Surveys of diseases of winter barley in England and Wales, 1981–1991

Samples from 200–400 randomly selected winter barley crops were taken annually at growth stage 71–73 from 1981 to 1991, with the exception of 1984 and 1985. The number of samples from each region was proportional to the area of barley growth in each region. The percentage of the area of the top two leaves affected by diseases and the severity of stem base diseases were recorded. Mildew (Erysiphe graminis) was the most widespread of the foliar diseases and in three years (1982, 1986 and 1991) was also the most severe. Rhynchosporium (Rhynchosporium secalis), net blotch (Pyrenophora teres) and brown rust (Puc-cinia hordei) were also prevalent in some years. Of the stem base diseases, fusarium was often the most widespread. Eyespot (Pseudocercosporella her-potrichoides) severity varied widely from year to year ranging from 1.2% of stems affected by moderate or severe symptoms in 1982 to 24.1% in 1988. There were regional differences in the severity of mildew, rhynchosporium, brown rust, halo spot (Selenophoma donacis) and eyespot. Cultivar resistance affected disease severity and previous cropping affected eyespot and less frequently mildew, rhynchosporium and net blotch. Eyespot, and to a lesser extent, sharp eyespot, were less severe in late- than in early-sown crops. The percentage of crops treated with a fungicidal spray increased from 72% in 1981 to 95% in 1991. The use of benzimidazole fungicides for the control of eyespot declined in response to the development of resistance, and more recently the use of prochlo-raz also declined. Broad spectrum DMI fungicides were widely used, and the use of morpholines to improve mildew control increased significantly. The proportion of crops grown from seed treated with a non-mercurial fungicidal seed dressing reached a peak of 47% in 1986 but subsequently declined to 22% in 1990 and 1991.     

Interaction between barley yellow dwarf virus and rust in wheat, barley and oats, and the effects on grain yield and quality

In three separate experiments, the upper leaf surface of the fifth formed leaf of wheat cv. Highbury, the fourth and fifth leaves of barley cv. Julia and the third and fourth leaves of oat cv. Mostyn were inoculated in a spore settling tower with wheat brown rust (Puccinia recondita f. sp. tritici), barley brown rust (P. hordei) or oat crown rust (P. coronata f. sp. avenae), respectively. Fewer pustules developed on distal portions of leaves of plants infected with barley yellow dwarf virus (BYDV) than on similar portions of leaves from virus-free plants. There were no significant differences in the number of pustules on proximal leaf portions. In barley and oats, the number of pustules on distal leaf portions was negatively correlated with the amount of yellowing of the leaf areas scored. In wheat, symptoms of BYDV were mild and leaves were little affected by yellowing. The latent period of rust on wheat and oats was not affected by BYDV. In barley, BYDV reduced the latent period of rust on leaf 5, but not on leaf 4, and reduced it on proximal, but not distal, leaf portions. In other experiments, BYDV reduced the yield of wheat and oats by 44% and 66%, respectively, while BYDV-infected barley was almost sterile. The appropriate rust reduced the yield of wheat, barley and oats by 33%, 13% and 86%, respectively. When infected with both BYDV and rust, yield of wheat and oats was reduced by 63% and 91%, respectively. Neither BYDV nor rust affected the percentage crude protein content of wheat grain, nor did rust affect that of barley. In oats, BYDV and rust each significantly increased crude protein of grain, but rust infection of BYDV-infected plants tended to reduce it.     

Surveys of diseases of winter wheat in England and Wales, 1976–1988

Samples from 300 – 400 randomly selected winter wheat crops were taken annually at growth stage 73 – 75 from 1976 to 1988 with the exception of 1983 and 1984. The number of samples from each region was proportional to the area of wheat grown in each region. The percentage of the area of the top two leaves affected by diseases, the severity of ear and stem base diseases and, in 6 years, the severity of take-all were recorded. Septoria tritici and Septoria nodorum were, on average, the most severe of the foliar diseases and eyespot (Pseudocercosporella herpotrichoides) was the most severe of the stem base diseases. Regional differences in levels of S. tritici, brown rust, sharp eyespot and nodal fusarium were significant. Cultivar resistance affected disease severity, and previous cropping patterns particularly affected take-all and eyespot. Eyespot and sharp eyespot were less severe in late- than in early-sown crops. The percentage of crops treated with a fungicidal spray increased from 14% in 1976 to over 90% between 1983 and 1985. Use of benzimidazole fungicides applied at growth stage 31 declined, while use of morpholines from flag leaf emergence onwards increased between 1985 and 1988.     

Role of topography sensing for infection-structure differentiation in cereal rust fungi

Over 90% of the germ tubes of Puccinia graminis tritici (wheat stem rust) and Puccinia hordei (barley brown rust) differentiate appressoria on encountering stomata.There has been controversy as to the role of host topographical signals in the highly precise and efficient induction of these infection structures over stomata by cereal rusts. In the present study, polystyrene replicas of microfabricated silicon wafers, bearing precise microtopographies of defined dimensions, were used to investigate the influence of ridge spacing and height on infection-structure induction by P. graminis tritici and P. hordei. It was found that artificial topographical signals alone can induce a reproducibly high percentage (83–86%) of germ tubes to differentiate infection structures. Multiple, closely spaced (1.5 μm) ridges which were 2.0 μm high provided the most inductive topography. Differentiation on flat surfaces and over single ridges was < 4%. Appressorium induction commonly initiated a cascade of differentiation events involving the formation of infection pegs, vesicles, infection hyphae, and occasionally haustorial mother cells. It is suggested that the close spacing of cell junctions associated with the dumbbell-shaped guard cells of cereal stomatal complexes provide inductive signals for infection-structure formation by cereal rusts in vivo. Received: 17 October 1996 / Accepted: 5 December 1996     

Apoplastic Sugar Concentration and pH in Barley Leaves Infected with Brown Rust

Soluble sugars were extracted by low speed centrifugation fromthe apoplast of leaves of barley (Hordeum distichum L.) infiltratedwith water. Infection of the leaf with the brown rust fungus(Puccinia hordeii) resulted in a reduction in the concentrationof sucrose, glucose and fructose in the apoplast. Sugars werepresent in an apoplastic space occupying 12 and 17 cm³ m^–2of leaf area in healthy and infected tissue, respectively. Uptakeof hexoses by intercellular hyphae is suggested as a cause ofthis reduction. The pH of apoplastic sap extracted from rust-infectedleaves was increased to pH 7·3 from pH 6·6 incontrols. The effect of a reduced apoplastic sugar pool andincreased pH on export from infected leaves is discussed. Key words: Apoplast, barley (Hordeum distichum L.), brown rust (Puccinia hordeii Otth.), pH, sucrose, hexose     

AB-QTL analysis in spring barley. I. Detection of resistance genes against powdery mildew, leaf rust and scald introgressed from wild barley

The objective of this study was to map new resistance genes against powdery mildew (Blumeria graminis f. sp. hordei L.), leaf rust (Puccinia hordei L.) and scald [Rhynchosporium secalis (Oud.) J. Davis] in the advanced backcross doubled haploid (BC₂DH) population S42 derived from a cross between the spring barley cultivar Scarlett and the wild barley accession ISR42-8 (Hordeum vulgare ssp. spontaneum). Using field data of disease severity recorded in eight environments under natural infestation and genotype data of 98 SSR loci, we detected nine QTL for powdery mildew, six QTL for leaf rust resistance and three QTL for scald resistance. The presence of the exotic QTL alleles reduced disease symptoms by a maximum of 51.5, 37.6 and 16.5% for powdery mildew, leaf rust and scald, respectively. Some of the detected QTL may correspond to previously identified qualitative (i.e. Mla) and to quantitative resistance genes. Others may be newly identified resistance genes. For the majority of resistance QTL (61.0%) the wild barley contributed the favourable allele demonstrating the usefulness of wild barley in the quest for resistant cultivars.     

Effect of Host Plant Resistance on Haustorium Formation in Cereal Rust Fungi

Haustoria of Puccinia triticina (wheat leaf rust fungus) and P. hordei (barley leaf rust fungus) were isolated from susceptible and partially resistant wheat lines, and susceptible, hypersensitive and partially resistant barley lines. Haustoria were counted and measured. The size of haustoria was similar in the partially resistant and susceptible genotypes but haustoria were smaller in the hypersensitive barley line L94+Pa7. The number of haustoria was reduced in both partially and hypersensitive lines when compared with susceptible ones. Therefore it seems that the reduction in the number of haustoria is a consequence of the resistance that can be attributable either to early abortion of infection units or reduced colony growth. The reduction of the number of haustoria was more pronounced in the adult plant stage.     

Immunogold localization of THRGP-like epitopes in the haustorial interface of obligate,biotrophic fungi on monocots

Summary Immunoelectron microscopy was used to determine the subcellular distribution of threonine-hydroxyproline-rich glycoprotein (THRGP) epitopes in host-parasite interactions between obligate, biotrophic fungi and cereals. Infection sites of stem rust (Puccinia graminis f. sp.tritici) and leaf rust (Puccinia recondita) on primary leaves of wheat (Triticum aestivum), as well as of powdery mildew (Erysiphe graminis f. sp.hordei) on coleoptiles of barley (Hordeum vulgare), wete probed with a polyclonal antiserum to maize THRGP. A few immunogold particles were found over the cell walls of wheat mesophyll tissue and barley coleoptile epidermis. Unlike previous examples in dicot plants, no enhanced accumulation of THRGP was observed in cereal cell walls adjacent to sites of pathogen ingress. Instead, the most pronounced accumulation of THRGP-like molecules occurred over the extrahaustorial matrix in both incompatible and compatible plant-pathogen interactions. For powdery mildew of barley, immunogold staining was distinctly increased over the center of the penetration sites; however, no labeling was found over papillae that formed during incompatible and compatible interactions. In addition, no cross-reactivity of the anti-THRGP antiserum with intercellularly growing rust pathogens was observed. The highly localized deposition of THRGP-like molecules in the extrahaustorial matrix suggests that the host plant establishes a modified barrier between itself and the pathogen.Abbreviations C chloroplast - EC plant epidermal cell - EM extrahaustorial membrane - EMA extrahaustorial matrix - GO Golgi body - GRP glycine-rich protein - HP high pressure - HRGP hydroxyprolinerich glycoprotein - Hyp hydroxyproline - LT low temperature - PBS phosphate-buffered saline - PBST PBS with Tween-20 - THRGP threonine-hydroxyproline-rich glycoprotein - VA vesicular arbuscular     

Effects of Infection Density on the Size of Barley and Wheat Leaf Rust Colonies before and on the Size of Uredia after the Start of Sporulation

Three barley genotypes were exposed to four different inoculum densities of barley leaf rust, Puccinia hordei. Colony area well before first sporulation and uredia size well after the start of sporulation were measured. In a second series two barley and two wheat cultivars were exposed to four different inoculum densities of barley and wheat leaf rust (P. recondita f. sp. tritici), respectively. Before the sporulation is initiated the colony size was independent of the uredia density. This was valid for densities ranging from approximately 7 to over 200 uredia per cm² leaf area. After the sporulation had started the uredia size was strongly dependent on the uredia density. The size of the uredia was approximately halved when the uredia density increased from about 10 to about 150 per cm². The urediospore production per uredium decreased much stronger with increased uredia density.     

Selecting a set of wild barley introgression lines and verification of QTL effects for resistance to powdery mildew and leaf rust

A set of 59 spring barley introgression lines (ILs) was developed from the advanced backcross population S42. The ILs were generated by three rounds of backcrossing, two to four subsequent selfings, and, in parallel, marker-assisted selection. Each line includes a single marker-defined chromosomal segment of the wild barley accession ISR42-8 (Hordeum vulgare ssp. spontaneum), whereas the remaining part of the genome is derived from the elite barley cultivar Scarlett (H. vulgare ssp. vulgare). Based on a map containing 98 SSR markers, the IL set covers so far 86.6% (1041.5 cM) of the donor genome. Each single line contains an average exotic introgression of 39.2 cM, representing 3.2% of the exotic genome. The utility of the developed IL set is illustrated by verification of QTLs controlling resistance to powdery mildew (Blumeria graminis f. sp. hordei L.) and leaf rust (Puccinia hordei L.) which were previously identified in the advanced backcross population S42. Altogether 57.1 and 75.0% of QTLs conferring resistance to powdery mildew and leaf rust, respectively, were verified by ILs. The strongest favorable effects were mapped to regions 1H, 0–85 cM and 4H, 125–170 cM, where susceptibility to powdery mildew and leaf rust was decreased by 66.1 and 34.7%, respectively, compared to the recurrent parent. In addition, three and one new QTLs were localized, respectively. A co-localization of two favorable QTLs was identified for line S42IL-138, which holds an introgressed segment in region 7H, 166–181. Here, a reduction effect was revealed for powdery mildew as well as for leaf rust severity. This line might be a valuable resource for transferring new resistance alleles into elite cultivars. In future, we aim to cover the complete exotic genome by selecting additional ILs. We intend to conduct further phenotype studies with the IL set in regard to the trait complexes agronomic performance, malting quality, biotic stress, and abiotic stress.     

The use of bulk segregant analysis to identify a RAPD marker linked to leaf rust resistance in barley

An F₂ population from a cross between barley accession Q21861 and the Australian barley variety Galleon was used to develop RAPD markers for resistance to barley leaf rust (Puccinia hordei). Resistant and susceptible DNA bulks were constructed following the classification of F₂ plants by leaf rust infection type. Bulked segregant analysis was then used to identify a 2.7-kb marker, designated OU02₂₇₀₀ and located approximately 12cM from RphQ, a leaf rust resistance gene in Q21861. The marker was generated by PCR with the oligonucleotide primer OPU-02 (Operon). Infection types of F₃ progeny were used to confirm assignment of F₂ genotypes. OU02₂₇₀₀ was shown, retrospectively, to be useful in the identification of individual F₂ plants that had been originally misclassified as having susceptible infection types. Both the RAPD marker and RphQ will be potentially useful in the development of new barley cultivars.     

Effect of chronic irradiation on plant resistance to biotic stress in 30-km chernobyl nuclear power plant exclusion zone

It was established in greenhouse experiments that infection with powdery moldew (Erysiphe graminis DC. f. sp. tritici Em. Marchal) and brown rust (Puccinia triticana Erikss. & Henn.) of three wheat (Triticum aestivum L.) cultivars (Mironovskaya 808, Polesskay 70, and Kiyanka) grown from seeds, collected in the Chernobyl exclusion zone, was 1.5–2.0 times higher than of plants grown from control seeds. On field trials in the Chernobyl zone, wheat plant resistance to biotic stress was reduced. At artificial infection with brown rust, the disease development was enhanced on plots with increased radiation background. One of the mechanisms of the declined phytoimmunity potential under the action of low doses of chronic irradiation is evidently a reduced activity of plant proteinase inhibitors. Thus, in wheat and rye (Secale cereale L., cv. Saratovskaya) grains, their activity reduced by 35–60% as compared to control. Active form and race formation in the population of the cereal stem rust causal agent (Puccinia graminis Pers.) was observed in the Chernobyl zone. A “new” population of this fungus with high frequency of more virulent clones than in other Ukraine regions was distinguished. The results obtained independently in greenhouse and field trials performed in the Chernobyl zone demonstrated radiation stress influence on the pathogen-plant interactions. They indicate a necessity of monitoring the microevolutionary processes occurring in both plants and their pathogens under conditions of technogenic stresses.     

The maize rp1 rust resistance gene identifies homologues in barley that have been subjected to diversifying selection

A number of agronomically important grasses (sorghum, wheat, panicum, sugar cane, oats, rice and barley) are shown to contain sequences homologous to rp1, a maize gene that confers race-specific resistance to the rust fungus Puccinia sorghi. Mapping of rp1-related sequences in barley identified three unlinked loci on chromosomes 1HL, 3HL and 7HS. The locus located on chromosome 7HS comprises a small gene family of at least four members, two of which were isolated and are predicted to encode nucleotide binding site-leucine-rich repeat (NBS-LRR) proteins that are respectively 58% and 60% identical to the maize rp1 protein. Evidence of positive selection for sequence diversification acting upon these two barley genes was observed; however, diversifying selection was restricted to the carboxy terminal half of the LRR domain. One of these rp1 homologous genes cosegregated with the barley Rpg1 stem rust resistance gene amongst 148 members of the Steptoe × Morex double haploid mapping family. Three other unrelated resistance gene-like sequences, potentially encoding NBS-LRR proteins, are also shown to be linked to the Rpg1 locus but not cosegregating with the gene. Received: 2 August 1999 / Accepted: 28 September 1999     

A survey of foliar diseases of spring barley in England and Wales in 1967

A total of 5,250 acres of spring barley was sampled in proportion to the barley acreage in each of eight regions. The percentage leaf area ‘affected’ by each disease was recorded for the first (flag) and second leaves at a growth stage when the grain was milky ripe. The operation was automated by punching the data directly on to computer paper tape and a programme was written to analyse the results. Mildew was found to be the most important disease, causing an average loss in yield of approximately 18 %, followed in descending order by brown rust 3%, leaf blotch 1 %, yellow rust 1 %, halo spot 1 %. The total loss due to foliar diseases was estimated to be 20–25% of the national barley yield. Diseases were more severe in the southern than northern regions, e.g. mildew severity was three times higher because of the greater popularity of mildew-susceptible varieties, and the prevalence of weather more conducive to mildew development. Disease severity was not related to previous cropping, and in general the February-sown crops had more mildew than the April-sown crops.     

Accumulation of genes for susceptibility to rust fungi for which barley is nearly a nonhost results in two barley lines with extreme multiple susceptibility

Nonhost resistance is the most common type of resistance in plants. Understanding the factors that make plants susceptible or resistant may help to achieve durably effective resistance in crop plants. Screening of 109 barley (Hordeum vulgare L.) accessions in the seedling stage indicated that barley is a complete nonhost to most of the heterologous rust fungi studied, while it showed an intermediate status with respect to Puccinia triticina, P. hordei-murini, P. hordei-secalini, P. graminis f. sp. lolii and P. coronata ff. spp. avenae and holci. Accessions that were susceptible to a heterologous rust in the seedling stage were much more or completely resistant at adult plant stage. Differential interaction between barley accessions and heterologous rust fungi was found, suggesting the existence of rust-species-specific resistance. In particular, many landrace accessions from Ethiopia and Asia, and naked-seeded accessions, tended to be susceptible to several heterologous rusts, suggesting that some resistance genes in barley are effective against more than one heterologous rust fungal species. Some barley accessions had race-specific resistance against P. hordei-murini. We accumulated genes for susceptibility to P. triticina and P. hordei-murini in two genotypes called SusPtrit and SusPmur, respectively. In the seedling stage, these accessions were as susceptible as the host species to the target rusts. They also showed unusual susceptibility to other heterologous rusts. These two lines are a valuable asset to further experimental work on the genetics of resistance to heterologous rust fungi.Electronic Supplementary Material Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00425-004-1319-1Abbreviations ff. spp Formae speciales - RIL Recombinant inbred line - DC Double cross - DC-S Progeny produced by selfing of double-cross plants     

Powdery mildew resistance and Lr34/Yr18 genes for durable resistance to leaf and stripe rust cosegregate at a locus on the short arm of chromosome 7D of wheat

The incorporation of effective and durable disease resistance is an important breeding objective for wheat improvement. The leaf rust resistance gene Lr34 and stripe rust resistance gene Yr18 are effective at the adult plant stage and have provided moderate levels of durable resistance to leaf rust caused by Puccinia triticina Eriks. and to stripe rust caused by Puccinia striiformis Westend. f. sp. tritici. These genes have not been separated by recombination and map to chromosome 7DS in wheat. In a population of 110 F₇ lines derived from a Thatcher × Thatcher isogenic line with Lr34/Yr18, field resistance to leaf rust conferred by Lr34 and to stripe rust resistance conferred by Yr18 cosegregated with adult plant resistance to powdery mildew caused by Blumeria graminis (DC) EO Speer f. sp. tritici. Lr34 and Yr18 were previously shown to be associated with enhanced stem rust resistance and tolerance to barley yellow dwarf virus infection. This chromosomal region in wheat has now been linked with resistance to five different pathogens. The Lr34/Yr18 phenotypes and associated powdery mildew resistance were mapped to a single locus flanked by microsatellite loci Xgwm1220 and Xgwm295 on chromosome 7DS.     

Tissue culture of the alternate hosts of wheat,rye and oat rusts and their response to in vitro inoculation with the rust pathogens

Alternate host plants of cereal rust fungi are necessary for studying the rust sexual cycle and pathogenicity. These plants are usually difficult to propagate through cloning, while seed-propagated plants may have variable responses to the pathogen. To overcome these obstacles, tissue culture, under controlled and aseptic conditions, was utilized for clonal propagation and in vitro inoculation of the following species: Rhamnus palaestinus Boiss., the alternate host of oat (Avena spp.) crown rust (Puccinia coronata Corda); Thalictrum speciosissimum L., the alternate host of brown leaf rust of wheat (Puccinia recondita f. sp. tritici Eriks. & Henn.); and Lycopsis arvensis L., the alternate host of rye (Secala spp.) leaf rust (Puccinia recondita f. sp. recondita Rob. & Desm.). Shoot culture procedures for initial establishment and proliferation were developed for all three alternate host species. Shoot cultures were multiplied at rates ranging from 0.3 to 1.7 shoots/week. Successful infection following inoculation with teliospores of the corresponding rust fungi was obtained for R. palaestinus and T. speciosissimum but not for L. arvensis. The hardening and acclimatization efficiency of rooted T. speciosissimum and L. arvensis was of 80–90%. The propagation efficiency for R. palaestinus was not successful because of the low rate and poor quality of its rooting. It is concluded that the in vitro system might be used as an alternative method for inoculation and multiplication of alternate hosts of cereal rusts, although more experimentation is needed to define accurately the appropriate conditions for the proper infection response. This revised version was published online in June 2006 with corrections to the Cover Date.     

Incidence, importance and virulence of Puccinia hordei on barley in New Zealand

Brown rust of barley (Puccinia hordei) is widespread in New Zealand and causes crop losses in some years. The cultivars presently grown lack adequate genetic resistance but a highly resistant line has been produced. Twelve different virulence combinations were identified in the pathogen population using the Welsh differential lines; there were differences between North Island and South Island isolates; and no virulence was observed for resistance conferred by factors Pa 3 and Pa 7.     

Seedling and adult‐plant stage resistance of a world collection of barley genotypes to stripe rust

Barley stripe rust caused by Puccinia striiformis f.sp. hordei (PSH) is one of the major diseases in barley production regions worldwide. A total of 336 barley genotypes with diverse genetic backgrounds targeted for low‐input barley production were tested for seedling and adult‐plant stage resistance against six PSH races (0S0, 0S0‐1, 1S0, 4S0, 5S0 and 7S0) originated from India. The seedling resistance was evaluated by inoculating the barley genotypes with six races separately under controlled conditions in Shimla, India. The same barley genotypes were evaluated for adult‐plant stage resistance in the Agricultural Research Station (ARS) of Rajasthan Agriculture University, Durgapura, Rajasthan, India. Out of the 336 barley genotypes tested for seedling resistance, 119 (35.4%), 101 (30.1%), 87 (25.9%), 100 (29.8%), 91 (27.1%) and 70 (20.8%) genotypes were resistant to races 0S0, 0S0‐1, 1S0, 4S0, 5S0 and 7S0, respectively. In the field, 102 (30.3%) genotypes showed the resistance response of which 18 (5.3%) genotypes were highly resistant to PSH. Barley genotypes AM‐14, AM‐177, AM‐37, AM‐120, AM‐300, AM‐36, AM‐103, AM‐189, AM‐291, AM‐275 and AM‐274 showed resistance response to all six races at seedling and adult‐plant stages. Seedling resistance reported in the current study is effective against the newly emerged race 7S0 and previously reported five races in India. Therefore, resistant barley genotypes identified in the current study provided effective protection against all six races at seedling and adult‐plant stages. The stripe rust resistance identified in the current studies may be potential donors of stripe rust resistance to barley breeding programmes in India and elsewhere.     

Interaction between crown rust (Puccinia coronata) and two viruses of ryegrass

In Lemtal Italian and S.24 perennial ryegrass plants, two isolates of ryegrass mosaic virus (RMV) suppressed the amount of crown rust emerging on leaves inoculated with Puccinia coronata uredospores by up to 75% compared with the amount on virus-free plants. Severity of rust infection on barley yellow dwarf virus (BYDV) infected plants generally did not differ significantly from that on virus-free plants. When both RMV and BYDV were present, rust was restricted in Lemtal plants to a level intermediate between those occurring on plants infected by either virus alone, and in S.24 plants to a level below that obtained with either virus alone. The mean water soluble carbohydrate (WSC) content of Lemtal plants was reduced more than 20% by RMV, but was not significantly altered by BYDV. In S.24 plants the WSC content was increased by 10% by RMV and by 60% by BYDV. Rust reduced the WSC content of healthy and virus-infected plants, the reduction being positively correlated with the level of rust on the sampled leaves. In plants of Lemtal, but not of S.24, the degree of rust infection was positively correlated with the WSC content of leaves from rust-free control plants.