R.L.C. Wijesundera

Global diversity and geography of soil fungi

Introduction The kingdom Fungi is one of the most diverse groups of organisms on Earth, and they are integral ecosystem agents that govern soil carbon cycling, plant nutrition, and pathology. Fungi are widely distributed in all terrestrial ecosystems, but the distribution of species, phyla, and functional groups has been poorly documented. On the basis of 365 global soil samples from natural ecosystems, we determined the main drivers and biogeographic patterns of fungal diversity and community composition. Direct and indirect effects of climatic and edaphic variables on plant and fungal richness. Line thickness corresponds to the relative strength of the relationships between the variables that affect species richness. Dashed lines indicate negative relationships. MAP, mean annual precipitation; Fire, time since last fire; Dist. equator, distance from the equator; Ca, soil calcium concentration; P, soil phosphorus concentration; pH, soil pH. Rationale We identified soil-inhabiting fungi using 454 Life Sciences (Branford, CN) pyrosequencing and through comparison against taxonomically and functionally annotated sequence databases. Multiple regression models were used to disentangle the roles of climatic, spatial, edaphic, and floristic parameters on fungal diversity and community composition. Structural equation models were used to determine the direct and indirect effects of climate on fungal diversity, soil chemistry, and vegetation. We also examined whether fungal biogeographic patterns matched paradigms derived from plants and animals—namely, that species’ latitudinal ranges increase toward the poles (Rapoport’s rule) and diversity increases toward the equator. Last, we sought group-specific global biogeographic links among major biogeographic regions and biomes using a network approach and area-based clustering. Results Metabarcoding analysis of global soils revealed fungal richness estimates approaching the number of species recorded to date. Distance from equator and mean annual precipitation had the strongest effects on richness of fungi, including most fungal taxonomic and functional groups. Diversity of most fungal groups peaked in tropical ecosystems, but ectomycorrhizal fungi and several fungal classes were most diverse in temperate or boreal ecosystems, and many fungal groups exhibited distinct preferences for specific edaphic conditions (such as pH, calcium, or phosphorus). Consistent with Rapoport’s rule, the geographic range of fungal taxa increased toward the poles. Fungal endemicity was particularly strong in tropical regions, but multiple fungal taxa had cosmopolitan distribution. Conclusions Climatic factors, followed by edaphic and spatial patterning, are the best predictors of soil fungal richness and community composition at the global scale. Richness of all fungi and functional groups is causally unrelated to plant diversity, with the exception of ectomycorrhizal root symbionts, suggesting that plant-soil feedbacks do not influence the diversity of soil fungi at the global scale. The plant-to-fungi richness ratio declined exponentially toward the poles, indicating that current predictions—assuming globally constant ratios—overestimate fungal richness by 1.5- to 2.5-fold. Fungi follow similar biogeographic patterns as plants and animals, with the exception of several major taxonomic and functional groups that run counter to overall patterns. Strong biogeographic links among distant continents reflect relatively efficient long-distance dispersal compared with macro-organisms. Fungi play major roles in ecosystem processes, but the determinants of fungal diversity and biogeographic patterns remain poorly understood. Using DNA metabarcoding data from hundreds of globally distributed soil samples, we demonstrate that fungal richness is decoupled from plant diversity. The plant-to-fungus richness ratio declines exponentially toward the poles. Climatic factors, followed by edaphic and spatial variables, constitute the best predictors of fungal richness and community composition at the global scale. Fungi show similar latitudinal diversity gradients to other organisms, with several notable exceptions. These findings advance our understanding of global fungal diversity patterns and permit integration of fungi into a general macroecological framework. Global metagenomics detects hotspots of fungal diversity and macroecological patterns and indicates that plant and fungal diversity are uncoupled. [Also see Perspective by Wardle and Lindahl] Assessing fungal diversity worldwide Fungi are hyperdiverse but poorly known, despite their ecological and economic impacts. Tedersoo et al. collected nearly 15,000 topsoil samples from 365 sites worldwide and sequenced their genomes (see the Perspective by Wardle and Lindahl). Overall, they found a striking decline in fungal species richness with distance from the equator. For some specialist groups though, diversity depended more on the abundance of host plants than host diversity or geography. The findings reveal a huge gap between known and described species and the actual numbers of distinct fungi in the worlds soils. Science, this issue 10.1126/science.1256688; see also p. 1052

Use of sodium bicarbonate and Candida oleophila to control anthracnose in papaya during storage

Abstract The potential of using the biocontrol agent Candida oleophila and sodium bicarbonate solution alone or in combination to reduce anthracnose caused by Colletotrichum gloeosporioides on papaya (Carica papaya L.) in storage was investigated. Treatment with 2% sodium bicarbonate did not significantly affect the growth of the biocontrol agent The efficacy of sodium bicarbonate at 2% for control of anthracnose increased when combined with C. oleophila strain (1–182) resulting in significant reduction of disease incidence at 13.5°C and 95% RH for 14 days and afterwards by 2 days at simulated marketing conditions (25°C, 75%). The growth of C. gloeosporioides was reduced significantly in the presence of C. oleophila in both inoculated and naturally infected fruits. Thus, use of sodium bicarbonate at 2% with the antagonist C. oleophila is a promising alternative to chemicals to control anthracnose, a major postharvest disease on papaya during storage.

New species from the Fusarium solani species complex derived from perithecia and soil in the Old World tropics

A large collection of strains belonging to the Fusarium solani species complex (FSSC) was isolated from soil and perithecia in primary forests in Sri Lanka (from fallen tree bark) and tropical Australia (Queensland, from fallen tree fruits and nuts). Portions of the translation elongation factor 1-alpha (tef1) gene, the nuclear large subunit (NLSU) and internal transcribed spacer regions (ITS) of the nuclear ribosomal RNA genes were sequenced in 52 isolates from soil and perithecia. The FSSC was divided previously into three clades with some biogeographic structure, termed Clades 1, 2 and 3. All Sri Lankan and Australian soil isolates were found to be members of Clade 3, most grouping with the cosmopolitan soil-associated species F. falciforme. All but two Sri Lankan perithecial isolates were associated with a set of five divergent phylogenetic lineages that were associated with Clade 2. Australian perithecial isolates resided in a subclade of Clade 3 where most of the previously defined mating populations of the FSSC reside. Isolates from perithecia and those cultured from soil were always members of different species lineages, even when derived from proximal locations. The previous biogeographic assignment of Clade 2 to South America is now expanded to the worldwide tropics. Sri Lanka appears to be an important center of diversity for the FSSC. Nectria haematococca is epitypified with a collection from the type locality in Sri Lanka; its anamorph is described as a new species, Fusarium haematococcum. Neocosmospora E.F. Smith is adopted as the correct genus for Nectria haematococca. These new species are described: F. kurunegalense/Neo. kurunegalensis, F. rectiphorus/Neo. rectiphora/, F. mahasenii/Neo. mahasenii/, F. kelerajum/Neo. keleraja.

Antagonistic effect of Trichoderma harzianum on postharvest pathogens of rambutan (Nephelium lappaceum.

Trichoderma harzianum (TrH 40) isolated from soil samples from rambutan orchards (Nephelium lappaceum) had antagonistic effects against three postharvest pathogens of rambutan:Botryodiplodia theobromae, Colletotrichum gloeosporioides andGliocephalotrichum microchlamydosporum, causative fungi of stem end rot, anthracnose and brown spot, respectively. The effects were due to both antibiosis and mycoparasitism.T. harzianum (TrH 40) treatment, while reducing the occurrence of the three postharvest diseases, also retained the overall quality and color of the fruits.

Genetic variation in Corynespora cassiicola: a possible relationship between host origin and virulence

Genetic variation of 42 isolates of Corynespora cassiicola, a destructive fungal pathogen of many economically important crop plants including rubber, was investigated using RAPD-PCR analysis. Five genetic groups were identified using RAPD-PCR profiles generated by eight random primers. Results indicate that there is a significant genetic variation among C. cassiicola isolates collected from different host plants. These results should facilitate the development of rubber clones with enhanced resistance against all genetic groups of C. cassiicola.

Effect of the combined application of chitosan and carbonate salts on the incidence of anthracnose and on the quality of papaya during storage

Summary The potential of chitosan (1%) alone, or in combination with ammonium carbonate (3%) or sodium bicarbonate (2%), to reduce the incidence of anthracnose (Colletotrichum gloeosporioides) on papaya (Carica papaya L.) and maintain quality was investigated during storage at 13.5°C and 95% RH for 14 d, and for 2 d under simulated market conditions (25°C, 75% RH). Chitosan alone or in combination with sodium bicarbonate or ammonium carbonate significantly (P < 0.005) reduced the severity of anthracnose in both inoculated and naturally-infected fruit. The effect of chitosan with ammonium carbonate on the incidence and severity of anthracnose was greater than chitosan alone, or chitosan with sodium bicarbonate. Chitosan with ammonium carbonate significantly (P < 0.005) reduced the incidence of anthracnose in, and the recovery of C. gloeosporioides from naturally-infected fruit compared to control fruit. Eating quality was not affected by these post-harvest dip treatments. Chitosan with ammonium carbonate retained high fruit quality, significantly (P < 0.005) retarded colour development of skin and flesh, increased fruit firmness and reduced weight loss. Internal carbon dioxide concentrations increased in chitosan-coated fruit to reach 7% in chitosan and ammonium carbonate-treated fruit. Thus, chitosan combined with ammonium carbonate represents a commercially acceptable, economically viable and effective alternative for post-harvest control of anthracnose during storage of papaya.

Helvolic acid, an antibacterial nortriterpenoid from a fungal endophyte, Xylaria sp. of orchid Anoectochilus setaceus endemic to Sri Lanka.

An endophytic fungus was isolated from surface sterilized leaf segments of Anoectochilus setaceus, an orchid endemic to Sri Lanka, and was identified as Xylaria sp. by morphological characters and DNA sequencing. Bioassay-guided chromatographic fractionation of the organic extract of a laboratory culture of this fungus led to the isolation of the known antibacterial helvolic acid. Helvolic acid was active against the Gram-positive bacteria, Bacillus subtilis [minimal inhibitory concentrations (MIC), 2 µg mL−1] and methicillin-resistant Staphylococcus aureus (MIC, 4 µg mL−1).

Factors affecting spore production, germination and viability of Colletotrichum acutatum isolates from Hevea brasiliensis

Colletotrichum acutatum is cosmopolitan and causes diseases in many crops including Hevea brasiliensis. It sporulated freely on PDA at 10–40 °C with peaks at around 15° and 25°. Self-inhibition of spore germination occurred at concentrations above 8 x 10 6 spores ml −1 . Ultraviolet radiation (especially 254 nm) inactivated the spores. Spore germination was around 90% between 15–35°. Free water promoted spore germination but was not essential; high relative humidity (95%) was sufficient. Spores could also withstand temperatures up to 35°. It is concluded that the most favourable conditions for the spread of C. acutatum are prevalent in the major rubber growing areas in Sri Lanka throughout the monsoons.

Dhilirolides A-D, meroterpenoids produced in culture by the fruit-infecting fungus Penicillium purpurogenum collected in Sri Lanka.

Dhilirolides A (1) to D (4), a family of secondary metabolites with a putative meroterpenoid biogenetic origin and the unprecedented dhilirane and isodhilirane carbon skeletons, have been isolated from laboratory cultures of the fruit-infecting fungus Penicillium purpurogenum collected in Sri Lanka. The structures of 1 to 4 were elucidated by interpretation of NMR data and a single crystal X-ray diffraction analysis of 1.

Combined effect of generally regarded as safe (GRAS) compounds andTrichoderma harzianum on the control of postharvest diseases of rambutan

Botryodiplodia theobromae, Colletotrichum gloeosporioides andGliocephalotrichum microchlamydosporum are the causal fungi of the rambutan postharvest diseases stem-end rot, anthracnose and brown spot, respectively. Two different treatments of rambutan fruits(Nephelium lappaceum) against the three pathogens were compared: potassium metabisulphite (250 ppm) or cinnamaldehyde (30 ppm), each combined withTrichoderma harzianum (TrH 40). The application of TrH 40 and potassium metabisulphite effectively controlled the incidence and severity of the three postharvest diseases and maintained the overall quality and color of the fruit under low temperature storage at 13.5°C and 95% r.h. for 18 days. The greatest effect of this treatment was shown onG. microchlamydosporum. Cinnamaldehyde affected the growth and germination of TrH 40, whereas potassium metabisulphite did not.