40 BULLETIN 957, U. S. DEPARTMENT OE AGRICULTURE. 



Doran (28), in 1919, found that the peciospores germinate well in 

 distilled-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 ^^ 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 ceiit 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 ^^ found that the incubation period on Ribes in the 

 eastern Adirondack region with both seciospores 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 yoimg 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 ^^ in many cases has obtained telia directly from seciospore as 

 well as urediniospore inoculations upon overmature -* leaves. 



22 Pennington, L. H. Op. cit. 



23 York, H. H. Op. cit. 



24 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 begim to decUne in 

 photosynthetic activity. 



