Level of partial resistance to leaf rust,Puccinia hordei,in West-European barley and how to select for it

Summary The partial resistance to leaf rust (Puccinia hordei) of 40 West-European spring barley cultivars was measured in plots isolated from one another to reduce inter plot interference. The leaf area affected by leaf rust was also measured in small plots of 0.5 m² adjacent to each other, and on individual plants. The latent period was measured in the seedling stage and the adult plant stage, the infection frequency in the seedling stage only. The cultivars varied widely for partial resistance, many cultivars carrying a considerable level. Both the small adjacent plots and the single plants showed a marked inter plot interference strongly reducing the difference between cultivars. H wever, the ranking order of the cultivars was hardly, if at all, affected. Both latent period and the infection frequency showed large differences between cultivars, the latent period in the adult plant stage being highly correlated (r=0.82) with partial resistance, infection frequency in the seedling stage only rather weakly (r=–0.33).Selection for partial resistance appeared very effective in all stages tested; the seedling, the single adult plant, and the small plot stage. Selection in the small plot stage was the most effective followed by selection in the seedling stage. Selection for partial resistance therefore appears very well possible at all stages of the selection program.     

Variation for resistance to Puccinia hordei in Ethiopian barley landraces

Summary Up to 100 single plant derived lines of 18 Ethiopian barley landraces were evaluated for infection type in the seedling and adult plant stage, and for latent period in the adult plant stage only. A low infection type indicates the presence of race-specific resistance genes of the hypersensitive type, while the latent period is the major component of the polygenic, partial resistance.In the seedling stage 1721 of these single plant derived landrace lines were assessed for infection type against two barley leaf rust races. In the adult plant stage 1227 from these 1721 lines were evaluated for infection type against one race. In the seedling stage 2 (against race 1-2-1) and 29 against race A) out of the 1721 lines showed an infection type lower than 6–7 on the 0 to 9 scale. In the adult plant stage none of the 1227 lines had an infection type lower than 6–7 against race 1-2-1.The variation between and within the landraces for latent period in the adult plant stage was large. Some landraces such as landrace 212845 showed a highly significant and longer mean latent period than most other landraces. Virtually all plants in all landraces carry at least some partial resistance.The near-absence of race-specific, major, resistance genes and the high frequency of moderate levels of partial resistance indicates that the durability of leaf rust resistance in Ethiopian barley landraces is due to the latter type of resistance, and that the multiline principle does not operate.     

Partial resistance of barley to leaf rust,Puccinia hordei. IV. Effect of cultivar and development stage on infection frequency

Summary The cultivar effect on infection frequency (IF) was studied in the seedling and adult plant stages of 15 spring barley cultivars. In both stages the cultivar effects were highly significant. The cultivars L94 and Vada represented the extremes. Vada having about 2 1/2 times fewer uredosori than L94. Between cultivars and development stage clear interactions occurred. Pauline f.i. had the same low IF as Vada in the seedling stage, but in the adult plant stage its IF was about 70% higher. Also other effects could influence the cultivar effects. Increasing leaf age appears to increase IF. The cultivar effect also seemed to depend on the level of IF. At high levels the cultivars differed far less than at low levels of IF. The cultivar effect on IF appeared correlated with partial resistance in the field (r=0.7) through a high correlation with the cultivar effect on latent period (r=0.8).     

Identification of growth stage dependent expression of partial resistance of barley to powdery mildew

Summary Genotypic differences for growth stage dependent expression of partial resistance to barley powdery mildew have been identified on the basis of two components of resistance in the glasshouse and the pattern of epidemic development in the field.Differences for infection frequency and proportion of sporulating colonies were highly significant between the 16 genotypes investigated at four stages of plant development in the glasshouse. Both resistance components were significantly correlated mutually (r=0.73 to 0.86) and with the infection level on leaves developed at comparable growth stages in the field (r=0.52 to 0.73). The infection level of seedlings in the glasshouse was significantly correlated with the infection level of the leaves of the first until the third node in the field (r=0.70 to 0.73). Adult plant resistance was predominantly expressed at the uppermost leaf. A clear genotype × growth stage interaction was apparent for infection level; some genotypes showed partial resistance predominantly at the seedling stage and others predominantly at the adult plant stage.Abbreviations IF Infection Frequency, number of colonies per cm² leaf area, glasshouse experiment - IL Infection Level, number of colonies per cm² leaf area, field experiment - PSC Proportion Sporulating Colonies from total number of colonies, glasshouse experiment     

Accumulating polygenes for partial resistance in barley to barley leaf rust,Puccinia hordei. II. Field evaluation

Summary Eight lines from the cross between Vada and Cebada Capa with long to very long latent periods and four barley cultivars representing the known range of partial resistance to barley leaf rust, caused by Puccinia hordei, were evaluated in the field for partial resistance and in the greenhouse for the latent period (LP) in the young flag leaf.Each of the 12 entries was sown (15-4-1983) on a plot of 1.0 m². There were four replicates. To reduce interplot interference the plots were separated from each other by 4.0 m of spring rye. The number of urediosori per tiller was evaluated at 27-6, 4-7, 12-7 and ten days after heading. The LP was measured on 10 to 15 plants per entry in 1982 and on 10 plants in 1983.The levels of partial resistance varied greatly. The difference in number of sori per tiller between the most susceptible cultivar, Akka, and the most resistant cultivar, Vada, was about 50 times. Between Akka and the most resistant line this was approximately 5000 times. The LP's varied similarly. Vada had a LP 64% longer than that of Akka, the LP of line 26-6-11 was 15% longer. The range of partial resistance has been extended more than twofold.The correlation coefficients between LP and the level of barley leaf rust, expressed in transformed scale units, varied from -0.99 for the first sampling date to -0.97 for the third sampling date. Sampling the same development stage, ten days after heading, did not improve the r-value (r=–0.98). The LP evaluated in the young flag leaf is shown to be a very reliable criterion for partial resistance in the barley-Puccinia hordei pathosystem.Earliness tends to be associated with susceptibility. The correlation of days to heading with LP was 0.63, and with the level of barley leaf rust in the field 0.64.     

Influence of development stage and host genotype on three components of partial resistance to leaf rust in spring wheat

Summary Latency period (LP), infection frequency (IF) and urediosorus size (US) of leaf rust were determined on primary leaves and young flag leaves of 18 spring wheat cultivars. A large growth stage effect and a large cultivar effect on all three components were observed. Partial resistance as measured by the three components was generally better expressed in the adult plant stage than in the seedling stage. Associated variation of the components was observed: long LP, low IF and small US tended to go together. The association was not complete, cultivars with clear deviations of this association for one of the components were found suggesting the existence of at least partly different genetic factors controlling the respective components. LP measured on flag leaves gave the most reliable results and, therefore, could best be used as a selection criterion in breeding programs for partial resistance.     

Partial resistance in lettuce to downy mildew (Bremia lactucae). 2. Differential interactions between plant genotypes and fungus races and the relationship of latent period,infection frequency and number of infected leaves with the level of partial resistance

Summary In three experiments differential interactions between lettuce genotypes and Bremia lactucae races were investigated and also the influence of latent period, infection frequency and number of infected leaves on the level of resistance.In experiment 1 very significant differential interactions were observed mainly caused by two plant genotypes (PIVT 82 and PIVT 315) and race NL 7. In experiments 2 and 3 differential interactions were absent or of minor importance. Results suggest that partial resistance often was of a race-non-specific character. Latent period appeared to be a major component of resistance in the lettuce-Bremia lactucae relationship. It determines the onset of the next cycle of spore production and is also indicative of fungus growth in host tissue. Compared with infection frequency and number of infected leaves, latent period shows a small error variation and can efficiently be measured. The closeness of the relationship of the number of primary infections and the number of infected leaves with resistance level varied, depending on the experiment. The above three components appeared to be associated but the degree of the association varied. The variable behaviour of the three components suggests that they are governed by different genes.For the level as well as for the complexity and durability of the resistance, accumulation of as many components of resistance as possible is desirable. Therefore, in parents all such components should be analysed before crosses are carried out to improve resistance level. In the final products of crossing and selection the relative importance of components should also be investigated to determine whether an increased resistance level results from additional effects of the parents on one component (e.g. latent period) or from accumulation of different components in a plant genotype.Students at the Agricultural University Wageningen     

Effect of leaf age on components of partial resistance in rice to leaf blast

Summary Ten rice genotypes were inoculated with a virulent isolate of Magnaporthe grisea to study the effect of leaf age on components of partial resistance, and evaluate associations between these components. The number of sporulating lesions per cm² leaf declined with increase of leaf age in all genotypes. The number of lesions per cm² leaf area in one week old leaves was about 25% that of the number in very young leaves in the susceptible cultivar CO39, but less than 2% in the more resistant cultivars IR36, IR60 and IR62. Large differences between genotypes were found for the number of sporulating lesions that developed, and this factor was closely related to the period that leaves remained susceptible after appearance. The number of lesions in the most susceptible cultivar CO39 was about 7 times that in the cultivars IR60 and IR64. Differences between genotypes were also found for lesion size. The effect of aging on average lesion size was less pronounced than on lesion density. Lesion size and lesion density were positively correlated, but a rapid decline of density was not necessarily accompanied by a rapid decline of size. No apparent differences between genotypes were observed for latent period. Genotypes with leaves that became highly resistant soon after appearance expressed higher levels of partial resistance in the field.     

Evaluation of sources of partial resistance to mildew in barley using enzyme-linked immunosorbent assay and other assessment methods

Summary Twenty-nine spring barley genotypes ranging from primitive landraces to modern cultivars were characterised for partial resistance to powdery mildew on detached leaves and in the field. Detached seedling and adult plant leaves were produced under spore-free glasshouse conditions and were assessed for infection frequency (IF) and then total mildew biomass using enzyme-linked immunosorbent assay (ELISA), and thereby biomass per colony (BMC), following controlled infection. Field plants were assessed visually for infection and for total mildew biomass using ELISA. Several genotypes were identified as having below average IF and BMC on both seedling and adult detached leaves and field scores of leaf area infected with mildew. IF at the seedling stage was highly correlated with IF at adult plant stage and with mildew biomass in the field. Adult plant IF was highly correlated with visual field scores. BMC data were often negatively correlated with other assessments.     

Durable resistance to rice blast disease-environmental influences

Summary Blast is one of the most serious diseases of rice worldwide. The pathogen,Pyricularia grisea, can infect nearly all parts of the shoot and is commonly found on the leaf blade and the panicle neck node. Host resistance is the most desirable means of managing blast, especially in developing countries. Rice cultivars with durable blast resistance have been recognized in several production systems. The durable resistance of these cultivars is associated with polygenic partial resistance that shows no evidence of race specificity. This partial resistance is expressed as fewer and smaller lesions on the leaf blade but latent period does not appear to be an important component. Partial resistance to leaf blast is positively correlated with partial resistance to panicle blast, although some cultivars have been found showing leaf-blast susceptibility and panicle-blast resistance. A diverse set of environmental factors can influence the expression of partial resistance, including temperature, duration of leaf-wetness, nitrogen fertilization, soil type, and water deficit. Because of the great diversity of rice-growing environments, resistance that proves durable in one system may or may not prove useful in another. In highly blast-conducive environments, other means of disease management must be applied to assist host-plant resistance.     

Components of adult plant resistance to leaf rust in wheat

Summary Winter wheat genotypes were tested for resistance in the field by assessing the percentage sporulating leaf area after infection with wheat leaf rust. The disease level in the first field trial was too low for selection. In the second field trial a low sporulating leaf area was found on several genotypes showing a susceptible infection type. These genotypes possibly have partial resistance. Six genotypes possibly possess adult plant resistance, as they showed a resistant infection type and a low sporulating leaf area.The latency period, infection frequency and uredosorus size of sixteen genotypes were determined in the greenhouse after infection with two races of leaf rust at two temperature regimes. The temperature × genotype interaction, found for latency period and infection frequency, was mostly influenced by the cultivars Cerco, Tundra and Miller. Adult plant resistance was postulated for four genotypes whereas another four appeared to have partial resistance.Only one of the sixteen genotypes (Apexal) possessed adult plant resistance and two genotypes (Arminda and Cappelle Desprez) showed partial resistance in the field as well as in the greenhouse.     

Partial resistance to wheat leaf rust in 18 spring wheat cultivars

Summary Eighteen spring wheat cultivars were tested in microfields and race nurseries for their partial resistance PR to wheat leaf rust under low and high disease pressure respectively. Large differences existed between the 18 cultivars, Skalavatis 56 being the most susceptible and Ponta Grossa 1 being the most resistant cultivar. Of the three epidemic parameters, disease severity (DS) at the time that the susceptible check was severely diseased and area under the transformed disease severity curve (AUTC) and the logistic growth rate (r), AUTC and DS were highly correlated. Both seemed to be reliable estimators of PR but DS should be preferred for economical reasons. The logistic growth rate seemed to be unsuitable as an estimator of partial resistance.High and low disease pressure gave similar cultivar ranking. PR can be screened and selected equally well in race nurseries with low space, low time and low cost input as in microfields with high space, time and cost input.Cultivar differences in development rate had a large impact on the cultivar differences for amount of disease and can therefore greatly bias the estimation of cultivar resistance. The resistance of early cultivars tended to be underestimated whereas the resistance of late cultivars tended to be overestimated. The effect of differences in developmental rate was most pronounced in the flag leaf. It is advisable to avoid the assessment of disease levels on the flag leaf only and to incorporate in the tests several susceptible and resistant checks that cover the range of development rates in the material to be selected, because otherwise selection for resistance will tend to select also for lateness.Regression of the epidemiological parameters on three components of partial resistance revealed that latency period (LP) is an important factor in determining the resistance observed in the field explaining on average 67% of the observed variation. Adding infection frequency (IF) and urediosorus size (US) to the linear model increased the proportion of the observed variation in the field explained by the components to 80%. This result supports the idea that the components of PR inherit independently, at least, in part.     

Race-specific effects in major genic and polygenic resistance of barley to barley leaf rust in the field: identification and distinction

Summary Six partially resistant spring barley cultivars were exposed to four barley leaf rust (Puccinia hordei) races in the field and in the greenhouse. The 24 cultivar-race combinations were tested in field plots of 1.5×1.5 m² in two replications over two years. To reduce the interplot exchange of urediospores each plot was surrounded by winter rye.The level of barley leaf rust varied among cultivars, races and years. In both years the variance for cultivar-race interactions was highly significant and originating largely from the cultivar-race combinations Berac-22. Armelle-22, Armelle-A and Tyra-A. The Berac-22 interaction was towards higher, the other three interactions towards a lower level of barley leaf rust. The reduced rust levels of these three combinations were not due to interactions between the partial resistance of these cultivars and the aggressiveness of the races but to major genes for hypersensitivity not effective to the races 1-2-1 and F, common in Western Europe, but effective against the rare races 22 and A. This was revealed in the greenhouse experiments where all combinations had a susceptible infection type except Armelle-22, Armelle-A and Tyra-A, which showed low infection types in both the seedling and adult plant stages. The urediosori present in the field plots of these three combinations apparently arose from spores derived from other plots; this interplot interchange suggesting partial resistance.The interaction of Berac with race 22 truly was a small race-specific effect within the polygenic, partial resistance of barley to barley leaf rust like the one reported before between Julia and race 18.     

Interplot interference and the assessment of barley cultivars for partial resistance to leaf rust,Puccinia hordei

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 4×41/2 m (18 rows); C) adjacent plots of 4×11/2 m (6 rows); D) adjacent plots of 4×1/4 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 know 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 1 to 10 scale) of 1, 3–4, 7 and 8 respectively, based on isolated plots experiments.     

Components of rust resistance in lentil

Four components of resistance to Uromyces viciae-fabae, namely, latent period (LP), infection efficiency (IE), pustule size (PS) and spore production (SP) were evaluated on lentil (Lens culinaris) in glasshouse experiments. Four cultivars, Gudo, R-186, FLIP-87-66L and FLIP-89-60L, with different levels of resistance, and the susceptible check EL-142, were included in this study. The cultivars were also compared for area under the disease progress curve (AUDPC), area under the pustule density curve (APDC), apparent infection rate (r_G), disease severity (DS), pustule density (PD), LP and PS under field conditions. Gudo and R-186 had significantly smaller and fewer pustules, lower spore yield and longer latent period than EL-142. FLIP-87-66L was intermediate for infection efficiency and pustule size. In addition, significant differences were found between cultivars for AUDPC, APDC, DS and r_G. Estimates of AUDPC, APDC, DS and r_G were reduced in Gudo, R-186 and FLIP-87-66L compared with the susceptible check EL-142. FLIP-89-60L also showed low AUDPC, APDC and DS. Some of the components obtained in the field were significantly correlated with each other (r = 0.92–0.99, P 0.05) and those measured in the glasshouse. Most of the components studied in the glasshouse were significantly (P 0.05) correlated with AUDPC and DS. Data indicated the existence of incomplete [partial (PR)] resistance in the test cultivars, and the possibility of using IE, LP, SP and PS as selection criteria in the evaluation of PR to rust in lentil. Since there was an interdependence of the components, selection based on more than one component should help obtain lines with higher levels of PR. The AUDPC, DS and r_G could also be used for selecting lines with PR in the field.     

Variation in partial resistance to barley leaf rust (Puccinia hordei) and agronomic characters of Ethiopian landrace lines

An inventory of 481 lines derived from 12 Ethiopian barley (Hordeum vulgare L.) landraces and the checks was made for partial resistance to Puccinia hordei under greenhouse and field conditions at Adet, Ambo and Sinana Agricultural Research Centers in 2003 and 2004 cropping seasons in Ethiopia. The experiments were laid out in a triple lattice design. Each plot consisted of two rows of 1–m long with spacing of 0.20 m between rows. The overall mean leaf rust epidemics varied from area under disease progress curve (AUDPC) of 86 to 1,835. The disease was as high as AUDPC 1,378 on the susceptible check L94. Highly significant variations were recorded between and within the landraces/lines in leaf rust incidence, severity, days to heading, plant height, thousand seed weight and yield. Similarly, the variations between and within barley groups from three altitude areas and three ear-types were significant. Landraces 1686, 3255, 3262 and 3783 had the least and landraces 219900, 3975 and 3980 had the highest leaf rust severity. Of the 481 lines tested, 413 (86%) had significantly lower disease than the susceptible check, but not than the partial resistant check Vada. In contrast, the yields were more for lines with less disease than for those with high. The frequency of resistant landraces/lines was more in altitude 2,301–2,500 m, and irregular and two rows ear-types than in lower altitude areas and six rows ear-type. Nevertheless, the correlation and regression analysis revealed the adverse effect of the disease in the yields of barley. The 413 lines with high infection types at seedling stage and lower AUDPC under field conditions possess partial resistance to leaf rust.     

Accumulating polygenes for partial resistance in barley to barley leaf rust,Puccinia hordei. I. Selection for increased latent periods

Summary The barley cultivar Cebaba Capa was crossed to the cultivar L94, which is assumed to carry no genes for increased latent periods, and Vada, which is assumed to carry five to six minor genes for a longer latent period (LP). In the F2 selection was carried out for short and long LP's in the young flag leaves to Puccinia hordei in both crosses. In the F3, F4 and F5 the selection for short as well as for long LP continued by selecting the extreme plants in the extreme lines, a typical pedigree selection approach.The LP's are given relative to those of L94, set at 100 and of Vada, set at 185. From the cross with L94 homogeneous lines were obtained with relative LP's of 100 and of 220. From the cross with Vada the extreme lines had LP's of 135 and around or even beyond 300.Cebaba Capa is thought to carry four to six minor genes with an average gene effect slightly larger than those of the five to six minor genes in Vada. From the four to six minor genes one or two may be identical to or closely linked with minor genes of Vada, the others appeared to be different. In the lines with LP's of close to 300 or even more the number of minor genes accumulated is thought to be in the order of eight or nine. These gene number estimates are based on independent assortment. If linkage occurs the number of genes involved may be larger.Because of the high correlation between LP in the young flag leaf and the partial resistance in the field the selected lines are assumed to have a partial resistance to barley leaf rust far beyond that of Vada, which represents almost the highest level of partial resistance in European cultivars.     

Pathogenic variation among isolates of Rhynchosporium secalis from barley grass growing in South Eastern Australia

Summary The pathogenicity of 182 single spore isolates of Rhynchosporium secalis from Hordeum leporinum and 94 isolates from H. vulgare collected from throughout southeastern Australia was tested on 15 barley varieties, each having different combinations of resistance genes. Forty five percent of the barley grass isolates were pathogenic on 5 or more varieties but only 6% of the cultivated barley isolates attacked this range of varieties. On the basis of reaction type 20 different pathogenicity groups were recognised, with barley grass isolates being classified into 19 and the cultivated barley isolates into 5, four of which were the same as the barley grass isolates. Numerical analysis of data on leaf area damage inferred 33 groups, 24 of which were unique to barley grass isolates, two to cultivated barley isolates and 9 common to both groups. There was as much variation in pathogenicity among single spore isolates from the same lesion as between isolates from different lesions collected from the same or different locations.     

Components of partial resistance to Leptosphaeria nodorum among seven soft red winter wheats

Summary Seven soft red winter wheat cultivars were evaluated for partial resistance to Leptosphaeria nodorum under field conditions. The results demonstrate that resistance is available among cultivars that are adapted to the southeastern U.S. and that resistance is long lasting. Resistant cultivars had longer incubation and latent periods, slower rates of lesion development, and reduced the level of sporulation of L. nodorum. Seedlings of susceptible cultivars sustained severe disease with dew periods as short as 48 hr in greenhouse tests. One resistant cultivar sustained little damage with dew period as long as 144 hr.Latent period was shortest on the second leaf below the flag leaf (F-2 leaf) of all cultivars and longest on the flag leaf which reflected the effect of microclimate and leaf age. The range in length of latent period on the flag, F-1, and F-2 leaves of resistant cultivars was less than that for susceptible cultivars. Differences between cultivars were greatest (up to 6.8 days) for the F-2 leaf. A delay in production of inoculum on the F-2 and lower leaves of resistant cultivars should delay infection of the flag leaf and spike.The greatest differentiation among cultivars for sporulation of L. nodorum was on upper leaves at Feekes growth stage 11.2. Oasis consistently had less sporulation than other cultivars at all sampling dates and leaf positions. The rate of disease progress up the plant (disease severity) and area under the disease progress curve were also least on Oasis.There were significant correlations among components of resistance and associated components suggesting that a single or interrelated mechanisms control expression of resistance. Overall, Oasis was the most resistant cultivar followed by Coker 762 and Coker 747. There cultivars have remained resistant for ten or more years. Stacy was intermediate in resistance and was the most variable of the seven cultivars for the components tested. Holley, Omega 78, and Florida 301 were highly susceptible by all criteria tested. Plant height was not a factor in resistance. The resistant cultivars are mostly later in maturity than the susceptible cultivars. In this study, crop maturity was similar under existing environmental conditions during the two seasons when most data were collected.     

The importance of abortive stoma penetration in the partial resistance and nonhost reaction of adult barley and wheat plants to leaf rust fungi

Summary Early stages of the infection process of Puccinia hordei isolate 1.2.1 and of a P. recondita f.sp. tritici isolate were studied on adult plants of four barley lines and one wheat line. Two of the barley lines are extremely susceptible to P. hordei, the other two have a very high level of partial resistance.A histological study based on a trypan blue staining indicated that stoma penetration by P. hordei isolate 1.2.1 was equally successful on the susceptible as on the partially resistant adult barley plants. Abortion of substomatal vesicles was rare in all lines. These results do not support a hypothesis that mechanisms of partial resistance in adult plants differ from those in seedlings by a substantial abortive stoma penetration.Also in the nonhost combinations wheat-P. hordei and barley-P. recondita f.sp. tritici inhibition of stoma penetration and of substomatal vesicle development appears to play a biologically insignificant role in adult plants.The proportion of stoma penetration on the leaf sheaths of two of the barley lines was as high as on the leaf blades of the flag leaf and the leaf below the flag leaf. There was no evidence for stomatal exclusion as a crucial factor in the relatively low infectibility of leaf sheaths to leaf-blade specialized rust species.