Field experiments on sugar-beet downy mildew (Peronospora farinosa)

The numbers of sugar-beet plants infected with downy mildew in experimental crops increased slowly over a period of 8–15 weeks from late May. Few new infections appeared after the middle of August, although less than half the plants were then infected. Most infected plants recovered, and recovery was most rapid and complete in young plants, irrespective of the weather. Plants infected when young produced small roots, but their sugar content and juice purity at harvest was similar to those of uninfected plants. Plants first infected in late July and early August had the smallest sugar content and most juice impurities.     

Study on DNA diversity of Iranian populations of Erysiphe betae causal agent of sugar beet powdery mildew

107 samples of E. betae were collected on infected leaves from all over Iranian beet cultivation areas. Their choosing were based on geographical and host origin(sugar beet, red beet, fodder beet and wild beet). 30 isolates were single colonized and grown on sugar beet susceptible genotype 7233. 107 specimens were analyzed by restriction fragment length polymorphism (RFLP) of the ribosomal internal transcribed spacers (ITS) and 5.8s DNA which previously amplified by the polymerase chain reaction (PCR) with 2 universal primers, ITS1 and ITS4. PCR product was affected by 9 different restriction enzymes. PCR product was a 645 bp band for all of the isolates. 3 restriction enzymes; CfoI, MspI and HaeIII could cut this fragment into smaller bands, but electrophoretic patterns were identical for all of the isolates. 30 single colonized isolates were used in RAPD experiments. In RAPD-PCR experiment genetic diversity was investigated with 30 isolates from different parts of the country. 59 random primers were used and then 21 primers that displayed good consistency and reproducibility were selected. Most of the primers revealed identical patterns between 3 to 14 bands. 5 primers that showed more polymorphism were selected to analyze 30 isolates. For these 5 primers 61 distinct bands were obtained which 62% of these bands were polymorphic. Results indicated that there is no relationship between cluster grouping and geographical origin and the isolates showed a high similarity.     

Study on the overwintering of cleistothecia and conidia of Erysiphe betae causal agent of sugar beet powdery mildew in Iran

Sugar beet leaves covered by sexual (cleistothecia) and asexual forms (mycelia and conidia) of Erysiphe betae were gathered at harvest time and maintained under natural outdoor conditions. In order to determine the function of cleistothecia and also conidia in the overwintering of E. betae some experiments were performed. The results showed that ascospores were unable to be released in petri dishes but their release under natural conditions occurred after 4 months. Under In vitro conditions ascospores did not germinate but on the leaves germination was rarely possible, however these ascospores were degraded after 7 days and didn't have pathogenicity. Conidia could induce pathogenicity after 3 but not 4 months. The period after inoculation until appearance of disease symptoms increased with aging of conidia. The results for conidial germination showed that fresh conidia had 80 percent germination on glass slides but it decreased sharply after 2 weeks and reached to 0 percent after 4 weeks. Although their germination on the leaves was not more than 46 percent of fresh conidia but they had good germination after 2 and 4 weeks. The results for the experiment to observe the first appearance of the disease in the field suggested that the first conidia were trapped by spore-trap in early June and the first symptoms appeared 20 days later. The conclusive results showed that ascospores had no function in the survival of the fungus and air-borne conidia are the main source of primary infections.     

Laboratory experiments on sugar-beet downy mildew (Peronospora farinosa)

The optimum conditions for Peronospora farinosa betae to produce spores were temperature 8–10 °C and relative humidity 90 % or more, but many spores were produced between 5 and 20 °C and between 80 and 90 % R.H. Most spores were formed in darkness after leaves were exposed to light for 6–8 h. Spores survived exposure to 60 % R.H. for up to 5 days, but were soon killed by temperatures above 20 °C. The germination capacity of spores collected from the field was often very small, but this could not be related to the weather. Most seedlings were infected when inoculated at the growing point and incubated in a saturated atmosphere between 3 and 15 °C for at least 8 h.     

Molecular phylogenetic analyses reveal a close relationship between powdery mildew fungi on some tropical trees and Erysiphe alphitoides, an oak powdery mildew

To investigate the phylogenetic relationships among the powdery mildew fungi of some economically important tropical trees belonging to Oidium subgenus Pseudoidium, we conducted molecular phylogenetic analyses using 30 DNA sequences of the rDNA internal transcribed spacer (ITS) regions and 26 sequences of the domains D1 and D2 of the 28S rDNA obtained from the powdery mildews on Hevea brasiliensis (para rubber tree), Anacardium occidentale (cashew), Bixa orellana, Citrus spp., Mangifera indica (mango), and Acacia spp. The results indicate that the powdery mildew fungi isolated from these tropical trees are closely related to one another. These powdery mildews are also closely related to E. alphitoides (including Erysiphe sp. on Quercus phillyraeoides). Because of the obligate biotrophic nature of the powdery mildew fungi, the relationship between powdery mildews and their host plants is conservative. However, the present study suggests that a particular powdery mildew species has expanded its host ranges on a wide range of the tropical trees. This article also suggests that a powdery mildew fungus distributed in temperate regions of the Northern Hemisphere expanded its host ranges onto tropical plants and may be a good example of how geographical and host range expansion has occurred in the Erysiphales.     

Intraspecies differentiation in the powdery mildew Erysiphe cichoracearum determined with rDNA RFLPs

The powdery mildew species Erysiphe cichoracearum has a described host range of over 300 plant species from among several families. Host-range testing indicates host-specialized subdivision within this taxonomic species. However, the extent of subdivision remains largely undetermined among host-limited forms. We have characterized diversity among field collections of E. cichoracearum from a variety of hosts, and from other powdery mildew species, with RFLPs from a PCR amplified ribosomal DNA (rDNA) segment The E. cichoracearum samples expressed six distinct RFLP haplotypes. Each haplotype was specific to either a single host or to a set of related host species. These haplotypes formed a continuum of divergence ranging from about 18–35% average pairwise distance from one another, while those from other mildew species clustered at consistently higher average pairwise distances from E. cichoracearum and from each other. Our findings support earlier suggestions, based on host-range and morphological characterizations, that E. cichoracearum is a complex of morphologically similar, but host-limited forms. Also, comparisons of rDNA haplotype distance between E. cichoracearum and Blumeria (Erysiphe) graminis were consistently greater than between E. cichoracearum and Sphaerotheca fulginea. This result supports earlier questions concerning the monophyletic nature of Erysiphe.     

Germination of conidia of poppy powdery mildew (Erysiphe polygoni DC)

Mode of germination and effect of various substrates, temperature, humidities, light and darkness on germination of poppy powdery mildew conidia have been studied.Conidia germinated on all substrates tested. Highest germination (89.41 %) was on agar 2 %. Germination was equally good on dry slide and in water. Generally one, rarely two germ tubes emerged from each conidium from the corner or occasionally from other places. Germination started after two hours of incubation and was almost over in 5 hours. Germination energy was 86.2.Maximum temperature for germination was 32.5 C. Optimum lied near 20 C.Light and darkness did not affect germination.Conidia germinated at all relative humidities tested including 0 and 100 %.Department of Plant Pathology, Agricultural Experiment Station, University of Udaipur, Udaipur, India.Based on a part of Ph.D. thesis of the senior author.     

First record of dill powdery mildew caused by Erysiphe heraclei DC in Egypt

In Egypt, powdery mildew was observed for the first time on dill plants, during annual disease surveys of March–May 2003 and 2005. Typical symptoms of powdery mildew of dill plant (Anethum graveolens L.) were observed in Gharbeia Governorate. Symptoms of powdery mildew became common on leaves, stems inflorescences and fruits as white irregular areas. These symptoms appeared at vegetative and early flowering stages then gradually increased through fruiting and pre-maturity stages. Samples of infected leaflets, stem, inflorescences and fruits were collected for examination by light and scanning electron microscope (SEM). Microscopic examination revealed that conidiophores were short, erect–69 × 6–10 μm in dimension, conidia were observed without conspicuous fibrosin bodies singly, elliposid to ovoid 25–33 × 10–16 μm in dimension, and the length to width ratio of conidia ranged from 1.7 to 2.0 and were produced singly. Cylindrical foot cells (22.0 × 8.0 μm) were followed by one or two shorter cells (12.5 × 7.5 μm). In spring, the sexual stage (cleistothecia) appeared on infected leaves and stems in spherical, gregarious measures 105–117 (111) × 100– 87.5 μm in diameter. Each cleistothecium contained (2–4) round to ovoid asci, 45–55 (50) × 45–25 (35) μm in dimension. The ascus contained (3–4) ellipsoid to ovoid ascospores, 20–17.5 × 15–10 (13.2) μm. Cleistothecia appendages are simple myceloid branched tips measuring 80–200 (140) μm in length and 3–5 (4) μm in diameter. Based on the observations of the morphology of its anamorph and teleomorph stages, the causal agent of dill powdery mildew was identified as Erysiphe heraclei which is reported for the first time in Egypt.     

Phylogeny and taxonomy of powdery mildew fungi of Erysiphe sect. Uncinula on Carpinus species

A phylogenetic analysis of the Erysiphe with uncinuloid ascoma appendages (Erysiphe section Uncinula, Erysiphales, Ascomycota) on Carpinus spp. was done using sequences of the rDNA ITS regions and the D1/D2 domains of the 28S rDNA. These results, combined with morphological data, revealed a complex consisting of several distinct taxa. These included the already described Erysiphe carpinicola on C. japonica distinguishable from the Erysiphe sp. on C. betulus and C. tschonoskii as well as the one on C. laxiflora. Thus, it was shown that Oidium carpini, described from Europe on Carpinus betulus, the powdery mildew with uncinula-like ascomata, recently found in Europe on this host, as well as an Erysiphe on C. tschonoskii in Japan, described previously as E. carpinicola, all belong to a single new species, named E. arcuata in this paper. As the powdery mildew on C. laxiflora was also distinct from other known species, it is named E. carpini-laxiflorae in this paper. The already described E. pseudocarpinicola and Erysiphe sp. on Carpinus cordata are two additional taxa, which are morphologically and genetically distinguished from the other species of Erysiphe sect. Uncinula on Carpinus spp.     

Polyamines in discrete regions of barley leaves infected with the powdery mildew fungus, Erysiphe graminis

The concentrations of putrescine, spermidine and spermine and the activities of arginine decarboxylase (ADC; EC 4.1.1.19) and ornithine decarboxylase (ODC: EC 4.1.1.17) were determined in discrete regions of barley leaves ( Hordeum vulgare L. cv. Golden Promise) infected with the powdery mildew fungus ( Erysiphe graminis f.sp. hordei Marchal). Polyamine concentrations and the activities of both enzymes were always greatest within the region surrounding the fungal pustule, with the lowest values always being found in the region furthest away from the pustule. Although the concentrations of the three amines and ADC and ODC activities within the fungal pustule were always less than values from the zone surrounding the pustule, these differences were never significant. Polyamine concentrations and ODC activity were not significantly reduced, and ADC activity remained unchanged in mildewed leaves with all surface fungal growth removed. It would appear therefore that not only does most of the increase in amines and ODC activity reside in the leaf itself, but that very little of this increase is due to fungal growth and sporulation. Furthermore, it seems possible that the increase in polyamines in mildewed barley could be involved in 'green-island' formation, where regions around mildew pustules remain green and physiologically active while the rest of the leaf senesces.     

Mapping of five resistance genes to sugar-beet powdery mildew using AFLP and anchored SNP markers

Sugar-beet powdery mildew, caused by the fungus Erysiphe betae, now occurs in all sugar-beet growing areas and can reduce sugar yield by up to 30%. Powdery mildew resistant plants from three novel sources were crossed with sugar beet to generate segregating populations. Evaluation of resistance was carried out in artificially inoculated field and controlled environment tests. The resistance level in two of the sources was found to be significantly higher than that in currently available sugar-beet cultivars. AFLP analysis was used in combination with bulked segregant analysis to develop markers linked to the resistant phenotype in each population. Five dominant major resistance genes were identified and assigned the proposed symbols Pm2 to Pm6. Pm3 conferred complete resistance to powdery mildew; the other genes conferred high levels of partial resistance. From the use of anchoring SNP markers, two genes were located to chromosome II and three to chromosome IV. Two of the genes on chromosome IV mapped to the same location and one of the genes on chromosome II mapped to the same region as the previously identified Pm1 gene. With the availability of these genes there is now excellent potential for achieving durable resistance to sugar-beet powdery mildew, thus reducing or obviating the need for chemical control.     

Some observations on assessing Phoma betae infection of sugar-beet seed

Near-ultraviolet or ‘black’ light applied continuously from the start of incubation, facilitates tests for Phoma betae on sugar-beet seed by stimulating the production of pycnidia and restricting mycelial growth of P. betae and other fungi. Pretreatment of the seed with dilute sodium hypochlorite decreases the number of seeds with P. betae by removing superficial infection, but some of this is of significance in the field. Rubbing beet seed also decreased counts of P. betae in the laboratory and increased field emergence, primarily by removing the fungus. Griseofulvin sprayed on beet-seed plants either 2 wk or 2 days before harvest significantly decreased seed infection with P. betae, but not to a level at which further seed treatment could be omitted.     

Identity of a powdery mildew fungus occurring on Paeonia and its relationship with Erysiphe hypophylla on oak

A powdery mildew fungus found on Paeonia lutea at the Botanical Garden of Geneva (Switzerland) was identified as Erysiphe hypophylla based on morphological observations. The occurrence of E. hypophylla on Paeonia seemed curious, because host plants of this species have been restricted to a few Quercus species of the family Fagaceae. In this study, we determined the rDNA sequences of the powdery mildew specimens on Paeonia and E. hypophylla on Quercus to confirm the identity of the Paeonia fungus. The three sequences from the specimens on P. lutea were identical to one another in both ITS and 28S rDNA regions and also to the sequences of E. hypophylla on Q. robur, which supports the identification that the fungus on P. lutea is E. hypophylla. However, these sequences were also identical to the sequences of E. alphitoides on Quercus spp. and Oidium mangiferae on mango. This result suggests a possibility that E. hypophylla is conspecific to E. alphitoides. Further study is required to clarify whether E. hypophylla is a synonym of E. alphitoides or a distinct species.     

The influence of powdery mildew (Erysiphe graminis) infection on the development of four spring wheats

Infection with Erysiphe graminis reduced the weight of the roots and aerial parts of four wheat cultivars of contrasting susceptibility at the three-leaf stage. It had little effect on the more resistant varieties at the five-leaf and flag-leaf stages. When ¹⁴C was supplied to the youngest fully expanded leaves at successive stages of growth, the proportion recovered from the roots, and that translocated to other tillers when present, was reduced, especially in the more susceptible cultivars.