Subcellular localization of β-1,3-glucanase in Puccinia recondita f.sp. tritici-infected wheat leaves

An antiserum raised against the purified 33-kDa β-1,3-glucanase of wheat (Triticum aestivum L.) was employed to investigate the ultrastructural localization of the enzyme in wheat leaves infected with Puccinia recondita Rob. ex Desm. f.sp. tritici Eriks. and Henn. using a post-embedding immunogold labelling technique. In both compatible and incompatible interactions, β-1,3-glucanase was detected in the host plasmalemma and in the domain of the host cell wall near the plasmalemma of the mesophyll cells, but higher concentrations of the enzyme were detected in infected resistant wheat leaves than in infected susceptible ones. β-1,3-Glucanase was also found in the secondary thickening of xylem vessels and in the walls of guard cells, epidermal cells and phloem elements, while no labelling was observed in host organelles, viz. vacuoles, mitochondria, endoplasmic reticulum, Golgi bodies, nuclei and chloroplasts. A low concentration of the enzyme was detected on the intercellular hyphal wall and in the hyphal cytoplasm. In the compatible interaction, β-1,3-glucanase was demonstrated to accumulate predominantly in the haustorial wall and extrahaustorial matrix. In the incompatible interaction, strong labelling for β-1,3-glucanase was found in host cell wall appositions, in the extracellular matrix in the intercellular space, and in electron-dense structures of host origin which occurred in the incompatible interaction only. Received: 22 July 1997 / Accepted: 16 August 1997     

Ultrastructural comparison of incompatible and compatible interactions in the barley powdery mildew disease

Summary In the powdery mildew disease of barley,Erysiphe graminis f. sp.hordei forms an intimate relationship with compatible hosts, in which haustoria form in epidermal cells with no obvious detrimental effects on the host until late in the infection sequence. In incompatible interactions, by contrast, the deposition of papillae and localized host cell death have been correlated with the cessation of growth byE. g. hordei. With the advent of improved, low temperature methods of sample preparation, we felt that it was useful to reevaluate the structural details of interactions between barley andE. g. hordei by transmission electron microscopy. The haustoria that develop in susceptible barley lines appear highly metabolically active based on the occurrrence of abundant endoplasmic reticulum, Golgi-like cisternae, and vesicles. In comparison, haustoria found in the resistant barley line exhibited varying signs of degradation. A striking clearing of the matrix and loss of cristae were typical early changes in the haustorial mitochondria in incompatible interactions. The absence of distinct endoplasmic reticulum and Golgi-like cisternae, the formation of vacuoles, and the occurrence of a distended sheath were characteristic of intermediate stages of haustorial degeneration. At more advanced stages of degeneration, haustoria were dominated by large vacuoles containing membrane fragments. This process of degeneration was not observed in haustoria ofE. g. hordei developing in the susceptible barley line.Abbreviations b endoplasmic reticulum extension, blebbing - er endoplasmic reticulum - f fibrillar material - g Golgi-like structure - h haustorium - hb haustorial body - hcw haustorial cell wall - hcy haustorial cytoplasm - hf haustorial finger - hocw host cell wall - hocy host cytoplasm - 1 lipid-like droplet - m mitochondrion - mt microtubule - mve multivesicular body - n nucleus - p papilla - ph penetration site of an infection peg - pl plasma membrane - s sheath - sm extrahaustorial membrane - v vacuole - ve vesicle     

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.     

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.     

Extracellular beta-1,3-Glucanases in Stem Rust-Affected and Abiotically Stressed Wheat Leaves : Immunocytochemical Localization of the Enzyme and Detection of Multiple Forms in Gels by Activity Staining with Dye-Labeled Laminarin

Endo-β-1,3-glucanase activity in intercellular washing fluid (IWF) from leaves of wheat (Triticum aestivum) increased 10-fold 4 days after leaves were infected with the wheat stem rust fungus (Puccinia graminis f.sp. tritici), while exo-β-1,3-glucanase activity remained unchanged at a low level. Heat and ethylene stress had no effect, whereas mercury treatment resulted in a 2-fold increase in endo-β-1,3-glucanase activity. With a new method of activity staining using laminarin-Remazol brilliant blue as substrate in overlay gels, 18 electrophoretic forms of endo-β-1,3-glucanase were detected in IWF from unstressed leaves and up to 24 forms in IWF from stem rust-infected leaves. Most of the increase in β-1,3-glucanase activity and in the number of β-1,3-glucanases after rust infection was due to a nonspecific, stress-related effect on the plant, but two major forms of the enzyme probably originated from the fungus. β-1,3-Glucanase was localized cytochemically with anti-barley-β-1,3-glucanase antibodies. With preembedding labeling, the enzyme was demonstrated on the outside of host and fungal cell walls. Postembedding labeling localized the enzyme in the host plasmalemma and in the domain of host cell walls adjoining the plasmalemma, throughout walls of intercellular hyphal cells and haustoria, in the fungal cytoplasm, and in the extrahaustorial matrix. Cross-reactivity of β-1,3-glucanases from wheat and germinated uredospores of the rust fungus with the anti-barley-β-1,3-glucanase antibodies was confirmed in dot blot assays and on Western blots.     

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 adult plant rust resistance loci Lr34/Yr18 and Lr46/Yr29 are important determinants of partial resistance to powdery mildew in bread wheat line Saar

Powdery mildew, caused by Blumeria graminis f. sp. tritici is a major disease of wheat (Triticum aestivum L.) that can be controlled by resistance breeding. The CIMMYT bread wheat line Saar is known for its good level of partial and race non-specific resistance, and the aim of this study was to map QTLs for resistance to powdery mildew in a population of 113 recombinant inbred lines from a cross between Saar and the susceptible line Avocet. The population was tested over 2 years in field trials at two locations in southeastern Norway and once in Beijing, China. SSR markers were screened for association with powdery mildew resistance in a bulked segregant analysis, and linkage maps were created based on selected SSR markers and supplemented with DArT genotyping. The most important QTLs for powdery mildew resistance derived from Saar were located on chromosomes 7DS and 1BL and corresponded to the adult plant rust resistance loci Lr34/Yr18 and Lr46/Yr29. A major QTL was also located on 4BL with resistance contributed by Avocet. Additional QTLs were detected at 3AS and 5AL in the Norwegian testing environments and at 5BS in Beijing. The population was also tested for leaf rust (caused by Puccinia triticina) and stripe rust (caused by P. striiformis f. sp. tritici) resistance and leaf tip necrosis in Mexico. QTLs for these traits were detected on 7DS and 1BL at the same positions as the QTLs for powdery mildew resistance, and confirmed the presence of Lr34/Yr18 and Lr46/Yr29 in Saar. The powdery mildew resistance gene at the Lr34/Yr18 locus has recently been named Pm38. The powdery mildew resistance gene at the Lr46/Yr29 locus is designated as Pm39.     

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.     

Observations on the ultrastructure of the uredinial stage ofPuccinia polypogonis onPolypogon monspeliensis

The ultrastructure of the uredinial stage of the rust fungus,Puccinia polypogonis onPolypogon monspeliensis is described, using scanning and transmission electron microscopy. This study examined the urediniospores, intercellular hyphae, and haustoria of the fungus. The formation and structure of urediniospores is similar to those of otherPuccinia species. The ultrastructure of intercellular hyphae and haustoria is similar to those of other rust fungi, but with some differences. No modifications are observed in the wall of the haustorial mother cells during penetration. A collar is found only around old haustoria. In most cases, one nucleus is detected inside the haustorial body and no nucleoli are seen in the nuclei of intercellular hyphae and haustoria. The host-parasite interface, including extrahaustorial matrix and extrahaustorial membrane, is also discussed and compared with those of other rust fungi.     

Specific binding of a hypersensitive lignification elicitor from Puccinia graminis f. sp. tritici to the plasma membrane from wheat (Triticum aestivum L.)

We have recently reported the isolation and characterization of a glycoprotein (M_r 67 000) from germ-tube walls of Puccinia graminis f. sp. tritici which elicits the cellular hypersensitive lignification response in wheat (G. Kogel et al., 1988, Physiol. Mol. Plant Pathol. 33, 173–185). The present study uses this glycoprotein, referred to as Pgt elicitor, to identify putative elicitor targets in wheat cell membranes. In enzyme-linked immunosorbent assays using anti-Pgt elicitor antibodies, specific binding sites for Pgt elicitor were detected in highly purified plasma-membrane vesicles of wheat (Triticum aestivum L.) primary leaf cells. Binding proved to be independent of the presence or absence in wheat of the Sr5 gene for rust resistance, and also occurred on barley (Hordeum vulgare L.) plasma membrane. The binding sites have an Mr of 30 000 and 33 000, respectively, and binding activity was not lost in the presence of sodium dodecyl sulfate. [¹⁴C]imido-Pgt elicitor was used to determine the apparent K _d value for specific binding, found to be 2.0 M, and the maximum content of binding sites, found to be 250 pmol per mg of plasma-membrane protein. The relevance of the elicitor binding for the outcome of the interaction of P. graminis and wheat is discussed.Abbreviations BSA bovine serum albumin - ELISA enzyme linked immunosorbent assay - IDPase inosine 5-diphosphatase - MPLC medium-pressure liquid chromatography - MF microsomal fraction - Pgt elicitor elicitor of Puccinia graminis f. sp. tritici - SDS sodium dodecyl sulfate - Pre U₃, Pre U₁ pure plasma membrane from wheat cultivar Prelude and plasma membrane contaminated by intracellular membrane, respectively This work was supported by the Deutsche Forschungsgemeinschaft. We wish to thank C. Larsson, Lund, Sweden for his kind support in the preparation of plasma membrane.     

Cultivar-related differences in the distribution of cell-wall-bound thionins in compatible and incompatible interactions between barley and powdery mildew

Leaf-specific thionins of barley (Hordeum vulgare L.) have been identified as a novel class of cell-wall proteins toxic to plant-pathogenic fungi and possibly involved in the defence mechanism of plants. The distribution of these polypeptides has been studied in the host-pathogen system of barley and Erisyphe graminis DC.f.sp. hordei Marchal (powdery mildew). Immunogold-labelling of thionins in several barley cultivars indicates that resistance or susceptibility may be attributed to the presence or absence of thionins at the penetration site in walls and papillae of epidermal leaf cells.All of the leaf-specific thionin genes are confined to the distal end of the short arm of chromosome 6 of barley. None of the genes for cultivarspecific resistance to powdery mildew which have previously been mapped on barley chromosomes are found close to this locus.     

Proteins in intercellular washing fluid from noninoculated and rust-affected leaves of wheat and barley

Proteins in intercellular washing fluid (IWF) from wheat (Triticum aestivum) and barley (Hordeum vulgare) leaves were separated by two-dimensional isoelectric focusing-polyacrylamide gel electrophoresis and stained with Coomassie brilliant blue (CBB) or silver. Intracellular protein from the cut ends of leaves accounted for only a small proportion of total protein in IWF from wheat leaves. When these were heavily infected with the stem rust fungus (Puccinia graminis f. sp. tritici) and grown at 19°C, four infection-related CBB-stainable proteins were detected in IWF.

To compare IWF proteins from wheat and barley leaves infected with the same pathogen, conditions were established that permitted luxuriant growth of stem rust of wheat in barley (exposure to chloroform before inoculation and maintenance at 25°C thereafter). Under these conditions, at least 10 infection-related silver-stainable proteins were detected in IWF from infected wheat in addition to the more than 50 that were of host origin. The electrophoretic properties of 8 of the infection-related proteins were the same as those of 8 infection-related proteins in IWF from barley.

IWF from wheat and barley grown under these conditions was analyzed for Concanavalin A-binding glycoproteins immobilized on nitrocellulose membrane replicas made from gels. Of the many infection-related glycoproteins that were detected in IWF from stem rust-affected wheat, approximately 20 occupied the same positions as those from stem rust-affected barley. The glycoprotein pattern of IWF prepared from wheat leaves grown at 19°C and infected with the leaf rust fungus (P. recondita f. sp. tritici) was markedly different to that of IWF from the same host infected with the stem rust fungus. We conclude that IWF from rust-affected cereal leaves may be a useful source of surface or extracellular proteins from the parasitic mycelium.

     

CAPS and DHPLC Analysis of a Single Nucleotide Polymorphism in the Cytochrome b Gene Conferring Resistance to Strobilurins in Field Isolates of Blumeria graminis f. sp. hordei

A single nucleotide polymorphism in the wheat powdery mildew (Blumeria graminis f. sp. tritici) cytochrome b gene is responsible for resistance to inhibitors of the quinol outer binding site of the cytochrome bc₁ complex (QoI) fungicides. Analysis of a partial sequence of the cytochrome b gene from field isolates resistant and sensitive to QoI fungicides revealed the same point mutation in barley powdery mildew (B. graminis f. sp. hordei). Analysis of 118 and 40 barley powdery mildew isolates using a cleaved amplified polymorphic sequence assay and denaturing high performance liquid chromatography, respectively, confirmed that this single nucleotide polymorphism also confers resistance to QoI fungicides in barley powdery mildew.     

Identification of genes involved in stem rust resistance from wheat mutant D51 with the cDNA-AFLP technique

Wheat (Triticum aestivum L.) stem rust caused by Puccinia graminis f. sp. tritici is one of the main diseases of wheat worldwide. Wheat mutant line D51, which was derived from the highly susceptible cultivar L6239, shows resistance to the prevailing races 21C3CPH, 21C3CKH, and 21C3CTR of P. graminis f. sp. tritici in China. In this study, we used the cDNA-AFLP technology to identify the genes that are likely involved in the stem rust resistance. EcoRI/MseI selective primers were used to generate approximately 1920 DNA fragments. Seventy five differentially transcribed fragments (3.91%) were identified by comparing the samples of 21C3CPH infected D51 with infected L6239 or uninfected D51. Eleven amplified cDNA fragments were sequenced. Eight showed significant similarity to known genes, including TaLr1 (leaf rust resistance gene), wlm24 (wheat powdery mildew resistance gene), stress response genes and ESTs of environment stress of tall fescue. These identified genes are involved in plant defense response and stem rust resistance and need further research to determine their usefulness in breeding new resistance cultivars.     

Changes in the Hill Reaction and the Chlorophyll Fluorescence of Chloroplasts Isolated from Wheat Plants Infected with Stem Rust (Puccinia graminis f. sp. tritici)

Wheat plants with different resistance to stem rust were inoculated with urediospores of Puccinia graminis f. sp. tritici race 32. By means of isolated chloroplasts Hill activity and chlorophyll fluorescence were measured during the first 8–10 days of the infection. Highly resistant cultivars (infection type 0) showed no significant differences in Hill activity whereas resistant (infection type 1) and susceptible plants (infection type 4) exhibited a significant decrease in electron transport. This was associated with an apparent reduction of chlorophyll content. In both the incompatible as well as in the compatible interaction an additional factor had an effect on the inhibition of the Hill activity. Chlorophyll fluorescence was also affected in compatible and in incompatible interactions. However, the two interactions showed different patterns. Whereas the decline in variable fluorescence dominated in resistant varieties, susceptible cultivars showed an additional significant increase in basic fluorescence. The determination of Hill activity and chlorophyll fluorescence demonstrated that photosynthesis was inactivated in different ways in resistant and in susceptible cultivars.     

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     

小麦锌指蛋白基因TaLOL2的克隆及特征分析

通过电子克隆与RT-PCR相结合的方法,在条锈菌诱导的小麦叶片中克隆获得1个新的LSD1型锌指蛋白基因TaLOL2,并用qRT-PCR技术分析了其转录表达特征。结果显示:(1)小麦锌指蛋白基因TaLOL2的cDNA全长1 095bp,编码179个氨基酸。(2)TaLOL2含有3个典型的zf-LSD1型(CxxCxRxxLMYxxGASxVxCxxC)保守结构域,与水稻、拟南芥、大麦等植物LSD1型锌指蛋白序列具有高度相似性,其中与水稻OsLOL2相似度达86.0%。(3)进化树分析表明,TaLOL2与水稻、拟南芥和大麦中部分含有3个保守zf-LSD1锌指结构的基因亲缘关系较近,而与其它包含不同数目的zf-LSD1锌指结构的基因亲缘关系较远。(4)qRT-PCR定量分析表明,TaLOL2在条锈菌侵染前期呈上调表达,在亲和及非亲和反应中差异表达。研究表明,TaLOL2参与了条锈菌诱导的小麦抗病防卫反应,很可能作为正调控因子参与了小麦-条锈菌非亲和互作中对条锈菌的抗性信号途径。     

Ultrastructure of the host-pathogen interface in daylily leaves infected by the rust fungus Puccinia hemerocallidis

Summary. Transmission electron microscopy was used to examine details of the host–pathogen interface in daylily leaf cells infected by the rust fungus Puccinia hemerocallidis. Samples were prepared for study by high-pressure freezing followed by freeze substitution. The outstanding preservation of ultrastructural details afforded by this fixation protocol greatly facilitated the study of this host–pathogen interface. The extrahaustorial membrane that separated each dikaryotic haustorium from the cytoplasm of its host cell was especially well preserved and appeared almost completely smooth in profile. Large aggregations of tubular cytoplasmic elements were present near haustoria in infected host cells. Many of these tubular elements were found to be continuous with the extrahaustorial membrane and conspicuous electron-dense deposits present in the extrahaustorial matrix extended into these elements. The use of gold-conjugated wheat germ agglutinin for labeling of chitin revealed that these deposits were not part of the haustorial wall. Portions of many of the tubular elements associated with haustoria were conspicuously beaded in appearance. Some tubular elements were found to be continuous with flattened cisternae that in turn bore short beaded chains. Distinctive tubular-vesicular complexes previously reported only in cryofixed rust haustoria also were found in the haustoria of P. hemerocallidis. Received July 6, 2001 Accepted October 3, 2001     

Fungitoxic action of dimethirimol and ethirimol

The systemic fungicides dimethirimol and ethirimol were shown to be toxic to spores of Erysiphe graminis and Sphaerotheca fuliginea, in germination tests in vitro. Toxicity of dimethirimol to spores of Botrytis fabae, Phytophthora infestans, Puccinia recondita, Uncinula necator and Venturia inaequalis was relatively feeble or absent. When applied in foliage sprays or in root treatments, both compounds inhibited the emergence of germ tubes from powdery mildew spores on cucumber and barley plants. The specific disease control shown by these fungicides can be explained by their direct fungitoxic action. The effects of dimethirimol and ethirimol on powdery mildew infections and on spore germination were greatly decreased by the presence of riboflavin in the light. A rapid photo-chemical interaction between riboflavin and dimethirimol was demonstrated. Marked reversals both of disease control and of fungitoxicity were also given by folic acid, and it is suggested that folic acid overcomes a metabolic block induced in powdery mildew fungi by the fungicides.