Relationships Between Blueberry Flower Age, Pollination, and Conidial Infection by Monilinia vaccinii-corymbosi

Taxonomic Reclassification of the Fungal Pathogen Causing Dry Berry Disease of Caneberries into the Division Ascomycota as Monilinia rubi

As molecular genetic techniques improve and sequence data becomes available for more fungal species, taxonomic classifications historically based upon growth morphology alone are being revisited and occasionally reclassified. Herein, we present such an instance for the fungal pathogen that causes dry berry disease of caneberries. The organism was previously described as the basidiomycete fungus Rhizoctonia rubi based upon the pathogen's production of Rhizoctonia-like angular branching hyphae. Utilizing molecular genetic techniques unavailable when the pathogen was first characterized in 1959, three housekeeping gene regions (ITS, β-tubulin, and G3PDH) were sequenced across 13 contemporary dry berry isolates, as well as the original 1959 R. rubi type strain, CBS382.59. The resulting neighbor-joining, maximum likelihood, and Bayesian phylogenies for single and multilocus sequences provide strong evidence that the dry berry pathogen was misclassified. This data, in addition to revisiting in vivo macroscopic and microscopic growth morphology, again comparing contemporary dry berry isolates to the CBS382.59 type strain, suggests that the causal organism is a new species within the genus Monilinia that we propose be classified as Monilinia rubi. A transition from designation as a basidiomycete fungus to an ascomycete fungus could have implications on chemical management decisions, as well as the assumptions made about cell structure and the pathogen's putative life cycle.

The pollen virome of wild plants and its association with variation in floral traits and land use

Pollen is a unique vehicle for viral spread. Pollen-associated viruses hitchhike on or within pollen grains and are transported to other plants by pollinators. They are deposited on flowers and have a direct pathway into the plant and next generation via seeds. To discover the diversity of pollen-associated viruses and identify contributing landscape and floral features, we perform a species-level metagenomic survey of pollen from wild, visually asymptomatic plants, located in one of four regions in the United States of America varying in land use. We identify many known and novel pollen-associated viruses, half belonging to the Bromoviridae, Partitiviridae, and Secoviridae viral families, but many families are represented. Across the regions, species harbor more viruses when surrounded by less natural and more human-modified environments than the reverse, but we note that other region-level differences may also covary with this. When examining the novel connection between virus richness and floral traits, we find that species with multiple, bilaterally symmetric flowers and smaller, spikier pollen harbored more viruses than those with opposite traits. The association of viral diversity with floral traits highlights the need to incorporate plant-pollinator interactions as a driver of pollen-associated virus transport into the study of plant-viral interactions.

Fine-Scale Genetic Structure and Reproductive Biology of the Blueberry Pathogen Monilinia vaccinii-corymbosi

The fungus Monilinia vaccinii-corymbosi, a pathogen of Vaccinium spp., requires asexual and sexual spore production to complete its life cycle. A recent study found population structuring of M. vaccinii-corymbosi over a broad spatial scale in the United States. In this study, we examined fine-scale genetic structuring, temporal dynamics, and reproductive biology within a 125-by-132-m blueberry plot from 2010 to 2012. In total, 395 isolates of M. vaccinii-corymbosi were sampled from infected shoots and fruit to examine their multilocus haplotype (MLH) using microsatellite markers. The MLH of 190 single-ascospore isolates from 21 apothecia was also determined. Little to no genetic differentiation and unrestricted gene flow were detected among four sampled time points and between infected tissue types. Discriminant analysis of principal components suggested genetic structuring within the field, with at least K = 3 genetically distinct clusters maintained over four sampled time points. Single-ascospore progeny from eight apothecia had identical MLH and at least two distinct MLH were detected from 13 apothecia. Tests for linkage disequilibrium suggested that genetically diverse ascospore progeny were the product of recombination. This study supports the idea that the fine-scale dynamics of M. vaccinii-corymbosi may be complex, with genetic structuring, inbreeding, and outcrossing detected in the study area.

Population Structure of the Blueberry Pathogen Monilinia vaccinii-corymbosi in the United States

The fungus Monilinia vaccinii-corymbosi causes disease of blueberry (Vaccinium section Cyanococcus) shoots, flowers, and fruit. The objective of our research was to examine the population biology and genetics of M. vaccinii-corymbosi in the United States. A total of 480 samples of M. vaccinii-corymbosi were collected from 18 blueberry fields in 10 states; one field in Georgia, Massachusetts, Maine, Michigan, Mississippi, New Jersey, New York, Oregon, and Washington and nine fields in North Carolina. Analysis with 10 microsatellite markers revealed 247 unique multilocus haplotypes (MLHs), with 244 MLHs detected within 11 fields in the Northeast, Northwest, Midwest, and Southeast and three MLHs detected within seven fields in the Southeast United States. Genetic similarity and low genetic diversity of M. vaccinii-corymbosi isolates from the seven fields in the Southeast United States suggested the presence of an expansive, self-fertile population. Tests for linkage disequilibrium within 10 fields that contained ≥12 MLHs supported random mating in six fields and possible inbreeding and/or self-fertilization in four fields. Analysis of molecular variance, discriminate analysis of principal components, and Bayesian cluster analysis provided evidence for population structure and restricted gene flow among fields. This research represents the first comprehensive investigation of the genetic diversity and structure of field populations of M. vaccinii-corymbosi.

Arranging the bouquet of disease: floral traits and the transmission of plant and animal pathogens

Several floral microbes are known to be pathogenic to plants or floral visitors such as pollinators. Despite the ecological and economic importance of pathogens deposited in flowers, we often lack a basic understanding of how floral traits influence disease transmission. Here, we provide the first systematic review regarding how floral traits attract vectors (for plant pathogens) or hosts (for animal pathogens), mediate disease establishment and evolve under complex interactions with plant mutualists that can be vectors for microbial antagonists. Attraction of floral visitors is influenced by numerous phenological, morphological and chemical traits, and several plant pathogens manipulate floral traits to attract vectors. There is rapidly growing interest in how floral secondary compounds and antimicrobial enzymes influence disease establishment in plant hosts. Similarly, new research suggests that consumption of floral secondary compounds can reduce pathogen loads in animal pollinators. Given recent concerns about pollinator declines caused in part by pathogens, the role of floral traits in mediating pathogen transmission is a key area for further research. We conclude by discussing important implications of floral transmission of pathogens for agriculture, conservation and human health, suggesting promising avenues for future research in both basic and applied biology.

Infection of 'd'Anjou' Pear Fruit by Potebniamyces pyri in the Orchard in Relation to Phacidiopycnis Rot during Storage

Phacidiopycnis rot, caused by Potebniamyces pyri, is a recently recognized postharvest fruit rot disease of 'd'Anjou' pear (Pyrus communis) in the United States. To determine the timing of fruit infection in the orchard in relation to incidence of Phacidiopycnis rot during storage, fruit were inoculated in the orchard at different times during the growing season, harvested, and monitored for decay development during storage at 0°C. Fruit inoculated in the field and laboratory were also used to determine the infection courts and the importance of necrotic tissues to infection of sepals that may lead to calyx-end Phacidiopycnis rot. Phacidiopycnis rot was observed during cold storage on the fruit inoculated any time after bloom till near harvest but not on the fruit inoculated during bloom. Phacidiopycnis rot symptoms only developed at the stem and calyx end of the fruit during storage. Relatively more calyx-end rot was observed than stem-end rot on the fruit inoculated before August. Incidence of stem-end rot increased significantly on the fruit inoculated near harvest. Incidence of total Phacidiopycnis rot increased as the timing of fruit infection in the orchard approached harvest. Potebniamyces pyri was recovered more frequently from sepals than from styles and stamens of the fruit. Most infections on sepals were associated with the necrotic tissues. Naturally occurring necrotic tissues occurred on more than 85% and all sepals in the early fruit-growing and late growing seasons, respectively. Such necrotic tissues on sepals could serve as potential infection sites for P. pyri. The results may suggest that chemical control of Phacidiopycnis rot should focus on protecting the pedicel (stem) and floral parts of fruit and that fungicides applied near harvest are likely most important in controlling latent infections of pear fruit by P. pyri leading to Phacidiopycnis rot during storage.

Activity of Fungicides Against Monilinia vaccinii-corymbosi in Blueberry Flowers Treated at Different Phenological Stages

The activity of fenbuconazole and azoxystrobin applied to blueberry flowers at different phenological stages against subsequent gynoecial infection by the mummy berry fungus Monilinia vaccinii-corymbosi was evaluated. In the greenhouse, potted blueberry plants having flower clusters at five distinct stages (from bud scale separation to anthesis) were treated with the two fungicides. One day after anthesis (between 1 and 15 days after fungicide treatment), individual flowers were detached and inoculated with conidia of M. vaccinii-corymbosi in the laboratory. Four days after inoculation, hyphal ingress into the style was determined microscopically as a measure of fungicide efficacy. Results revealed a significant flower stage effect (P < 0.0001), whereby only fungicide application at anthesis but not at the four preanthesis stages reduced subsequent fungal ingress into the style. There was no significant difference between the two fungicides (P > 0.50) nor was there a significant fungicide-flower stage interaction (P > 0.30). In the field during 2 years, mature blueberry plants were treated with the two fungicides and exposed to natural pathogen inoculum. At the time of application, flower clusters at anthesis and at three preanthesis stages were selected and tagged. Mummy berry incidence in fruit developing from the tagged clusters was assessed to determine treatment effects. Whereas fenbuconazole lowered disease incidence for all preanthesis stages, azoxystrobin was effective only at the latest preanthesis stage. The discrepancy between these results and those of the greenhouse study (where there was no preanthesis activity of either fungicide) indirectly suggests post-infection fungicidal activity in the ovary, the base of which was exposed to the fungicide spray at the time of treatment for all flower phenology stages. Thus, although there appears to be insufficient translocation of the two fungicides in flowers treated at preanthesis stages to prevent stylar ingress by the pathogen, fungicidal activity in the ovary may be sufficient to halt subsequent fungal colonization, especially for fenbuconazole. To prescribe the most effective management program for flower-infecting fungi, translocation and post-infection activity of fungicides in floral tissues must be better understood.

Host Resistance to Monilinia vaccinii-corymbosi in Flowers and Fruits of Highbush Blueberry

Monilinia vaccinii-corymbosi, the causal agent of mummy berry disease, infects blueberry flowers via the gynoecial pathway. To describe the expression of host resistance in highbush blueberry (Vaccinium corymbosum), fungal growth in the styles and colonization of the locules were compared among five blueberry cultivars in a series of controlled greenhouse experiments. Styles were harvested 1 and 4 days postinoculation, and the length colonized by hyphae was determined using fluorescence microscopy. At 8 weeks after inoculation, fruit were harvested and scored for the presence of hyphae in the locules. The infection frequency of styles ranged from 0.33 to 0.71, and only cv. Weymouth had significantly lower infection frequency than the other cultivars. The mean length of the colonized portion of the stylar canal ranged from 0.126 to 0.434 mm after 1 day and 1.62 to 3.59 mm after 4 days. Hyphae in the styles of cv. Weymouth exhibited the least growth, whereas hyphae in the styles of cultivars Jersey and Rancocas were significantly longer. The distance of style penetrated for cultivars Bluecrop and Coville was intermediate. The mean disease incidence of locules differed significantly. Values for cultivars Weymouth and Jersey were the smallest (0.038 and 0.039) and largest (0.249 and 0.236), respectively. The results demonstrate that a component of resistance to infection by M. vaccinii-corymbosi is expressed during growth in the gynoecial pathway.

Mimicry in plant-parasitic fungi

Mimicry is the close resemblance of one living organism (the mimic) to another (the model), leading to misidentification by a third organism (the operator). Similar to other organism groups, certain species of plant-parasitic fungi are known to engage in mimetic relationships, thereby increasing their fitness. In some cases, fungal infection can lead to the formation of flower mimics (pseudo flowers) that attract insect pollinators via visual and/or olfactory cues; these insects then either transmit fungal gametes to accomplish outcrossing (e.g. in some heterothallic rust fungi belonging to the genera Puccinia and Uromyces) or vector infectious spores to healthy plants, thereby spreading disease (e.g. in the anther smut fungus Microbotryum violaceum and the mummy berry pathogen Monilinia vaccinii-corymbosi). In what is termed aggressive mimicry, some specialized plant-parasitic fungi are able to mimic host structures or host molecules to gain access to resources. An example is M. vaccinii-corymbosi, whose conidia and germ tubes, respectively, mimic host pollen grains and pollen tubes anatomically and physiologically, allowing the pathogen to gain entry into the host's ovary via stigma and style. We review these and other examples of mimicry by plant-parasitic fungi and some of the mechanisms, signals, and evolutionary implications.

Biology of flower-infecting fungi

The ability to infect host flowers offers important ecological benefits to plant-parasitic fungi; not surprisingly, therefore, numerous fungal species from a wide range of taxonomic groups have adopted a life style that involves flower infection. Although flower-infecting fungi are very diverse, they can be classified readily into three major groups: opportunistic, unspecialized pathogens causing necrotic symptoms such as blossom blights (group 1), and specialist flower pathogens which infect inflorescences either through the gynoecium (group 2) or systemically through the apical meristem (group 3). This three-tier system is supported by life history attributes such as host range, mode of spore transmission, degree of host sterilization as a result of infection, and whether or not the fungus undergoes an obligate sexual cycle, produces resting spores in affected inflorescences, and is r- or K-selected. Across the three groups, the flower as an infection court poses important challenges for disease management. Ecologically and evolutionarily, terms and concepts borrowed from the study of venereal (sexually transmitted) diseases of animals do not adequately capture the range of strategies employed by fungi that infect flowers.

Leaf and Flower Blight Caused by Monilinia vaccinii-corymbosi on Lowbush Blueberry: Effects on Yield and Relationship to Bud Phenology

ABSTRACT Naturally established lowbush blueberry clones in four fields were evaluated for the incidence of leaf and flower blight, proportion of mummy berries, and yield reductions caused by Monilinia vacciniicorymbosi. The relationship between the phenology of flower and leaf bud development and susceptibility also was examined. Three fields were examined over one crop year and one field was studied in two subsequent crop years. The incidence of stems with blight was correlated to incidence of leaf blight in all fields and to incidence of flower blight in one field. Incidence of leaf and flower blight and the proportion of mummy berries produced were not correlated. Lowbush blueberry clones with higher incidence levels of leaf blight had reduced fruit set and lower berry weights. For healthy stems, leaf-to-fruit ratios had no effect on berry weight in most fields. In contrast, blighted stems with higher leaf-to-fruit ratios had higher berry weights in three fields. Stems with slowerdeveloping leaf and flower buds had less leaf and flower blight, respectively, than stems with faster bud development. Some blueberry clones may avoid infection by delaying production of susceptible tissue until after ascospore release by M. vaccinii-corymbosi.

Effect of Temperature on Apothecial Longevity and Ascospore Discharge by Apothecia of Monilinia vaccinii-corymbosi

Pseudosclerotia of Monilinia vaccinii-corymbosi with apothecial initials were collected from blueberry (Vaccinium corymbosi cv. Jersey) fields in Grand Junction, MI, in the spring of 2002 and 2003 and monitored during apothecial development and maturation. Apothecia with cup openings as small as 1.2 mm in diameter were able to discharge ascospores. The number of ascospores discharged increased exponentially as the diameter of the cup opening increased, before leveling off at a cup diameter of about 6 mm. Cytological studies showed that ascospore discharge correlated well with the stage of development of asci and ascospores in the apothecium. The effect of temperature on the longevity of apothecia was studied at 10, 15, 20, and 25°C. Initial ascospore discharge was greater for apothecia incubated at higher temperatures (20 and 25°C). However, total discharge was greater in apothecia incubated at lower temperatures (10 and 15°C), as they persisted longer than those incubated at 20 and 25°C. Incubating apothecia at freezing temperatures (-2, -4, -6, and -8°C) for 1 h resulted in a 50 to 98% reduction in the number of ascospores discharged compared with before freezing. However, apothecia incubated down to -6°C were able to recover their ability to discharge ascospores by 2 days after the freezing treatment. Ascospores discharged from apothecia after freezing at -2°C for 1 h remained viable. However, the germination ability of ascospores from apothecia frozen at -4, -6, and -8°C decreased with decreasing temperature such that only 10% of ascospores from apothecia frozen at -8°C germinated successfully. The information obtained in this study may be useful in the development of scouting and management strategies for the control of mummy berry disease.