RACE DIFFERENTIATION IN CEREAL RUSTS 



inoculated with good rust material but obtained no success. We found 

 this especially striking during November and December 1968 and January 

 1969 (Table 9)'. 



The number of cultures of various origins used for inoculum on a 

 given day are shown in Table 9. The appearance of the mature inoculum 

 obtained from each culture for the next inoculation is shown to the 

 right. If all samples reappeared in the same quality as in the original 

 inoculum the number of cultures used would appear on the diagonal. In 

 the case of the December 18 inoculation date, most of the material was 

 shifted to the right upper corner, indicating a bad day for rust infec- 

 tion. In the case of December 25, the material was shifted to the lower 

 left corner, indicating a good day for rust infection. Similar observa- 

 tions in the same time were made in Wageningen (Stubbs, Personal com- 

 municatiori) , which may be a sign for weather factors over Europe. 



ADDITIONAL CHARACTERISTICS IN RACE DETERMINATION 



In the search for additional characteristics in race determination, 

 I attempted to utilize the variation in latent period 1 of the different 

 rust cultures. Certain cultures are always quick to sporulate (short 

 latent period) and some are slow to sporulate (long latent period) . 

 Cultures made up of individual races as defined by differentials and 

 samples of given races obtained from different geographic areas were used 

 as inoculum on Michigan Amber wheat. The inoculations were carried out 

 in the greenhouse with the same cultures and wheat variety on different 

 days. The results are shown in Table 10. Out of 95 inoculations on 

 November 10, 10 were mature enough to be used for new inoculations after 

 14 days, 70 after 16 days, 15 only after 18 days, i.e., the first 10 

 were "quick", the last 15 were "slow" and so on, always in relation to 

 the number of inoculations per day. The differences between 35 and 50 

 (November 12), 40 and 38 (November 17) and 42 and 52 (November 21) were 

 not great enough to allow a "quick" or "slow" classification. 



When these "periodicity" observations were summarized in relation 

 to the number of transfers per culture (Table H) they show additional 

 race-differentiation to that we got out of the differential varieties 

 tests. 



With certain Japanese cultures we found an interesting relationship 

 (Fuchs, 1965). All the cultures belong to race 42A as defined by viru- 

 lence to Lee, but they reacted differently on the Reichersberg 42 (Lee 

 supporting differential) . The reaction on Reichersberg 42 was correlated 

 to "quick" and "slow" latent periods. This observation may have been an 

 expression of qualitative differences, and was confirmed with the second 

 class differential Carsten V and some vigorous middle European races. 



Other methods of race identification without differential varieties 

 have been tried by Straib (1939) in using differences of spore germination 

 and germ tube growth for stripe rust as well as for Cronartium ribicola 

 J.C. Fisch. ex Rabenh. (Straib, 1953). 



- Zi'ne from urediospore inoculation to eruption of pustules for 

 release of new spores (van der Plank, 1963) . 



