A Review of Apothecial Production by Monilinia Fungi in Japan

Genome sequence of Monilinia vaccinii-corymbosi sheds light on mummy berry disease infection of blueberry and mating type

Mummy berry disease, caused by the fungal pathogen Monilinia vaccinii-corymbosi (Mvc), is one of the most economically important diseases of blueberries in North America. Mvc is capable of inducing two separate blighting stages during its life cycle. Infected fruits are rendered mummified and unmarketable. Genomic data for this pathogen is lacking, but could be useful in understanding the reproductive biology of Mvc and the mechanisms it deploys to facilitate host infection. In this study, PacBio sequencing and Hi-C interaction data were utilized to create a chromosome-scale reference genome for Mvc. The genome comprises nine chromosomes with a total length of 30 Mb, an N50 length of 4.06 Mb, and an average 413X sequence coverage. A total of 9399 gene models were predicted and annotated, and BUSCO analysis revealed that 98% of 1,438 searched conserved eukaryotic genes were present in the predicted gene set. Potential effectors were identified, and the mating-type (MAT) locus was characterized. Biotrophic effectors allow the pathogen to avoid recognition by the host plant and evade or mitigate host defense responses during the early stages of fruit infection. Following locule colonization, necrotizing effectors promote the mummification of host tissues. Potential biotrophic effectors utilized by Mvc include chorismate mutase for reducing host salicylate and necrotrophic effectors include necrosis-inducing proteins and hydrolytic enzymes for macerating host tissue. The MAT locus sequences indicate the potential for homothallism in the reference genome, but a deletion allele of the MAT locus, characterized in a second isolate, indicates heterothallism. Further research is needed to verify the roles of individual effectors in virulence and to determine the role of the MAT locus in outcrossing and population genotypic diversity.

Phenotyping Brown Rot Susceptibility in Stone Fruit: A Literature Review with Emphasis on Peach

Plant disease phenotyping methodologies can vary considerably among testers and often suffer from shortcomings in their procedures and applications. This has been an important challenge in resistance breeding to brown rot, one of the most severe pre-and postharvest stone fruit diseases caused by Monilinia spp. Literature about methodologies for evaluating stone fruit susceptibility to brown rot is abundant but displays significant variations across the described approaches, limiting the ability to compare results from different studies. This is despite the fact that authors largely agree on the main factors influencing brown rot development, such as Monilinia inocula, environmental conditions, cultivars, fruit stage, and management practices. The present review first discusses ways to control or at least account for major factors affecting brown rot phenotyping studies. The second section describes in detail the different steps of fruit infection assays, comparing different protocols available in the literature with the objective of highlighting best practices and further improvement of phenotyping for brown rot susceptibility. Finally, experimental results from multi-year evaluation trials are also reported, highlighting year-to-year variability and exploring correlations of evaluation outcomes among years and assay types, suggesting that choice of phenotyping methodology must be carefully considered in breeding programs.

Systematics, ecology, and application of Helotiales: Recent progress and future perspectives for research with special emphasis on activities within Japan

Helotiales is one of the most diverse groups of apothecial ascomycetes, including 3000-4000 taxa. Recent progress in the systematics, ecology, and their applications through research is herein reviewed based on the experiences of the author with a special emphasis on activities in Japan. In the past 30 y, more than 50 helotialean taxa have been added to the mycobiota of Japan, including new taxa. With the advent of molecular phylogeny, some families have been revisited, such as members with stroma (Sclerotiniaceae and Rutstroemiaceae) or hairs (Hyaloscyphaceae and Lachnaceae). Although the monophyly of Helotiales has not yet been demonstrated, our understanding of its phylogeny has greatly advanced. The unexpected ecological nature represented by endophytism has been revealed through barcoding and other molecular techniques. The research history of ash dieback is also reviewed, and the endophytism/saprophytism of the pathogen on its original host is discussed. Drug discoveries within Helotiales are reviewed, and successful examples are presented. As future perspectives, both the cumulation of occurrence and sequence data of Helotiales is greatly encouraged to elucidate this important group of fungi.

Preservation of Monilinia fructicola Genotype Diversity Within Fungal Cankers

Monilinia fructicola is a destructive pathogen causing brown rot on stone fruits worldwide. Though it is best known as a fruit rot pathogen, M. fructicola also causes blossom blight and, subsequently, twig cankers in the spring. Orchard management strategies often overlook cankers as an inoculum source, though they are an inoculum source of both blossom and fruit infections. In this study, we analyzed the role of cankers as storage structures for diverse genotypes of M. fructicola, examining whether multiple genotypes can be transmitted from blossom to canker. Fungal spores from blossoms, and 2 months later from their corresponding cankers, were collected from a conventional and an unsprayed orchard in 2015 and 2016. Simple sequence repeat markers were used to genotype 10 to 20 single spores from each of four blossom/canker pairs per orchard. Individual blossoms and cankers were detected containing up to four and five genotypes, respectively. The average number of genotypes in blossoms and corresponding cankers were not significantly different (P = 0.690) across both years and farms, showing that a bottleneck for genetic diversity was not generated during the transition from blossom to canker. The average number of genotypes unique to blossom or canker was not significantly different (P = 0.569) and no significant effect of farm (P = 0.961) or year (P = 0.520) was observed, although blossoms had a numerically greater number of unique genotypes in both cases. In conclusion, a single blossom may be infected by one or more genotypes of M. fructicola, and this diversity is being preserved in the corresponding canker. This information implicates M. fructicola cankers as diversity storehouses, and may also apply to other Monilinia spp. and fungal diseases that initiate in reproductive tissue.

Mating System in the Brown Rot Pathogens Monilinia fructicola, M. laxa, and M. fructigena

Monilinia fructicola, M. laxa, and M. fructigena are the most important pathogens responsible for brown rot disease of stone and pome fruits. Information on their mating system and sexual behavior is scant. A mating-type-specific PCR-based assay was developed and applied to 155 Monilinia isolates from 10 countries and 10 different host plants. We showed that single isolates carry only one of two opposite idiomorphs at the MAT1 locus consistent with a heterothallic mating system for all three species. MAT1-1 and MAT1-2 mating types were detected in similar proportions in samples of isolates of each species and hence there do not appear to be genetic obstacles to the occurrence of sexual reproduction in their populations. Inter simple sequence repeat markers suggested that asexual reproduction is prevalent, but that sexual recombination occurs in M. fructicola populations in Italy. The genetic architectures of the MAT1 loci of the three pathogens were analyzed. MAT1-1 and MAT1-2 idiomorphs are flanked upstream and downstream by the APN2 and SLA2 genes and resemble those of Botrytis cinerea and other heterothallic fungi in the family Sclerotiniaceae. Each idiomorph contains a specific couple of genes, MAT1-1-1 (with alpha-box domain) and MAT1-1-5 in MAT1-1, and MAT1-2-1 (with HMG-box domain) and MAT1-2-10 in MAT1-2. Small gene fragments (dMAT1-1-1 and dMAT1-2-1) from the opposite idiomorph were detected close to their flanking regions. Constitutive expression of the four MAT1 genes during vegetative growth was ascertained by transcriptomic analysis (RNA-Seq). Antisense transcription of the MAT1-1-1 and MAT1-2-1 genes and intergenic transcribed regions of the MAT1 locus were detected. These results represent new insights into the mating systems of these three economically-important pathogens which could contribute to improve the knowledge on their population biology.

Intraspecific Diversity of Monilinia fructicola and M. laxa Populations from Blossoms and Fruit of Different Hosts in Greece

The genetic variation among 145 isolates of Monilinia fructicola and 156 isolates of M. laxa collected from two distinct regions of Greece (Imathia and Larissa) was analyzed using intersimple-sequence repeat markers. The Monilinia spp. isolates had been collected from infected fruit or blossoms of peach, apricot, sweet cherry, and plum. Calculation of Nei's gene diversity and Shannon's diversity indices showed that M. fructicola populations had higher genetic diversity compared with M. laxa populations in both regions sampled. The levels of genetic diversity were similar between populations obtained from diseased blossoms and fruit for each species and the main variances were all from within rather than between populations for the respective regions, hosts, and organ of origin. Genetic distance (Nei's analysis) was lower between peach and apricot populations than between cherry and plum populations of M. fructicola. M. fructicola isolates from peach and apricot and from sweet cherry and plum were clustered together, while M. laxa isolates clustering based on the host of origin was not possible. The analysis of index of association showed the absence of sexual recombination for both species. The derived data support the hypothesis of a long presence of M. fructicola in Greece, and provide evidence of specialization of M. fructicola populations based on their host of origin.

Primary Inoculum Sources of Monilinia spp. in Spanish Peach Orchards and Their Relative Importance in Brown Rot

Immediately following the identification of Monilinia fructicola in a Spanish peach orchard in the Ebro Valley in 2006, this orchard and two other orchards in the same valley were intensively sampled for potential tree and ground sources of primary Monilinia inoculum before and during three growing seasons between 2006 and 2008. Overwintered Monilinia spp. produced inoculum from only mycelium, and no apothecia were found in any of the three orchards over the three growing seasons. Mummies on trees were the main source of primary inoculum. More than 90% of Monilinia isolates on all fruit mummies were M. laxa. Positive relationships were found between (i) the number of mummified fruit and the incidence of postharvest brown rot (P = 0.05, r = 0.75, n = 8), and (ii) the number of mummified fruit and nonabscised aborted fruit in the trees and the number of conidia on the fruit surface (P = 0.04, r = 0.71; P = 0.01, r = 0.94, respectively, n = 8) and the incidence of latent infection (P = 0.03, r = 0.75; P = 0.001, r = 0.99; respectively, n = 8). In addition, the numbers of mummified fruit and pruned branches on the orchard floor were correlated with the number of airborne conidia in the orchard. Based on the results of these surveys, the control of brown rot in stone fruit orchards is discussed.

Development of Apothecia from Stone Fruit Infected and Stromatized by Monilinia fructicola in California

Apothecia were produced in the orchard, lath house, and laboratory from peach and nectarine fruit infected and stromatized by Monilinia fructicola. Fully stromatized "mummies" and nonstromatized infected fruit were placed in the orchard either on the soil surface or buried to a depth of 2 to 3 cm. Mummies were placed in the orchard at monthly intervals from August to February in 1993-94 and 1994-95. Nonstromatized infected fruit, which were fleshy and decomposed rapidly, were soon unavailable and were only placed in the orchard in August and September. Apothecia developed in February and early March only from mummies that were placed in the orchard in either October, November, or December. Mummies placed in the field in August, September, January, and February did not produce apothecia. Leaving mummies on the soil surface versus burying them 2 to 3 cm did not affect the development of apothecia. Apothecia were never produced from nonstromatized or recently-infected (fleshy) fruit. In the laboratory, apothecia were only produced from mummies that were partially buried in moist sand and stored without light at 2°C and >97% relative humidity (RH) for more than 8 weeks prior to incubation for 2 weeks (12, 15, or 20°C) with a 12-h photoperiod. Mummies that were incubated at >97% RH for less than 8 weeks or incubated at <90% RH never produced apothe-cia when stored at 2°C and then transferred to warmer temperatures with light. In orchard experiments, apothecia were only observed in plots with nondisturbed orchard floor vegetation; whereas no apothecia were found in either herbicide-treated or rototilled plots. Apothecia in the San Joaquin Valley were only produced from mummies that were subject to an 8-week or greater cold-temperature incubation while in contact with soil.

Effect of Temperature on the Discharge and Germination of Ascospores by Apothecia of Monilinia fructicola

Naturally growing apothecia of Monilinia fructicola were collected from two commercial plum orchards near Reedley and Sanger, both in Fresno County, California. Ascospore discharges from 90 (1996) and 86 (1997) apothecia were monitored individually using spore traps at four constant temperatures. The period of discharge decreased as temperature increased from 10 to 25°C. However, daily discharge increased as temperature increased from 10 to 15°C and remained high at 20 and 25°C. The greatest discharge occurred with apothecia at 15°C, followed by those incubated at 20, 10, and 25°C. The germination of ascospores of M. fructicola and the length of germ tubes increased as temperature increased from 7 to 15°C; however, increasing temperatures above 15°C did not increase either ascospore germination or length of germ tubes. This information may help in the development of warning systems and management strategies for brown rot blossom blight of stone fruits.