Of 16 barley cultivars the partial resistance to leaf rust, Puccinia hordei, was measured in field plots isolated from each other by at least 30m of winter wheat. The partial resistance caused by a lower infection frequency, a longer latent period, a lower sporulation rate or a shorter sporulation period, or a combination of these, varied greatly among the cultivars, of which 9 were tested in 1973 and all 16 in 1974. The correlation with the assessments obtained from small, adjacent plots, where the epidemic is determined by the spreader rows and the susceptible cultivars, was very high (r = 0.95 to 0.96), The correlation with latent period in the seedling stage was moderate (r = 0.57) with latent period in the young flag leaves very high (r = 0.92). The assessments obtained from the small, adjacent plots, although evaluating the partial resistance of the cultivam in the right order did under estimate the partial resistance very strongly. As cultivar testing for variety-lists is generally done in this way, though the plots may be larger, the values for partial resistance entered into such variety-lists may often under estimate the resistance seriously.

Two spring barley composites, one based on eight West-European two-rowed cultivars (A) and the other, the predominantly six-rowed composite XXI, based on several thousands of barley cultivars (B), formed the starting point of a recurrent selection procedure . The aim was to study how effective a repeated process of mild selection against susceptibility followed by recombination of the remaining material was in accumulating partial resistance in four host-pathogen situations .i) Only partial resistance is present and the pathogen population is defined (a given race) .ii) Partial and major gene resistance both occur and the pathogen population is defined . iii) Only partial resistance is present and the pathogen population is not defined ; a mixture of races that varies over the years . iv) Partial and major gene resistance are both present and the pathogen population is not defined .The variation in partial resistance to barley leaf rust was large in both populations . Population A carried no effective major resistance genes, population B possibly a few at low frequency . The variation in partial resistance to powdery mildew was moderate in A and possibly large in B . As far as effective major resistance genes is concerned A did not carry any, although some recombinations of defeated genes might have been partially effective, while B seemed to carry many .During the selection procedure the populations were always exposed to race 1-2-1 of barley leaf rust and to the mixture of powdery mildew races that was naturally present .The selection procedure consisted of three cycles of recurrent selection . In the initial heterogeneous populations (SO) single plant selection was applied followed by line selection the next year . In both populations 12 lines were selected that were intercrossed in all directions within the two populations . Again single plant (S3) and line selection were exercised and 12 lines selected . The 12 A-lines were intercrossed in all directions with the 12 B-lines and the recombined population again exposed to single plant (S6) and line selection (S7) . The selection in this population was done within two-rowed entries (A*) and within six-rowed entries (B*) .The selection pressure was mild . In each selection stage about 30% plants or lines most affected by barley leaf rust and some 30% plants or lines most affected by powdery mildew were removed . Among the remaining plants or lines (ca . 45%) a selection for other useful agronomic characteristics was applied .The response to selection was measured in four evaluation trials . Both single plant and line selection contributed to the progress in resistance in both populations to both pathogens . 262The gain in partial resistance to barley leaf rust was the same in the two populations . The average amount of sporulating leaf tissue in the S7 was about twenty times less than that in the SO . The best S7 lines showed a sixty fold decrease compared with the SO . Corrected for the levelling effect of interplot interference, very strong with barley leaf ru...

Hordeum vulgare, barley, Puccinia hordei, leaf rust, interplot interference, partial resistance, wind borne leaf pathogens . SUMMARY The barley cultivars Akka, highly susceptible, and `Vada','partially resistant to barley leaf rust, Puccinia hordei, were evaluated for the amount of leaf rust in five experimental field plot situations over three successive years . The field plot situations were : A) plots well isolated from each other by distance and non-leaf rust contributing host plants ; B) adjacent plots of 4 x 4i m (18 rows); C) adjacent plots of 4 x lZ m (6 rows) ; D) adjacent plots of 4 x a m (1 row) ; E) adjacent plots of only one plant (cultivar mixtures) .The sporulating leaf area of each plot was measured from samples of 20 tillers taken at random from each plot . In each year the difference in sporulating area between 'Akka' and 'Vada' was large to very large in the absence of interplot interference in the isolated plots, ranging from 150 to 2100 times . In the adjacent plots the partial resistance of 'Vada' was greatly underestimated, 5 to 16 times in the situation B, 14 to 30 times in C, and 75 to 130 times in D and E .Testing lines or cultivars in adjacent plots is the standard procedure in use in breeding programs and in tests of cultivars for their agricultural value . To avoid such under estimation the following procedure is suggested . A few cultivars representing the known range of partial resistance and whose level of partial resistance is well known are evaluated together with the lines and cultivars whose partial resistance has to be assessed. This is demonstrated with a number of cultivars of which resistance values are known from the recommended variety lists for England and Wales . Cultivars have been assessed in Wageningen over four years together with the check cultivars Akka, Sultan, Julia and Vada representing the range of partial resistance with values (on a I to 10 scale) of 1, 3-4, 7 and 8 respectively, based on isolated plots experiments .

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