 10 days, 20.8 per cent in 11 days, and 19.7 
per cent in 12 days. The percentage rapidly drops after this to 9.7 
per cent in 13 days, 8.1 per cent in 14 days, 8.5 per cent in 15 days, 
8.1 per cent in 16 days, and 3.2 per cent in 17 days. 
Pennington 22 found that the incubation period on Ribes in the 
eastern Adirondack region with both aeciospores and urediniospores 
was 11 to 18 days; it was usually 13 to 15 in mature leaves and some- 
what longer in leaves which were very young when inoculated. 
There are times when the fungus produces only uredinia in the 
greenhouse as well as in the fields. The behavior of the fungus is 
not entirely controlled by weather conditions, but is greatly influ- 
enced by the condition of the host leaves. At other times the 
fungus will produce nothing but telia. At such times telia are pro- 
duced in a very short time after infection. Telia have been obtained 
in 9 or 10 days after infection. From 12 days upward they may be 
formed at almost any time up to 2 or 3 months after infection. 
York 23 in many cases has obtained telia directly from seciospore as 
well as urediniospore inoculations upon overmature 24 leaves. 
22 Pennington, L. H. Op. cit. 
23 York, H. H. Op. cit. 
2-4 The term "overmature"' is here used, to denote that stage of development of Ribes leaves where they 
have become tough, leathery in texture, and of maximum thickness, but have not begun to decline in 
photosynthetic activity. 
