A Novel Partitivirus in the Hypovirulent Isolate QT5-19 of the Plant Pathogenic Fungus Botrytis cinerea

A pink isolate (QT5-19) of Botrytis cinerea was compared with three gray isolates of B. cinerea for growth and morphogenesis on potato dextrose agar (PDA), and for pathogenicity on tobacco. A double-stranded (ds) RNA mycovirus infecting QT5-19 was identified based on its genome feature and morphology of the virus particles. The results showed that QT5-19 grew rapidly and established flourishing colonies as the gray isolates did. However, it is different from the gray isolates, as it failed to produce conidia and sclerotia asthe gray isolates did. QT5-19 hardly infected tobacco, whereas the gray isolates aggressively infected tobacco. Two dsRNAs were detected in QT5-19, dsRNA 1 and dsRNA 2, were deduced to encode two polypepetides with homology to viral RNA-dependent RNA polymerase (RdRp) and coat protein (CP), respectively. Phylogenetic analysis of the amino acid sequences of RdRp and CP indicated that the two dsRNAs represent the genome of a novel partitivirus in the genus Alphapartitivirus, designated here as Botrytis cinerea partitivirus 2 (BcPV2). BcPV2 in QT5-19 was successfully transmitted to the three gray isolates through hyphal contact. The resulting BcPV2-infected derivatives showed rapid growth on PDA with defects in conidiogenesis and sclerogenesis, and hypovirulence on tobacco. This study suggests that BcPV2 is closely associated with hypovirulence of B. cinerea.

A Reovirus Causes Hypovirulence of Rosellinia necatrix

White root rot, caused by Rosellinia necatrix, is a serious soilborne disease of fruit trees and other woody plants. R. necatrix isolate W370 contains 12 segments of double-stranded RNA (dsRNA) that is believed to represent a possible member of the family Reoviridae. W370 was weakly virulent and its hyphal-tip strains became dsRNA free and strongly virulent. The 12 segments of W370dsRNA were transmitted to hygromycin B-resistant strain RT37-1, derived from a dsRNA-free strain of W370 in all or none fashion through hyphal contact with W370. The W370dsRNA-transmitted strains were less virulent than their parent strain RT37-1 on apple seedlings, with mortality ranging between 0 to 16.7% in apple seedlings that were inoculated with the W370dsRNA-containing strains and 50 to 100% for seedlings inoculated with the dsRNA-free strains. Some W370dsRNA-containing strains killed greater than 16.7% of seedlings, but these were found to have lost the dsRNA in planta. These results indicate that W370dsRNA is a hypovirulence factor in R. necatrix. In addition, a strain lost one segment (S8) of W370dsRNA during subculture, and the S8-deficient mutant strain also exhibits hypovirulence in R. necatrix.

Ectomycorrhizal community structure on western hemlock (Tsuga heterophylla) seedlings transplanted from forests into openings

This study tested whether mature-forest ectomycorrhizal (ECM) communities could be maintained in forest openings on seedlings. Naturally regenerated western hemlock (Tsuga heterophylla (Raf.) Sarg.) seedlings were transplanted from mature forests into openings and the ECM fungal community was compared after 2 years with similar seedlings planted back into the forests or seedlings from openings planted back into openings. Fewer ECM morphotypes, lower average richness per seedling, and a steeper, less even species distribution curve were found, all of which suggest that the mature-forest ECM fungal community changed after transplanting forest seedlings into the openings. The increased abundance of pioneer fungi such as Thelephora terresteris suggested that many of the mature-forest ECM fungi were unable to maintain or continue root colonization in openings. Results suggest that many mature-forest ECM fungi require further stand development to maintain enough rooting density and hyphal contact to persist.Key words: ectomycorrhizal succession, disturbance, species-importance curves, multistage and late-stage fungi.

Transmissible Mitochondrial Hypovirulence in a Natural Population of Cryphonectria parasitica

A cytoplasmically transmissible hypovirulence syndrome has been identified in virus-free strains of the chestnut blight fungus Cryphonectria parasitica isolated from healing cankers on American chestnut trees in southwestern Michigan. The syndrome is associated with symptoms of fungal senescence, including a progressive decline in the growth potential and abundance of conidia, and elevated levels of respiration through the cyanide-insensitive alternative oxidase pathway. Conidia from senescing mycelia exhibited varying degrees of senescence ranging from normal growth to death soon after germination. Cytoplasmic transmission of hypovirulence between mycelia occurred by hyphal contact and coincided with the transfer of a specific restriction fragment length polymorphism from the mitochondrial DNA (mtDNA) of the donor strains into the mtDNA of virulent recipients. The transmission of the senescence phenotype was observed not only among vegetatively compatible strains but also among incompatible strains. Hypovirulence was present in isolates from the same location with different nuclear genotypes as identified by DNA fingerprinting. This study confirms that mitochondrial hypovirulence can occur spontaneously and spread within a natural population of a phytopathogenic fungus.

A comparative ultrastructural study of endophytic basidiomycetes in the parasitic achlorophyllous hepatic Cryptothallus mirabilis and the closely allied photosynthetic species Aneura pinguis (Metzgeriales)

This ultrastructural study of two closely related liverworts with contrasting modes of nutrition reveals very similar interactions with endophytic dikaryotic basidiomycetes. In both hepatics, collected from a variety of sites, the fungus is confined to specific regions of the gametophyte thallus, and hyphal contact with the substratum is via the rhizoids. The colonization cycle comprises a growth phase when the fungus forms large intracellular coils, host cytoplasm proliferates and the starch content of the plastids decreases, followed by senescence when the hyphae die back and aggregate into large masses. Repeated colonization cycles are frequent. Young hyphae contain abundant glycogen and sometimes amyloid deposits in Cryptothallus. In terms of dolipore substructure, hyphal dimensions, highly characteristic multilayered walls, absence of clamp connections, and the mode of hyphal degeneration, the endophyte in Cryptothallus is virtually identical to that in Aneura from alpine sites but very different from the fungus in Aneura from sand dunes and a chalk pit. It is suggested that Cryptothallus evolved from an Aneura-like ancestor through association with a fungal saprophyte of waterlogged peaty soils. Differences in dolipore morphology in the ectomycorrhizal fungus of Betula roots growing in association with Cryptothallus indicate that these two hosts do not share the same fungus. Remarkable similarities between the fungal associations in Cryptothallus and Aneura and orchidaceous mycorrhizae include the same colonization cycle, absence of polyphosphate granules, and separation of the host plasma membrane from thin-walled, exclusively intracellular hyphae by a prominent interfacial matrix. Key words: basidiomycetes, dolipores, liverworts, mycorrhiza, symbiosis, ultrastructure.

Gliocladium catenulatum: hyperparasite of Sclerotinia sclerotiorum and Fusarium species

Gliocladium catenulatum parasitizes Sclerotinia sclerotiorum and Fusarium spp. It kills the host by direct hyphal contact causing the affected cells to collapse or disintegrate. Pseudoappressoria are formed by the hyperparasite but hyphae derived from them do not penetrate the host cell walls. Vegetative hyphae of all species tested and macroconidia of Fusarium spp. are susceptible to this hyperparasite but chlamydospores of Fusarium equiseti are resistant.

Mycoparasitic relationships. III. Parasitism of Physalospora obtuse by Calcarisporium parasiticum

The host–parasite interaction between Physalospora obtusa and the biotrophic contact mycoparasite Calcarisporium parasiticum was studied by light and electron microscopy. After hyphal contact between the two fungi, a contact cell was delimited by a septum in the germ tube apex of C. parasiticum. Subsequently, a portion of the appressed walls of the host–parasite interface was dissolved, creating a large pore through which cytoplasmic exchange occurred. The results of this study explain, in part, how increased nutrient uptake from the host by the parasite is accomplished and dispells the hypothesis that the contact cell increases the permeability of the host plasmalemma to nutrients and required growth factors.