Erysiphe Species Research Articles

First Report of Erysiphe quercicola Causing Powdery Mildew on Ubame Oak in Korea.

Ubame oak (Quercus phillyraeoides A. Gray) is native to eastern Asia, including China, Korea, and Japan. In 2009 and 2010, a powdery mildew on Q. phillyraeoides growing in clusters and singly was observed in three locations on the campus of Chonnam National University, Gwangju, Korea. White superficial conidia of the powdery mildew fungus occurred on adaxial and abaxial surfaces. However, the white powdery growth was more abundant on the adaxial surface. Leaf symptoms commonly appeared white from May to October. Along with the typical white powdery mildew, spot and/or necrotic symptoms with irregular violet-to-wine red surfaces were also frequently observed on overwintered leaves. A voucher specimen has been deposited in EML (Environmental Microbiology Laboratory) herbarium collection, Chonnam National University (EML-QUP1). Conidia were commonly formed singly but also occurred in chains. Primary conidia were obovoid to ellipsoid, with a rounded apex and subtruncate base. Secondary conidia were generally obovoid to ellipsoid or sometimes cylindrical but dolioform when mature. The size was 30.1 to 43.2 (average 37.7) × 14.1 to 21.1 (average 18.1) μm with length/width ratio of 1.8 to 2.4 (average 2.1). Conidiophores were erect and up to 102.2 μm long. No chasmothecia were found. From extracted genomic DNA, the internal transcribed spacer (ITS) region inclusive of 5.8S rDNA was amplified with ITS1F (5'-CTTGGT CATTTAGAGGAAGT-3') and LR5F (5'-GCTATCCTGAGGGAAAC-3') primers (4). Sequence analysis by BLASTN search indicated that EML-QUP1 (GenBank Accession No. HQ328834) was closest to E. quercicola (GenBank Accession No. AB292691) with >99% identity (478 of 480), forming a monophyletic quercicola clade in the resulting phylogenetic analysis. The causal fungus was determined to be Erysiphe quercicola on the basis of morphology and sequence data analysis. Major genera including Cystotheca, Erysiphe, Microsphaera, and Phyllactinia have been reported to cause powdery mildews on Quercus plants. Until now, 22 Erysiphe species including E. abbreviata, E. alphitoides, E. calocladophora, E. gracilis, E. polygoni, and E. quercicola have been reported to cause powdery mildews on Quercus spp. (1). Of these, four Erysiphe species including E. alphitoides, E. gracilis, E. quercicola, and an unidentified Erysiphe sp. have been found on Q. phillyraeoides from Japan (1-3). E. quercicola was reported to occur on five Quercus species: Q. crispula, Q. phillyraeoides, and Q. serrata in Japan, Q. robur in Australia, and Quercus sp. in Australia, Iran, and Thailand (1). To our knowledge, this is the first report of leaf powdery mildew caused by E. quercicola on Q. phillyraeoides in Korea.

Powdery Mildew of Japanese White Birch Caused by Erysiphe ornata var. ornata in Korea

Betula platyphylla var. japonica (Miq.) Hara, or Japanese whitebirch, is a tree which is widely distributed in East Asia. In July 2009,some young trees showing typical symptoms of powdery mildewdisease on both sides of their leaves were first found at HongneungArboretum, Korea Forest Research Institute, Seoul, Korea. White superficial colonies developed amphigenously on affectedleaves (Fig. 1A & B). Appressoria were multi-lobed to moderatelylobed (Fig. 1C). Conidiophores were 60-100×10-14 µm, andcomposed of slightly curved to flexuous basal cells and one to threecylindrical cells (Fig. 1D & E). Conidia, devoid of fibrosin bodies,were oblong to cylindrical, with wrinkling surface, 32-42×12-16µm, and producing perihilar germ tubes (Fig. 1F & G). Chasmothe-cia were spherical, 70-95 µm in diameter. Appendages were 8-18per chasmothecium, 0.8-1.3 times as long as the chasmothecialdiameter, nearly straight, 3.5-9 µm wide, thick-walled at the base,but thinner upwards, aseptate or occasionally uniseptate at the base,apex 3-5 times branched, tips of the ultimate branchlets recurved.Asci were 3-6 per chasmothecium, 50-67×35-43µm, 5-8-spored.Ascospores were oval, 22-27×11-14µm (Fig. 1H & I). Morpho-logical characteristics of the material are in accordance with theprevious records of Erysiphe ornata var. ornata (U. Braun) U.Braun & S. Takam. (formerly Microsphaera ornata var. ornata U.Braun). This variety differs from var. europaea having fewer (ca 4-10) and shorter appendages (Braun, 1987).The complete ITS region of rDNA from KUS-F24679 wasamplified with primers ITS5 and P3 and sequenced, and theresulting sequence was deposited in GenBank (HM057441). ABLAST search showed that there was no comparable sequence of E.ornata and thus this was the first ITS sequence for the speciessubmitted in GenBank. To infer the relationship between E. ornatavar. ornata and other Erysiphe species, a phylogenetic tree (Fig. 2)was constructed using MEGA4, version 4.0 with neighbor-joiningmethod and Tajima-Nei model .According to Braun (1987), E. ornata var. ornata on Betula spp.is widespread in North America, known in Asia (Central Asia,Siberia, Far East of Russia) and very rare in Eastern Europe. InChina, Microsphaera betulae Magnus (actually E. ornata var.ornata in the current concept) on Betula spp. was recorded fromXinjiang by Zhao (1979) and from Inner Mongolia by Liu (1997).Interestingly, beginning in the 1990s there have been new records ofE. ornata var. ornata on Betula spp. from Estonia, Poland, Serbia,Switzerland and Ukraine (see Farr and Rossman, n.d.). Thissuggests that E. ornata var. ornata, the North American powderymildew fungus, has been spreading to Europe and Asia in recentyears. This is the first confirmed report of E. ornata var. ornata inEast Asia and the first record of B. platyphylla var. japonica as itshost plant.

Genetic variability and mycohost association of Ampelomyces quisqualis isolates inferred from phylogenetic analyses of ITS rDNA and actin gene sequences

Ampelomyces quisqualis complex is well known as the most common and widespread hyperparasite of the family Erysiphaceae, the cause of powdery mildew diseases. As commercial biopesticide products it is widely used to control the disease in field and plastic houses. Although genetic diversity within Ampelomyces isolates has been previously recognized, a single name A. quisqualis is still applied to all pycnidial intracellular hyperparasites of powdery mildew fungi. In this study, the phylogenetic relationships among Ampelomyces isolates originating from various powdery mildew fungi in Korea were inferred from Bayesian and maximum parsimony analyses of the sequences of ITS rDNA region and actin gene. In the phylogenetic trees, the Ampelomyces isolates could be divided into four distinct groups with high sequence divergences in both regions. The largest group, Clade 1, mostly accommodated Ampelomyces isolates originating from the mycohost Podosphaera spp. (sect. Sphaerotheca). Clade 2 comprised isolates from several genera of powdery mildews, Golovinomyces, Erysiphe (sect. Erysiphe), Arthrocladiella, and Phyllactinia, and was further divided into two subclades. An isolate obtained from Podosphaera (sect. Sphaerotheca) pannosa was clustered into Clade 3, with those from powdery mildews infecting rosaceous hosts. The mycohosts of Ampelomyces isolates in Clade 4 mostly consisted of species of Erysiphe (sect. Erysiphe, sect. Microsphaera, and sect. Uncinula). The present phylogenetic study demonstrates that Ampelomyces hyperparasite is indeed an assemblage of several distinct lineages rather than a sole species. Although the correlation between Ampelomyces isolates and their mycohosts is not obviously clear, the isolates show not only some degree of host specialization but also adaptation to their mycohosts during the evolution of the hyperparasite.

New insights into the identity and origin of the causal agent of oak powdery mildew in Europe

SummaryPowdery mildew is the most common disease on oaks in Europe where it was first recorded at the beginning of the 20th century. Yet, little is known about the origin of the causal agent. In this study, we analysed the variability of the ribosomal DNA (rDNA) of the pathogen. The internal transcribed spacer (ITS) region, the 5.8S rRNA coding gene and the intergenic spacer (IGS) of the rDNA of 33 European (mostly French) samples of oak powdery mildew were amplified using polymerase chain reaction (PCR) and PCR products were subsequently sequenced. Four different haplotypes were obtained for ITS among the various samples (ITSA, ITSB, ITSC and ITSD). Each ITS sequence corresponded to a different IGS sequence. The comparison of ITS sequences obtained with sequences accessible in the GenBank database revealed very high homologies with different taxa. Of these, three taxa had already been described on oaks in Europe, i.e. Erysiphe alphitoides (100% homology with ITSA), Erysiphe hypophylla (99.4% homology with ITSC) and Phyllactinia guttata (97.64% homology with ITSD). Our data also confirmed the 100% homology between ITSA and the sequence described for Oïdium mangiferae, the agent of mango powdery mildew. The fourth haplotype, i.e. ITSB, represented by nearly 25% samples, showed 100% homology with the recently described Erysiphe quercicola from Quercus spp. in Asia, and several tropical and sub‐tropical powdery mildew species, including Oïdium heveae, a major pathogen of rubber trees worldwide. Our results suggest that oak powdery mildew might originate from host shifts of tropical Erysiphe species introduced to Europe through infected exotic host plants.

Erysiphe monascogera sp. nov., an unusual powdery mildew fungus found on fruits of Styrax japonica

Morphological observations using light and scanning electron microscopes and molecular phylogenetic analysis revealed that the fungus growing on the surface of fruits or sepals of Styrax japonica collected at Nagano, Japan, is a new powdery mildew with an unusual morphology, described here as Erysiphe monascogera. This fungus has mainly a single ascus in a chasmothecium, but molecular phylogenetic analysis and the shape of the hyphal appressoria suggest that it is an Erysiphe species. Erysiphe monascogera is a sister-species to E. nomurae on Symplocos chinensis var. leucocarpa f. pilosa, although there are obvious morphological differences between the two species. This inconsistency between molecular phylogeny and morphology may be explained by the unique habitat of E. monascogera. Erysiphe monascogera and E. nomurae are included in a clade composed of the E. alphitoides complex, which suggests that these two species diverged by host jumping of the E. alphitoides complex, having oaks as major host plants.

First report of powdery mildew caused by Erysiphe lonicerae var. lonicerae on Lonicera sempervirens in Korea

Trumpet honeysuckle (Lonicera sempervirens) is an exotic ornamental vine, which was recently introduced and widely planted in gardens throughout Korea. In September 2006, during a survey of diseases of ornamental trees and shrubs, severe outbreaks of powdery mildew on this plant were noticed in two parts of Seoul. White superficial mycelia and conidia were present epiphyllously on both sides of the leaves as well as on young shoots, forming thin, irregular colonies. Infections often caused leaf distortions, withering and reduced growth. The hyphae were septate, branched and 4–6 µm in width. Conidiophores were unbranched, cylindrical, 50–95 µm and composed of 3–4 cells. Conidia produced singly, were ellipsoid or doliiform, 28–37(–42) × 18–22 µm, without distinct fibrosin bodies. Chasmothecia were dark brown, spherical, and scattered subgregariously, 86–120 µm diameter. Each chasmothecium contained 5–10 asci. Appendages arose equatorially, 6–14 per chasmothecium, and were 1·5–3·5 times as long as the chasmothecial diameter. They were substraight to somewhat flexuous, 6–9 µm in width, thick-walled at the base, gradually becoming narrower, aseptate or occasionally 1-septate. They were compactly and irregularly branched 3–5 times at the apex. Primary branches were often elongated. Asci were shortly stalked, 3-6-spored, 50–68 × 30–40 µm. Ascospores were ellipsoid-ovoid, 21–28 × 10–13·5 µm. Based on these characteristics, this fungus was identified as Erysiphe lonicerae var. lonicerae (Braun, 1987, 1995). Six species of Erysiphe are known to infect Lonicera spp. (Braun, 1987; Tanda, 2000): E. lonicerae (var. lonicerae and ehrenbergii), E. magnusii, E. erlangshensis, E. caprifoliacearum (var. caprifoliacearum and flexuosa), E. miurae and E. lonicerae-ramosissimae. These can be differentiated based on the length and septation of the chasmothecial appendages. Six native species of Lonicera are known to be associated with E. erlangshensis in Korea (Shin, 2000). The morphological patterns of the powdery mildew fungus on the exotic L. sempervirens, however, correspond to those of E. lonicerae var. lonicerae. Previously, this species was recorded on the trumpet honeysuckle from the USA and Estonia (Braun, 1995). This is the first report of the occurrence of powdery mildew on L. sempervirens in Asia. The voucher specimens are kept at Korea University (SMK 22270, 22300).

<i>Erysiphe monascogera</i> sp. nov., an unusual powdery mildew fungus found on fruits of <i>Styrax japonica</i>

Morphological observations using light and scanning electron microscopes and molecular phylogenetic analysis revealed that the fungus growing on the surface of fruits or sepals of Styrax japonica collected at Nagano, Japan, is a new powdery mildew with an unusual morphology, described here as Erysiphe monascogera. This fungus has mainly a single ascus in a chasmothecium, but molecular phylogenetic analysis and the shape of the hyphal appressoria suggest that it is an Erysiphe species. Erysiphe monascogera is a sister-species to E. nomurae on Symplocos chinensis var. leucocarpa f. pilosa, although there are obvious morphological differences between the two species. This inconsistency between molecular phylogeny and morphology may be explained by the unique habitat of E. monascogera. Erysiphe monascogera and E. nomurae are included in a clade composed of the E. alphitoides complex, which suggests that these two species diverged by host jumping of the E. alphitoides complex, having oaks as major host plants.

Molecular determination of anamorphic powdery mildew fungi on the Fabaceae in Australia

Powdery mildew fungi are common on the Fabaceae, but sexual states have not been found in Australia, resulting in an inadequate understanding of which species occur. The rDNA internal transcribed spacer (ITS) regions of thirty-two anamorphic powdery mildew specimens on the Fabaceae were analysed by restriction fragment length polymorphisms (RFLPs) and sequencing to reveal four species in Australia. Erysiphe trifolii was identified on a broad range of fabaceous hosts and E. pisi was found on Pisum sativum. Oidium hardenbergiae was found on Hardenbergia and was genetically similar to these two Erysiphe species. The fourth species, on Vigna and Phaseolus in tropical Australia, was determined to be Podosphaera xanthii.

Cellular interactions between biotrophic fungal pathogens and host or nonhost plants

Biotrophic fungi, such as the rust fungi and the powdery mildew fungi, represent a distinct and economically important group of plant pathogens. Studies of nonhost interactions with Uromyces vignae (cowpea rust fungus, monokaryotic stage) or Erysiphe cichoracearum (plantain powdery mildew fungus) suggest that for both types of biotrophs, penetration through the epidermal wall commonly fails in association with wall-associated defense responses, in part elicited by breakdown products of the plant cell wall. Data suggest that extracellular hydrogen peroxide plays a primary role in this penetration failure, acting either as a signaling agent or as a necessary adjunct to some other defense response within the wall. Protoplast involvement, but not necessarily transcription or translation, is necessary for these wall-associated responses, and they can be reduced by reducing the adhesion between the plant cell wall and the plasma membrane. The latter observation suggests that this adhesion is important for at least some types of defensive signaling between the wall and the cell contents. In host or nonhost plants, Erysiphe species increase this adhesion and wall-associated responses occur even in susceptible plants. However, in its host species, U. vignae locally reduces this adhesion around the penetration site and, as an apparent consequence, no wall-associated responses can be detected. Thus, this latter system is ideal for studying postpenetration cellular and molecular changes in resistant or susceptible host cells without the confounding effect of responses to the trauma of cell wall penetration. Such studies indicate that not only is penetration failure in nonhost plants determined while the fungus is growing through the epidermal cell wall, but that it is at this stage of infection in host species that cellular responses are initiated that determine whether the plant cell will become susceptible to infection or exhibit the hypersensitive response.

Two Erysiphe species associated with recent outbreak of soybean powdery mildew: results of molecular phylogenetic analysis based on nuclear rDNA sequences

Serious outbreaks of powdery mildew by a fungus belonging to the mitosporic genus Oidium subgenus Pseudoidium have been reported on soybean (Glycine max) in a wide area of eastern Asia since 1998. The taxonomic and phylogenetic placement of the causal fungus has not yet been determined because of lack of the perfect stage. We found ascomata having mycelioid appendages on a single leaf of soybean infested by powdery mildew. Molecular phylogenetic analysis was conducted based on a total of 14 sequences of the rDNA internal transcribed spacer (ITS) region from 13 soybean and wild soybean (Glycine soja) materials collected in Japan, Korea, Vietnam, and the United States, combined with 47 sequence data obtained from the DNA databases. It was revealed that two Erysiphe species were associated with the outbreak of soybean powdery mildew. There was 16% difference between the two species in genetic divergence of the ITS sequence. One species with perfect stage has an ITS sequence identical to that of Erysiphe glycines on Amphicarpaea and is identified as Erysiphe glycines based on the ITS sequence and morphology of ascomata. The second species, without the perfect stage, is likely to be Erysiphe diffusa (= Microsphaera diffusa), known as the fungus causing soybean powdery mildew in the United States, because the ITS sequences are identical to those from materials collected in the United States. However, we need materials having ascomata of E. diffusa to confirm the species name.

Evolutionary analysis of the powdery mildew fungi using nucleotide sequences of the nuclear ribosomal DNA

The nucleotide sequences of the nuclear ribosomal DNA including 18S, 5.8S, and 28S rDNA and the internal transcribed spacer regions were determined for 33 powdery mildew taxa spanning 15 genera to infer the phylogenetic relationships for these fungi. Phylogenetic analyses revealed that Uncinula septata was placed in the primitive base to the large clade composed of all other powdery mildew taxa. Powdery mildews excluding Unc. septata were split into five major lineages. The Pseudoidium lineage consisted of six genera, i.e., Erysiphe, Microsphaera, Uncinula, Uncinuliella, Brasiliomyces, and Typhulochaeta, all of which are characterized by polyascal cleistothecia and Pseudoidium-type anamorph. The Eudoidium lineage consisted of three Erysiphe species (E. cichoracearum, E. orontii and E. galeopsidis) and Arthrocladiella mougeotii, which are characterized by polyascal cleistothecia and Euoidium-type anamorph. The endophytic lineage consisted of two Phyllactinia species (Phy. moricola and Phy. Kakicola), Leveillula taurica and Pleochaeta shiraiana, which are characterized by the presence of endophytic or partly endophytic mycelia. The fibrosin lineage consisted of the genera Sawadaea, Cystotheca, Podosphaera, and Sphaerotheca, all of which are characterized by the presence of well-developed fibrosin bodies in the conidia and conidiophores. The monocot lineage consisted of a single species, Blumeria graminis, which is well characterized by being parasitic to monocots, especially to cereal plants. Character polarization was discussed on several major characteristics. The mycelioid appendage which has long been regarded as an ancestral character may be a derived character as a result of an event that may have occurred multiple times independently due to convergence. Euoidium-type anamorph and clavate appressorial germ tube are considered to be ancestral characters to Pseudoidium-type anamorph and lobed germ tube. Ectophytic nature and polyascal cleistothecia are also regarded as ancestral features to endophytic nature and monoascal cleistothecia.

Phylogeny of the Erysiphales (powdery mildews) inferred from internal transcribed spacer ribosomal DNA sequences

Phylogenetic relationships were inferred for 45 powdery mildews plus two outgroup species from internal transcribed spacer sequence data. Phylogenetic analyses revealed that the powdery mildews formed six evolutionary lineages, which corresponded well to recently revised mitosporic taxa. Clade 1 consisted of Erysiphe, Microsphaera, and Uncinula, all of which have an Oidium subgenus Pseudoidium mitosporic state. Clade 2 consisted of Erysiphe galeopsidis and Erysiphe cumminsiana, which have an Oidium subgenus Striatoidium mitosporic state. Clade 3 consisted of Erysiphe species with an Oidium subgenus Reticuloidium mitosporic state. Clade 4 consisted of Leveillula and Phyllactinia, which have Oidiopsis and Ovulariopsis mitosporic states, respectively. Clade 5 consisted of Sphaerotheca, Podosphaera, and Cystotheca, which have Oidium subgenera Fibroidium and Setoidium mitosporic states. Clade 6 consisted of Blumeria graminis, which has an Oidium subgenus Oidium mitosporic state. Seventeen morphological characters were analysed and found not to conflict with the molecular data, so the morphological and molecular data were combined. The combined data increased the overall support for the six clades.Key words: Ascomycete, Blumeria, Erysiphe, fungi, molecular evolution, plant pathogen.

Phylogeny of the Erysiphales (powdery mildews) inferred from internal transcribed spacer ribosomal DNA sequences

Phylogenetic relationships were inferred for 45 powdery mildews plus two outgroup species from internal transcribed spacer sequence data. Phylogenetic analyses revealed that the powdery mildews formed six evolutionary lineages, which corresponded well to recently revised mitosporic taxa. Clade 1 consisted of Erysiphe, Microsphaera, and Uncinula, all of which have an Oidium subgenus Pseudoidium mitosporic state. Clade 2 consisted of Erysiphe galeopsidis and Erysiphe cumminsiana, which have an Oidium subgenus Striatoidium mitosporic state. Clade 3 consisted of Erysiphe species with an Oidium subgenus Reticuloidium mitosporic state. Clade 4 consisted of Leveillula and Phyllactinia, which have Oidiopsis and Ovulariopsis mitosporic states, respectively. Clade 5 consisted of Sphaerotheca, Podosphaera, and Cystotheca, which have Oidium subgenera Fibroidium and Setoidium mitosporic states. Clade 6 consisted of Blumeria graminis, which has an Oidium subgenus Oidium mitosporic state. Seventeen morphological charact...

The nucleoside and nucleotide contents of uredospores from Puccinia graminis and Puccinia striiformis and of conidia from Erysiphe graminis, Erysiphe pisi, and Trichoderma harzianum

Nucleoside and nucleotide contents of uredospores from Puccinia graminis f. sp. tritici and Puccinia striiformis f. sp. tritici and conidia from Erysiphe graminis , f. sp. hordei, Erysiphe pisi , and Trichoderma harzianum were analyzed by high performance liquid chromatography using an effective method to homogenize the spores. The spores of the two Puccinia species showed large quantities of nucleotides ranging from 10 to 15 μmol (g dry wt) −1 , whereas the conidia from the two Erysiphe species had a lower nucleotide content, less than 5 μmol (g dry wt) −1 . Some of the spores contained large amounts of nucleosides; with E. graminis their total amount was larger than that of the nucleotides. All spores contained high amounts of the chitin precursor, UDP- N -acetylglucosamine, in the range of 1.3 to 4.6 μmol (g dry wt) −1 . The rust uredospores had relatively large amounts of nucleoside triphosphates, especially ATP, whereas the powdery mildew conidia had only very small amounts of them. For those species from which spore batches (obtained from different harvests) were available, the adenine nucleotide-derived energy charge showed a positive correlation with the germination efficiency only in the case of P. graminis .

Are primary germ tubes of conidia unique to Erysiphe graminis?

Conidia of Erysiphe graminis f. sp. hordei and E. graminis f. sp. tritici produce primary germ tubes that breach epidermal walls of host plants several hours before appressoria are formed. The present study, using micromanipulation in the scanning electron microscope, examines whether or not some other Erysiphe species, E. pisi, E. cichoracearum and E. polygoni also develop primary germ tubes or similar structures. For comparison, E. graminis f. sp. hordei and E. graminis f. sp. agropyri were also examined. Conidia of E. cichoracearum produced only a single germ tube during the initial 16hr after inoculation, and only a few per cent of E. pisi and E. polygoni conidia had more than one tube. Furthermore, penetration pores were only observed below appressorial tubes of these species. By contrast, conidia of E. graminis f. sp. hordei and E. graminis f. sp. agropyri produced both short primary germ tubes which breached epidermal walls of host plants and appressoria that penetrated and developed haustoria. From these results and the literature, we conclude that primary germ tubes may be unique to and thus represent a characteristic feature of E. graminis development.

Some effects of inoculation with yellowing viruses on the susceptibility of sugar beet to fungal pathogens: II. Susceptibility to Erysiphe species

Large differences in susceptibility to Erysiphe betae (Vañha) Weltzien (E. polygoni DC ex Merat) were observed, in field and glasshouse experiments, between individual virus-tolerant sugar beet lines and varieties. The severity of powdery mildew symptoms on plants of virus-tolerant varieties and of two commercial varieties was increased by infection with beet mild yellowing virus (BMYV) and decreased by infection with beet yellows virus (BYV). Simultaneous infection with both viruses did not apparently alter the susceptibility to powdery mildew, presumably because of the opposite effects of BYV and BMYV infection. It is suggested that leaves of plants infected with BMYV provide a more suitable substrate for the fungus than leaves of virus-free plants. The effect of BYV infection is obscure, but may be due to a reduction in the concentration of substances essential for the rapid development of the fungus, in BYV-infected leaves. The results of these experiments show that, although virus infection appeared to have a similar effect on the powdery mildew symptoms of all varieties, the efficiency of selection tests for resistance to E. betae will be seriously reduced if plants are not all of the same virus infection status.