Optimizing Use of DMI Fungicides for Management of Apple Powdery Mildew Caused by Podosphaera leucotricha in New York State

Powdery mildew, caused by the ascomycete Podosphaera leucotricha, is an endemic disease found wherever apples are grown that negatively impacts both tree vigor and fresh market yield. In the absence of durable host resistance, chemical management is the primary means of disease control. Demethylation inhibitor (DMI) fungicides are widely used to manage apple powdery mildew, but members within this fungicide class have been observed to differ in efficacy with respect to disease control. Moreover, debate exists as to the optimal timing of DMI fungicide applications for management of apple powdery mildew. In this regard, the goal of this study was to determine the best-use practices for DMI fungicides to manage apple powdery mildew in New York State. Multi-year trials were conducted to evaluate the potential differential efficacy performance of four common DMI fungicides, as well as additional trials to assess optimal application timing. In all years, we observed that treatments of flutriafol and myclobutanil consistently had the lowest incidences of powdery mildew compared to difenoconazole and fenbuconazole. In the 2018 and 2021 trials, the newly registered mefentrifluconazole was more comparable to the difenoconazole program with respect to powdery mildew disease incidence. We hypothesize that differences in DMI efficacy may be due to each fungicides’ water solubility and lipophilicity characteristics, and thus their ability to move systemically in the host or more easily penetrate the surface of germinating conidia. Applications timed between petal fall and first cover resulted in the lowest incidence of powdery mildew on terminal leaves of apple shoots compared to applications timed prior to petal fall. These observations are contrary to previous studies conducted in regions with differing climates. We also found that the incidence of secondary powdery mildew observed two weeks after petal fall was influenced by applications of DMI fungicides during the previous season. For example, management programs consisting of applications of flutriafol or myclobutanil in the prior season tended to have lower incidence of apple powdery in the following spring, presumably owing to reductions in overwintering inoculum. Despite reports of DMI resistance in other apple pathosystems, the DMI fungicide class is still relevant for the successful management of apple powdery mildew in New York State.

Types and composition of diseases and pests of restructured forest and pasture plants in the dry part of the Aral Sea

Studies on the dried bottom of the Aral Sea have shown that dominant disease from 3 species of them in Haloxylon plant is Podosphaera leucotricha and the dominant species from 34 species of pests is Big Dwarf Haloxylon Locust (D. albidula) and Small Haloxylon Locust (D. annulata roseipen). Chemical and biological control measures were taken against this disease and pests. When using anti-inflammatory drugs Redomil gold, Dnox, Fundazol and Sporagin, their effectiveness is as follows: Redomil gold - 68% sp (2.0–2.5 kg / ha), Fundazol -50% ke (2.0 l / ha), Dnox - 40% ke (2.0 l / ha) and Sporagin (4.0, l / ha) were tested using fungicides in the norms. Of the drugs used in the experimental variant, Redomil gold, 68% s.p. (2.0-2.5 kg / ha) was the highest biological efficacy against powdery mildew in the variant in which the fungicides were applied, was 91.3%.

An Examination of Apple Powdery Mildew and the Biology of Podosphaera leucotricha (Ellis & Everh.) E. S. Salmon from Past to Present

Within is a comprehensive review of current apple powdery mildew literature, with the taxonomy, morphology, and life cycle of the causal phytopathogen Podosphaera leucotricha described. A discussion of economic impact, environmental aspects contributing to apple powdery mildew epidemiology, options for cultural, chemical, and biopesticide disease control, and fungicide resistance in P. leucotricha is also detailed. The present status of breeding for host resistance and apple cultivar susceptibility are discussed. Finally, emerging research into the biology and management of apple powdery mildew are discussed along with how they may aid future research endeavors.

High Genetic Diversity in Predominantly Clonal Populations of the Powdery Mildew Fungus Podosphaera leucotricha from U.S. Apple Orchards

Apple powdery mildew (APM), caused by Podosphaera leucotricha, is a constant threat to apple production worldwide. Very little is known about the biology and population structure of this pathogen in the USA and other growing regions, which impacts APM management. Two hundred fifty-three P. leucotricha isolates, sampled from 10 apple orchards in Washington, New York, and Virginia, were genetically characterized with novel single sequence repeat and mating type markers. Eighty-three multilocus genotypes (MLGs) were identified, most of which were unique to a given orchard. Each isolate carried either a MAT1-1 or MAT1-2 idiomorph at the mating type locus, indicating that P. leucotricha is heterothallic. Virulence tests on detached apple leaves showed that the 10 most frequent P. leucotricha MLGs were not avirulent on a line containing a major resistance gene. Analysis of molecular variance showed significant differentiation (P > 0.001) among populations, a result supported by principal coordinate analysis revealing three genetic groups, each represented by non-overlapping MLGs from Washington, New York, and Virginia. A Bayesian cluster analysis showed genetic heterogeneity between Washington populations, and a relative migration analysis indicated substantial gene flow among neighboring orchards. Random mating tests indicated that APM epidemics during the active cycle were dominated by clonal reproduction. However, the presence of sexual structures in orchards, the likelihood that five repeated MLGs resulted from sexual reproduction, and high genotypic diversity observed in some populations suggest that sexual spores play some role in APM epidemics. IMPORTANCE Understanding the population biology and epidemiology of plant pathogens is essential to develop effective strategies for controlling plant diseases. Herein, we gathered insights into the population biology of P. leucotricha populations from conventional and organic apple orchards in the United States. We showed genetic heterogeneity between P. leucotricha populations in Washington, and structure between populations from different U.S. regions, suggesting that short-distance spore dispersal plays an important role in the disease’s epidemiology. We presented evidence that P. leucotricha is heterothallic, and that populations likely result from a mixed (i.e., sexual and asexual) reproductive system, revealing that the sexual stage contributes to apple powdery mildew epidemics. We showed that the major resistance gene Pl-1 is valuable for apple breeding because virulent isolates have most likely not emerged yet in U.S. commercial orchards. These results will be important to achieve sustainability of disease management strategies and maintenance of plant health in apple orchards.

In Vitro and In Vivo Evaluation of Silver Nanoparticles Phytosynthesized Using Raphanus sativus L. Waste Extracts

The aim of the current paper is the development of phytosynthesized silver nanoparticles mediated by Raphanus sativus L. extracts obtained through two extraction methods (temperature and microwave) and to test their potential application for controlling apple crops pathogens. The phytosynthesized materials were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. All the materials were evaluated in terms of antioxidant and in vitro antimicrobial activity (against bacteria, molds, and yeast: Escherichia coli ATCC 8738, Staphylococcus aureus ATTC 25923, Pseudomonas aeruginosa ATCC 9027, Salmonella typhimurium ATCC 14028, Candida albicans ATCC 10231, Venturia inaequalis, Podosphaera leucotricha, Fusarium oxysporum ATCC 48112, Penicillium hirsutum ATCC 52323, and Aspergillus niger ATCC 15475). Considering the results obtained in the in vitro assays, formulations based on nanoparticles phytosynthesized using Raphanus sativus L. waste extracts (RS1N) were evaluated as potential antifungal agents for horticultural crops protection, against Venturia inaequalis and Podosphaera leucotricha through in vivo assays. For the DPPH assay, the inhibition (%) varied between 37.06% (for RS1N at 0.8 mg/mL concentration) and 83.72% (for RS1N at 7.2 mg/mL concentration) compared to 19.97% (for RS2N at 0.8 mg/mL) and only 28.91% (for RS2N at 7.2 mg/mL). Similar results were obtained for RS3N (85.42% inhibition at 7.2 mg/mL) compared with RS4N (21.76% inhibition at 7.2 mg/mL). Regarding the ABTS assay, the highest scavenger activity values were recorded for samples RS1N (91.43% at 1.6 mg/mL) and RS3N (96.62% at 1.6 mg/mL).

No Evidence of Resistance to Trifloxystrobin, Triflumizole, and Boscalid in Podosphaera leucotricha Isolates from U.S. Commercial Apple Orchards

Apple powdery mildew, caused by Podosphaera leucotricha, continues to be a challenge in commercial apple orchards in the U.S. Pacific Northwest and worldwide. In this study, P. leucotricha isolates were collected in 2018 and 2019 from two organic (baseline) and eight conventional (exposed) apple orchards in Washington, New York, and Virginia, and assessed for their sensitivity to trifloxystrobin (TRI, n= 232), triflumizole (TFZ, n = 217), and boscalid (BOS, n = 240) using a detached leaf assay. Effective concentrations inhibiting 50% growth (EC50) were not significantly different between baseline and exposed isolates, and ranged from 0.001 to 0.105, 0.09 to 6.31, and 0.05 to 2.18 µg/ml, for TRI, TFZ, and BOS, respectively. Reduction in sensitivity by factors of 105, 63 and 22 to TRI, TFZ, and BOS, respectively, were observed in some isolates, but all isolates were controlled by the commercial label rates of the three fungicides on detached leaves. Sequencing of the cytochrome b (cytb), cytochrome P450 sterol 14α-demethylase (CYP51), and the iron-sulfur protein subunit (SdhB) genes in isolates with high EC50 revealed no mutation previously reported to confer resistance to these fungicides in other fungi, and presence of a group I intron after codon 143 in the cytb gene. Significant (P < 0.001) moderate positive correlations (r = 0.38) observed between sensitivity to TRI and TFZ warrant continuous rotations of fungicides with different modes of actions in conventional orchards. The established baseline sensitivities and the molecular markers will help in selecting discriminatory doses and by-passing the challenging in vivo testing for future sensitivity monitoring in P. leucotricha.

Mobilization of genetic diversity of the genus Malus on the basis of information technologies for the breeding of varieties with complex resistance to fungal pathogens

Breeding renewal of the southern assortment of apple trees for the Russian fruit growing industry based on the accelerated process of creating domestic genotypes with a complex increased resistance to the main fungal pathogens of the culture (Venturia inaequalis (Cooke) Wint., Podosphaera leucotricha (Ell. & Ev.) E.S. Salmon, Phyllosticta mali Pr. et. Del., Phyllosticta briardi Sacc.) is due to the systematic use of modern methods, innovative breeding techniques and improved techniques. The purpose of the study is to develop and create a database of representatives of the genus Malus Mill., which is promising for use in the production and accelerated breeding of new domestic genetic resources for the southern region of Russia. During the research work, Russian modern and generally accepted programs and methods of breeding, variety study for fruit plants were used. The formed database “Main economically valuable, morphological and molecular-genetic characters of representatives of the genus Malus Mill. with resistance to the main fungal pathogens in the south of Russia”; filed an application for its patent. Using the data of the obtained information system will significantly speed up the solution of practical selection problems and reduce the costs of organizing the breeding process.

A Genome Resource for the Apple Powdery Mildew Pathogen Podosphaera leucotricha

Powdery mildew, caused by Podosphaera leucotricha, is an economically important disease of apple and pear trees. A single monoconidial strain (PuE-3) of this biotrophic fungus was used to extract DNA for Illumina sequencing. Data were assembled to form a draft genome of 43.8 Mb consisting of 8,921 contigs, 9,372 predicted genes, and 96.1% of complete benchmarking universal single copy orthologs (BUSCOs). This is the first reported genome sequence of P. leucotricha that will enable studies of the population biology, epidemiology, and fungicide resistance of this pathogen. Furthermore, this resource will be fundamental to uncover the genetic and molecular mechanisms of the apple−powdery mildew interaction, and support future pome fruit breeding efforts.