David Guest

THE COMPLEX ACTION OF PHOSPHONATES AS ANTIFUNGAL AGENTS

C O N T E S T S

Prevalence and residential determinants of fungi within homes in Melbourne, Australia

Recent epidemiological studies suggest that the adverse respiratory health effects caused by the inhalation of fungal propagules are substantial. Knowledge of the prevalence and environmental determinants of indoor fungal levels is essential in designing effective avoidance measures.

Mouldy houses influence symptoms of asthma among atopic individuals

Background The influence of current levels of indoor fungi on asthma is a controversial issue that needs to be resolved in order to advise patients appropriately.

Phytoalexin accumulation, phenylalanine ammonia lyase activity and ethylene biosynthesis in fosetyl-Al treated resistant and susceptible tobacco cultivars infected with Phytophthora nicotianae var. nicotianae

Abstract NC 2326, a cultivar of tobacco resistant to race 0 of the black shank pathogen Phytophthora nicotianae var. nicotianae , responds to stem inoculation by rapidly accumulating sesquiterpenoid phytoalexins and activating phenylalanine ammonia lyase activity at the infection front. In cv. Hicks, a near-isogenic susceptible cultivar, both responses are slower. Pretreatments of leaf discs with propylene oxide, which kills the cells, mevinolin, a specific inhibitor of sesquiterpenoid biosynthesis, or the non-specific amino-transferase inhibitor, aminooxyacetic acid (AOA), inhibit post-infection phytoalexin accumulation in both cultivars, and induce susceptibility in cv. NC 2326. Aminohydrazinophenylpropionic acid (AHPP), a specific inhibitor of phenylalanine ammonia lyase activity and aminoethoxyvinylglycine (AVG), an inhibitor of ethylene biosynthesis, do not affect the susceptibility of either cultivar. Plants of the cv. Hicks are protected from infection by the systemic phosphonate plant protectant, fosetyl-Al. Sesquiterpenoid phytoalexins, lignin and ethylene accumulate, and phenylalanine ammonia lyase activity increases more rapidly in fosetyl-Al-treated Hicks stems than in untreated stems. Propylene oxide, mevinolin and AOA inhibit sesquiterpenoid phytoalexin accumulation and the effectiveness of fosetyl-Al in cv. Hicks. Fosetyl-Al does not enhance sesquiterpenoid phytoalexin biosynthesis in cv. NC 2326, and only marginally reduces pathogen growth in the initial stage of infection, before resistance is expressed. Mevinolin and AOA do not induce total susceptibility in fosetyl-Al-treated NC 2326, indicating that factors other than sesquiterpenoid phytoalexins are also involved in the mode of action of fosetyl-Al in this cultivar.

The complex mode of action of phosphonates

Phosphonates control many plant diseases caused by Phytophthora, at concentrations in planta which only partially inhibit pathogen growth in vitro. One of the consequences of phosphonate treatment is that it induces a strong and rapid defence response in the challenged host plant, and this is sufficient in tobacco, capsicum and cowpea to fully limit the pathogen. We conclude that the combined effects of direct inhibition of the pathogen and enhanced host defence responses are responsible for the ability of phosphonates to provide effective, durable control of plant diseases at low application rates.

Fungal Planet description sheets: 107–127

Novel species of microfungi described in the present study include the following from Australia: Phytophthora amnicola from still water, Gnomoniopsis smithogilvyi from Castanea sp., Pseudoplagiostoma corymbiae from Corymbia sp., Diaporthe eucalyptorum from Eucalyptus sp., Sporisorium andrewmitchellii from Enneapogon aff. lindleyanus, Myrmecridium banksiae from Banksia, and Pilidiella wangiensis from Eucalyptus sp. Several species are also described from South Africa, namely: Gondwanamyces wingfieldii from Protea caffra, Montagnula aloes from Aloe sp., Diaporthe canthii from Canthium inerne, Phyllosticta ericarum from Erica gracilis, Coleophoma proteae from Protea caffra, Toxicocladosporium strelitziae from Strelitzia reginae, and Devriesia agapanthi from Agapanthus africanus. Other species include Phytophthora asparagi from Asparagus officinalis (USA), and Diaporthe passiflorae from Passiflora edulis (South America). Furthermore, novel genera of coelomycetes include Chrysocrypta corymbiae from Corymbia sp. (Australia), Trinosporium guianense, isolated as a contaminant (French Guiana), and Xenosonderhenia syzygii, from Syzygium cordatum (South Africa). Pseudopenidiella piceae from Picea abies (Czech Republic), and Phaeocercospora colophospermi from Colophospermum mopane (South Africa) represent novel genera of hyphomycetes. Morphological and culture characteristics along with ITS DNA barcodes are provided for all taxa.

Black Pod: Diverse Pathogens with a Global Impact on Cocoa Yield

ABSTRACT Pathogens of the Straminipile genus Phytophthora cause significant disease losses to global cocoa production. P. megakarya causes significant pod rot and losses due to canker in West Africa, whereas P. capsici and P. citrophthora cause pod rots in Central and South America. The global and highly damaging P. palmivora attacks all parts of the cocoa tree at all stages of the growing cycle. This pathogen causes 20 to 30% pod losses through black pod rot, and kills up to 10% of trees annually through stem cankers. P. palmivora has a complex disease cycle involving several sources of primary inoculum and several modes of dissemination of secondary inoculum. This results in explosive epidemics during favorable environmental conditions. The spread of regional pathogens must be prevented by effective quarantine barriers. Resistance to all these Phytophthora species is typically low in commercial cocoa genotypes. Disease losses can be reduced through integrated management practices that include pruning and shade management, leaf mulching, regular and complete harvesting, sanitation and pod case disposal, appropriate fertilizer application and targeted fungicide use. Packaging these options to improve uptake by smallholders presents a major challenge for the industry.

Incidence ofCladosporium, alternaria and total fungal spores in the atmosphere of melbourne (Australia) over three years

A Burkard spore trap was used to monitor daily fungal spore counts in the atmosphere of Melbourne (Australia) between October 1991 and December 1994. Annual total spore counts varied widely (range 345 770 in 1994 to 1 106 037 in 1992), of which approximately half were identified asCladosporium sp. and only about 1%Alternaria sp. Highest daily total spore counts were recorded late-summer through to mid-winter, probably corresponding to senescence of annual grasses and leaf fall. Spore counts were negatively correlated with rainfall (P<0.05), significantly correlated with average temperature (P<0.001), and showed a highly significant linear relationship with cumulative temperature throughout the year.

Evidence from light microscopy of living tissues that Fosetyl-Al modifies the defence response in tobacco seedlings following inoculation by Phytophthora nicotianae var nicotianae

Pretreatment of tobacco seedlings with 100 μg ml −1 of the systemic anti-oomycetc compound, Fosetyl-Al, or 70 μg ml −1 of its in vivo breakdown product, phosphorous acid, induced microscopically visible changes in the response to infection by zoospores of a compatible race of Phytophthora nicotianae var nicotianae . Periodic observations of unstained living seedlings, supported by observations of fixed, stained seedlings, reveal that Fosetyl-Al induces rapid cytoplasmic aggregation and deposition of papillae around, and migration of host nuclei toward, the penetration point, which in 86% of observed instances was at an anticlinal cell wall junction on the epidermis. Hyphal growth for the first 18 h was primarily intercellular, but by 24 h host cells around these hyphae underwent hypersensitive cell death and fungal growth ceased. In the remaining 14% of observations the infection hypha directly penetrated the cuticle and epidermal cell wall, and was restricted to the epidermal cell, in which it elicited a rapid hypersensitive response. In untreated seedlings 83% of penetrations were intercellular. Small papillae were deposited on the cell walls under, and then adjacent to, the penetration peg, but these did not prevent penetration and continued growth of intercellular hyphae. By IS h, haustoria and intracellular hyphae had formed in the cells of the cortex. By 36–48 h, sporangia were produced. In seedlings treated with 1 μg ml −1 Metalaxyl, fungal growth ceased within 5 h of inoculation. This was followed by cytoplasmic responses such as the formation of aggregates in cells adjacent to the infection peg; however, this response appeared to be a consequence of, rather than a primary event in, the mode of action of Metalaxyl. Results presented in this paper support an indirect mode of action of Fosetyl-Al utilizing enhanced host defence responses, and a direct antifungal mode of action of Metalaxyl.

The 5S ribosomal RNA gene is linked to large and small subunit ribosomal RNA genes in the oomycetes, Phytophthora vignae, P. cinnamomi, P. megasperma f.sp. glycinea and Saprolegnia ferax

Southern hybridization and polymerase chain reaction data indicate that the 5S ribosomal RNA gene is linked to the ribosomal RNA gene repeat unit in the oomycetes, Phytophthora vignae, P. cinnamomi, P. megasperma f.sp. glycinea and Saprolegnia ferax, and is apparently transcribed in the same direction as the large and small subunit ribosomal RNA genes. The polymerase chain reaction has been used to amplify all components of the entire ribosomal RNA gene repeat unit for each of these oomycetes. The total size of all amplified products is identical to the size of the ribosomal RNA gene repeat unit, as determined by Southern analysis.