Relationships between Oak powdery mildew incidence and severity and commensal fungi

Oak (Quercus robur) powdery mildew is a common and damaging fungal disease. In a local survey at Reading, UK, oak powdery mildew was common on trees of all height classes but was most common on trees of 3–9 m. A variety of other fungal species were commonly found growing in association with oak powdery mildew colonies. The abundance of such fungi was estimated through stratified sample surveys for 2.5 years. The taxa most commonly associated with oak powdery mildew were Acremonium sp., Trichoderma sp., Ampelomyces/Phoma sp. and Leptosphaerulina australis. Nearly 90% of mildew colonies were associated with L. australis, which is not generally considered as a mycoparasite or antagonist, in contrast with the other three fungi. Abundance varied between June and October surveys. Acremonium sp. abundance was greater in summer samplings, whereas L. australis and Trichoderma sp. abundances were greater in autumn samplings. Ampelomyces/Phoma sp. was never observed in the absence of powdery mildew. Relationships between the mildew‐associated fungi and oak powdery mildew appeared curved and differed significantly between sampling years. L. australis was positively correlated with the other three associated fungi studied when powdery mildew was also present . The variety and high population densities of the mildew‐associated fungi suggest that they may be important in determining the final density of oak mildew and the damage caused by it.     

Identification,pathogenicity and control of powdery mildew caused by Erysiphe elevata on Eucalyptus pellita in Sumatra,Indonesia

Powdery mildews are biotrophic ascomycetes that do not typically kill their hosts, but instead interfere with normal plant growth. In Indonesia, white powdery spots were observed forming on the upper part of fully developed leaves of Eucalyptus pellita planted in sand beds of a clonal nursery. Thus, this study aimed to identify the causal agent of eucalypt powdery mildew in Indonesia as well as to develop control strategies to better manage the disease. Pathogen identification was based on morphology, molecular characterization of DNA sequencing the ITS region and pathogenicity test. In addition, considering that some powdery mildews are inhibited by free moisture on leaves, efficacy of foliar watering was tested for disease control. Phylogenetic analyses showed that isolates collected from infected eucalypt plants grouped together with other sequences of the Erysiphe elevata clade with bootstrap support of 99%; a similar result was also confirmed by morphological observations. Pathogenicity of powdery mildew on eucalypt plants was confirmed through artificial inoculation, thus fulfilling the Koch's postulates. For disease control, foliar watering reduced the incidence of powdery mildew on mother plants. The reduction of the disease incidence began to be more evident at 2 weeks after trial establishment, and in the fourth week, the incidence was below 10% in all three treatments, while the control was above 20%. Therefore, we concluded that E. elevata is the causal agent of powdery mildew in Indonesia and that one application of foliar watering per week was enough to minimize the disease impact in the nursery.     

Erysiphe australiana: the cause of powdery mildew on Lagerstroemia speciosa in Brazil

Powdery mildew symptoms and signs were observed on seedlings of Lagerstroemia speciosa within an ornamental plant nursery in Viçosa, State of Minas Gerais, Brazil. Signs were most prominent as intense mycelial growth and sporulation on the shoots, new branches and leaves, which led to tissue necrosis. Based on morphological characteristics and phylogenetic analyses of ITS and 28S rDNA sequences, the powdery mildew pathogen of L. speciosa was identified as Erysiphe australiana, a well‐known powdery mildew pathogen infecting Lagerstroemia spp. worldwide. To our knowledge, this is the first report of E. australiana causing powdery mildew on L. speciosa in Brazil.     

Erysiphe alphitoides causes powdery mildew on Eucalyptus gunnii

In June 2016, pot‐grown Eucalyptus gunnii plants showing typical symptoms of powdery mildew disease were found in a greenhouse in Korea. Morphological characteristics of the anamorph and molecular sequence analyses of the ITS regions and 28S rDNA verified the fungus as Erysiphe alphitoides, which is known as common and widespread powdery mildew on Quercus spp. Although E. alphitoides has been reportedly associated with powdery mildew diseases of tropical trees and shrubs other than Quercus spp., this is the first finding of E. alphitoides on a Eucalyptus tree worldwide.     

Powdery mildew disease of rubber tree

Powdery mildew disease of rubber trees affects young rubber leaves, buds, inflorescences and other immature tissues reducing rubber latex yields by up to 45%. The disease is present in all rubber‐growing regions, with a high incidence rate in subtropical environments. The causal agent of powdery mildew disease of rubber trees was first described as Oidium heveae, but later research on the morphological characteristics suggests that O. heveae was in the past confused with Erysiphe quercicola. However, the most appropriate classification is still under debate between the genera Golovinomyces or Podosphaera. Molecular and phylogenetic analyses have established some relationships between rubber powdery mildew fungus and other powdery mildew species, but the generic sexual state of rubber powdery mildew fungus has not yet been established. Rubber powdery mildew conidia are wind‐spread spores that are produced in great numbers when growth conditions of the disease are favourable. The optimum conditions for conidial germination are 97–100% relative humidity and temperatures between 25 and 28°C. While some newly bred rubber clones have shown resistance to the disease, it can also be controlled with a number of fungicides and biological control agents. It is clear from recent knowledge about climate–fungus relationships that changes in weather strongly influence disease incidence and severity. The aim of this review is to highlight the classification conflicts, main causes and influencing factors behind the disease spreading, as well as draw attention to the impact of weather changes on the outbreaks of the disease. The information in this review will be helpful to adopt better control measures of the powdery mildew disease of rubber, especially in higher humidity areas, thereby minimizing the loss of rubber yields due to this disease.     

Coffee‐leaf extract and phosphites on the curative control of powdery mildew in eucalyptus mini‐stumps

The aim of this study was to evaluate the effectiveness of a plant extract obtained from the leaves of coffee plants infected by Hemileia vastatrix (NEFID), different formulations of phosphites and the combination of the extract and phosphites in controlling powdery mildew in eucalyptus mini‐stumps, as well as to study the effects on the production of mini‐cuttings and their rooting percentage. The experiments were conducted on two eucalyptus hybrids (‘urocam’ and ‘urograndis’) in a clonal garden and greenhouse. First, the NEFID plant extract and phosphites of copper, zinc, potassium and potassium/manganese were evaluated to determine their effectiveness in controlling powdery mildew and their toxic effect on the fungus' morphology. Subsequently, we analysed the direct fungitoxicity, curative efficiency and anti‐sporulating action of the NEFID extract and various combinations of phosphites with or without the extract, and we also evaluated their effects on shoot production and rooting in eucalyptus mini‐cuttings. The NEFID plant extract and phosphites of Cu, Zn and K/Mn were more effective than the fungicide pyraclostrobin + epoxiconazole in the control of powdery mildew. Potassium phosphite was not effective in controlling the pathogen. Among the tested mixtures, the combination of 50% NEFID plant extract and 50% copper phosphite was the most effective because it exhibited direct toxicity on powdery mildew, a high curative efficiency, anti‐sporulating action and no adverse effects on the production of shoots or rooting in eucalyptus mini‐cuttings.     

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

Powdery 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 (ITS_A, ITS_B, ITS_C and ITS_D). 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 ITS_A), Erysiphe hypophylla (99.4% homology with ITS_C) and Phyllactinia guttata (97.64% homology with ITS_D). Our data also confirmed the 100% homology between ITS_A and the sequence described for Oïdium mangiferae, the agent of mango powdery mildew. The fourth haplotype, i.e. ITS_B, 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.     

Outbreak of powdery mildew caused by Sawadaea negundinis on boxelder in Iran

Boxelder (Acer negundo) is planted as ornamental and shade tree in many countries including Iran. During July 2016, boxelder trees in parks and streets of different localities in East Azarbaijan province, Iran were found to be heavily infected with powdery mildew. Whitish powdery fungal mass developed mainly on the lower leaf surface. The infected young leaves became bubbled, curled and crinkled, giving affected plants an unpleasant appearance. During September, scattered, dark brown, spherical chasmothecia were produced on the lower leaf surface. The causal agent was identified as Sawadaea negundinis based on a combination of morphological characteristics of sexual and asexual morphs and sequence data of the rDNA internal transcribed spacer region (ITS1‐5.8S‐ITS2). This study provides the first report on the occurrence of S. negundinis on boxelder in Iran.     

Molecular identification of the powdery mildew fungus infecting rubber trees in China

Powdery mildew is a foliar disease of rubber trees. It is one of the most important leaf diseases in rubber plantations worldwide. The causal agent of this powdery mildew, previously known as Oidium heveae, has been identified under various names. To precisely identify the causal agents of this disease, 57 conidial samples were collected from 2013 to 2017 in Hainan, Yunnan and Guangdong provinces, the main rubber tree planting regions in China. Based on morphological observations using light and scanning electron microscopy, pathogenicity testing, and nrDNA ITS and 28S sequences, Erysiphe quercicola is suggested as the only causal agent of rubber tree powdery mildew in China.     

First report of Erysiphe kenjiana causing powdery mildew of elm in Romania

Erysiphe kenjiana is an Asian fungus causing powdery mildew of Ulmus spp. In 2017, 10 years after its introduction in Europe, it was detected in Romania on Ulmus pumila, the Siberian elm, which is cultivated in urban environments as an ornamental and shade tree. The identification of the fungus was based on morphological characteristics and molecular data (rDNA internal transcribed spacer region). This is the first report of E. kenjiana in Romania, and only the third from a European country.     

Current status of the Dutch elm disease pathogen populations affecting Ulmus minor in Greece

One hundred and eight Ophiostoma ulmi sensu lato isolates were collected from field elm trees with symptoms in 14 Prefectures of Greece. The purpose of this study was to assign Greek isolates to species and afterwards subspecies of the DED fungi and to analyse the genetic variability within the Greek populations of these pathogens. Isolates were compared with six reference strains belonging to O. ulmi and the two subspecies of O. novo‐ulmi. The structure of the population has been analysed by means of morpho‐physiological features (growth rates, colony morphology and fertility responses) and by DNA sequencing and PCR‐RFLP amplification of the cerato‐ulmin (cu) and the colony type (col1) gene regions. According to fertility tests, both subspecies of O. novo‐ulmi were detected in Greece, but none of the isolates collected was identified as O. ulmi. O. novo‐ulmi ssp. novo‐ulmi occurred more frequently than ssp. americana (73 and 35 isolates, respectively) and their ranges overlapped. All isolates that behaved as ssp. novo‐ulmi in the fertility tests had the cu, as well as the col1 profile of ssp. novo‐ulmi. Surprisingly, all isolates that behaved as O. novo‐ulmi ssp. americana in the fertility test had the cu, as well as the col1 (with one exception) profile of O. novo‐ulmi ssp. novo‐ulmi. A possible explanation for this inconsistency could be the occurrence of hybridization between the two subspecies in Greece.     

Photosynthetic traits and growth of Quercus mongolica var. crispula sprouts attacked by powdery mildew under free-air CO2 enrichment

To predict the performance of coppice forests with Japanese oak (Quercus mongolica var. crispula) in future changing environment, we studied the growth, photosynthesis, and powdery mildew (Erysiphe alphitoides) infection of sprouts of Japanese oak under free-air CO₂ enrichment. Elevated CO₂ reduced powdery mildew infection in both leaves of the shoot emerged in spring (1st flush) and the lammas and proleptic shoots (2nd flush) of sprouts. We observed significant increase in the net photosynthetic rate at growth CO₂ concentration (i.e., 370 and 500 μmol mol^?1 for ambient and elevated CO₂ treatments, respectively) in both 1st and 2nd flush leaves of sprouts grown under elevated CO₂. On the other hand, no significant increase in net photosynthetic rate under elevated CO₂ was found before cutting. The photosynthetic activity of 2nd flush leaves in the sprouts under ambient condition was greatly reduced by severe infection to powdery mildew. Growth of sprouts was enhanced in the elevated CO₂ condition. We conclude the growth enhancement in Japanese oak sprouts under elevated CO₂ in the present study was achieved not only by physiological response (i.e., photosynthetic stimulation) but also by disease interaction.     

First report on the Dutch elm disease pathogen Ophiostoma novo‐ulmi from Latvia

During 2017, Dutch elm disease was investigated on 67 Ulmus glabra and 29 Ulmus laevis samples from 28 sites in Latvia. The presence of Ophiostoma novo‐ulmi was detected in 61.2% of the samples from U. glabra and in 34.5% of the samples from U. laevis. In 11% of cases, hybrids between subsp. novo‐ulmi and subsp. americana were identified, particularly in the sites in the southern part of Latvia. This study presents the first verified record of Ophiostoma novo‐ulmi and its subspecies in Latvia.     

Quambalaria eucalypti a pathogen of Eucalyptus globulus newly reported in Portugal and in Europe

The Eucalyptus pathogen Quambalaria eucalypti has been reported from several subtropical and tropical countries of the Southern Hemisphere. During the course of a nationwide monitoring programme aimed at the detection of pathogens in commercial eucalypt plantations in Portugal, Q. eucalypti was found affecting Eucalyptus globulus. The identity of the pathogen was confirmed by sequence analyses of the internal transcribed spacer (ITS) region of the rDNA operon, as well as morphological characteristics. Quambalaria eucalypti is widespread throughout the eucalypt‐producing areas of Portugal. Pathogenicity studies revealed that different host genotypes showed various degrees of susceptibility to the pathogen. To our knowledge, this is the first report of Q. eucalypti on Eucalyptus spp. in Portugal and in the Northern Hemisphere.     

Erysiphe diffusa causing powdery mildew on Mimosa caesalpiniifolia in Brazil: A soya bean pathogen found on an important native Brazilian tree

Mimosa caesalpiniifolia: Fabaceae (sansão‐do‐campo or sabiá, in Brazil) is a small tree or shrub native to the Brazilian semi‐arid forest and widely used as a hedge plant in Brazil. Although of economic relevance, little is known of fungal pathogens infecting this plant. Recently, plants growing in an urban situation at Ouro Preto, and also on the roadside in Viçosa (state of Minas Gerais, Brazil), were found to be severely attacked by a powdery mildew which led to progressive leaf blight and serious defoliation. Based on morphology and molecular phylogenetic analysis of nuclear rDNA sequences, the fungus was identified as Erysiphe diffusa—a pathogen best known attacking soya bean, particularly in highland plantings.