Eric Cother

CLIMEX and DYMEX simulations of the potential occurrence of rice blast disease in south-eastern Australia

Rice blast, caused by Magnaporthe grisea does not occur in Australia. The potential for infection and sporulation events by M. grisea under Australian conditions was investigated using two software programs, DYMEX and CLIMEX. The climate of Deniliquin, NSW, representative of the southern Australian rice-growing areas, was projected to the rest of the world and compared, using CLIMEX, with foreign regions where rice blast occurs. Most of the locations whose climates matched that of Deniliquin were within the distribution range of rice blast and the potential for establishment of the disease therefore appears high. A model was also developed and run under DYMEX to predict the potential number of infection and sporulation events of the pathogen. The model was run for the period 1988–1999 with the meteorological data of four representative locations in the Australian ricegrowing region. Out of a possible 11 rice-growing seasons, the number of years favourable for M. grisea ranged from two at Griffith to nine at Yanco. The rice blast model confirmed the CLIMEX results and highlighted the potential threat of rice blast to the Australian rice industry. This paper reports the first disease model developed using DYMEX.

Prevalence and survival, with emphasis on stubble burning, of Rhizoctonia spp., causal agents of sheath diseases of rice in Australia

Aggregate sheath spot and sheath spot of rice were found in Australia in 2001. A disease survey revealed that both diseases are already well distributed within the Australian rice growing areas and that disease severity can be relatively important in some crops. Epidemiological studies showed that under Australian conditions, both Rhizoctonia oryzae and R. oryzae-sativae could overwinter as mycelium on straw debris, regardless of whether the straw is left on the ground or buried. Mycelium of R. oryzae-sativae present on rice straw was also found to be able to produce sclerotia, as a saprophyte, during the overwintering period. Results also strongly suggest that overwintered hyphal fragments present in the debris supplement the sclerotia as a primary source of inoculum, and also highlight the importance of straw management to reduce the inoculum of both pathogens in rice paddocks. The effect of burning stubble on the survival of laboratory-produced sclerotia of R. oryzae-sativae was investigated and it was shown that the vast majority of the sclerotia present on the soil surface survived stubble burning regardless of whether it was a ‘cold burn’ or a ‘hot burn’. A threshold temperature for sclerotial mortality was found to be between 93 and 12 PC.

Plectosporium alismatis comb. nov. a new placement for the Alismataceae pathogen Rhynchosporium alismatis

The phytopathogen Rhynchosporium alismatis, occurring on Alisma, Sagittaria and other genera in the Alismataceae, is under investigation as a mycoherbicide for alismataceous weeds in Australian rice crops. The type species of Rhynchosporium, R. secalis, belongs in the Helotiales while the phylogenetic relationships of R. alismatis were unknown. To identify fungi related to R. alismatis, the internal transcribed spacer (ITS) region of the ribosomal DNA (ITS1, 5.8S rRNA gene, ITS2) of 56 isolates was sequenced and compared to those available in databases. Analysis of ITS sequences revealed close relationships between R. alismatis and the teleomorph genus Plectosphaerella, as well as several anamorphic fungi which were primarily species of Verticillium. Rhynchosporium alismatis and Plectosphaerella cucumerina clustered together with 98 % bootstrap support. Morphological comparisons supported this relationship indicating that R. alismatis and the anamorphic genus Plectosporium are congeneric. Rhynchosporium alismatis is transferred to Plectosporium, a name proposed for conservation.

Yield loss in rice caused by Rhizoctonia oryzae and R. oryzae-sativae in Australia

Rhizoctonia oryzae-sativae and R. oryzae are the causal agents of aggregate sheath spot and sheath spot of rice, respectively, in Australia. Both diseases are well distributed within the Australian rice growing area. The effects of six fungicides (metalaxyl-methyl, azoxystrobin, kresoxim-methyl, pyraclostrobin, propiconazole and toclofos-methyl) on mycelial growth of R. oryzae-sativae and R. oryzae were tested in vitro. Pyraclosotrobin and propiconazole were strong inhibitors of both pathogens and all isolates tested were sensitive to these two fungicides. Both fungicides significantly reduced disease development in field tests but failed to increase rice yield. As well, aggregate sheath spot caused yield losses as high as 20.3% and sheath spot reduced yields by up to 10%. This is the first report quantifying potential yield losses caused by aggregate sheath spot and sheath spot of rice under Australian conditions.

DISEASE NOTES OR NEW RECORDS: First report of Waitea circinata causing sheath spot and Rhizoctonia oryzaesativae causing aggregate sheath spot on rice in south-eastern Australia

A survey conducted during the summer of 2001 in the rice-growing areas of south-eastern Australia revealed for the first time the presence of Rhizoctonia oryzae-sativae (Sawada) Mordue, the causal agent of aggregate sheath spot and Waitea circinata Warcup & Talbot, the causal agent of sheath spot, on stems of rice plants.

Polyphasic identification of Pseudomonas fuscovaginae causing sheath and glume lesions on rice in Australia

Pathogenic fluorescent pseudomonads associated with sheath brown rot disease symptoms on near-mature rice were characterised by a polyphasic study. Twelve strains of Pseudomonas fuscovaginae from the International Collection of Microorganisms from Plants (ICMP), which represent those strains lodged in several world culture collections, were used for comparison. Two strains, ICMP 9997 and 9999, were considered to have no similarity at all to P. fuscovaginae and a further two strains, ICMP11283 and 11284, were considered, by fatty acid analysis, to be more closely related to P. putida. These two strains were related to P. marginalis according to Biolog, and to P. tolaasii according to 16s and rpoB data. The Australian rice isolates were all identified as P. putida Biotype A by fatty acid analysis or as P. asplenii (4 isolates), P. fuscovaginae (2 isolates) or P. fluorescens (1 isolate) by Biolog. Sequencing of the 16s rRNA gene placed the rice isolates with P. fuscovaginae and P. asplenii, whereas rpoB gene sequence analysis showed a higher similarity to P. fuscovaginae than to P. asplenii. This is the first report of P. fuscovaginae in Australia.

Aspects of jojoba agronomy and management

Abstract Jojoba is a relatively new crop that is adapted to hot, dry climates. The crop is grown for its seed, which contains a wax with a high melting point. This wax is used in a variety of products, including lubricants, pharmaceuticals, and cosmetics. Various aspects of the crop agronomy and management are reviewed, including a botanical description, propagation from seed and cuttings, the genetics of the plant and wax production, and the various physical characteristics that affect the crops growth and yield, such as soil, water, fertilizer requirements, salt, pH, temperature tolerance, and mycorrhizal status of the crop. Last, the pests (insects, arthropods, and diseases) of jojoba are reviewed. This chapter is intended to provide scientists and managers with a reference on the agronomy and management of jojoba.

Characterisation of Pseudomonas syringae Strains Associated with a Leaf Disease of Leek in Australia

A necrotic leaf disease of leek (Allium ampeloprasum Porrum Group) is reported in Australia for the first time. The fluorescent pseudomonad consistently associated with diseased tissue was identified as Pseudomonas syringae by LOPAT tests (+,−,−,−,+), carbon utilisation, bean and lemon inoculations and fatty acid methyl ester analysis. It was confirmed as P. syringae pv. porri by pathogenicity to leeks, bulb onions, spring onions, shallots and garlic, and by genetic analysis using 16S rDNA PCR, REP, ERIC and BOX PCR, and IS50 PCR. Comparison with reference strains of pv. porri from other countries showed similarity to known strains of pv. porri. The Australian leek strains were generally uniform in their biochemical reactions although three strains tested varied in their pathogenicity to other Allium spp. and varied from published data. All Australian strains shared the same genetic profile with strains from New Zealand, France and California. However, Japanese strains from leek and onion were distinct from the Australian strains and those from New Zealand, France and California. Data strongly support the hypothesis that the pathogen is seed-borne.

Production of Chlamydospores and Conidia in Submerged Culture by Rhynchosporium alismatis, a Mycoherbicide of Alismataceae in Rice Crops

The objective of this work was to evaluate the production of chlamydospores and conidia of Rhynchosporium alismatis in a liquid Czapex-Dox based medium supplemented with increasing concentrations of sodium nitrate and malt extract. In addition, the germination of chlamydospores was evaluated. A high concentration of malt extract (4.4 g L−1) as the sole carbon source and a high level of sodium nitrate as the sole nitrogen source (3.3 g L−1) were shown to increase chlamydospore production while agitation (150 rpm) enhanced conidial yields. Maximum chlamydospore production (2.03×105±0.7 total chlamydospores mg DW−1) was achieved in cultures grown in a medium supplemented with 8.8 g L−1 malt extract and 5.74 g L−1 sodium nitrate. Two days growth was required for maximum chlamydospore and conidial production, while 6 days was necessary to obtain maximum dry weight accumulation (350 mg per flask). Germination of chlamydospores (90%) was significantly higher than germination of conidia (47%) after 2 days growth.

Early infection process of Damasonium minus by the potential mycoherbistat Rhynchosporium alismatis

Rhynchosporium alismatis is a fungal pathogen that promises to be a suitable biological control agent to manage the rice weed, starfruit (Damasonium minus). Light and scanning electron microscopy were used to study the infection process. Conidial germination, appressorium formation, and penetration by R. alismatis were compared at 25 and 30°C on starfruit leaves, and in separate experiments, conidial germination and appressorium formation were compared at both temperatures on cellophane paper. Appressorium formation was significantly (P < 0.005) higher at 25 than 30°C after 8 h on leaves and after 10 h on cellophane paper. Sessile appressoria were frequently observed on leaves, while no such structures were observed on cellophane paper. Penetration through stomata appeared to be a random event. Subepidermal hyphae were frequent after 24 h, and conidial formation was observed 48 h after inoculation of leaves at 25°C. Deformation of the leaf surface around the appressoria indicated signs of physical pressure. Fast conidial germination, penetration, and infection is an advantage, since the fungus is not exposed for long periods of time to adverse environmental conditions. The formation of conidia on the leaves can lead to further disease spread.