Vincent Lanoiselet

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.

Studies on the seed transmission of Wheat streak mosaic virus

Wheat streak mosaic, caused by Wheat streak mosaic virus (WMSV), is a recent disease of wheat in Australia. This study was conducted to investigate seed transmission of WSMV and is the first publication to report on the rate of WSMV seed transmission in artificially inoculated wheat cultivars (0.4%). Furthermore, seed transmission was shown to occur at very low levels (with a maximum of 0.22%) in infected commercial wheat crops of New South Wales, Australia. Seed infection was shown not to be correlated with seed size, and no relationship was determined between seed size and transmission. These results confirm the recent discovery that WSMV is a seedborne disease, and suggest that sizeselection of seed is not an option for growers to eliminate WSMV infected seed from seed lot.

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.

Survival of Sclerotinia sclerotia under fire

A series of experiments was performed to determine the effectiveness of stubble burning and dry heat on reducing the viability of sclerotia of Sclerotinia sclerotiorum, the causal agent of Sclerotinia stem rot of canola. An intense fire in a uniformly dense triticale stubble led to the death of all sclerotia. However, in another burn with wheat stubble of varying densities, sclerotia survived well when the amount of stubble cover on the ground was 70% or less and sclerotial viability was only reduced when 90% or more of the surface was covered in stubble. Further experiments in dense rice straw showed that temperatures in a stubble fire are variable within a field and also within the rice stubble profile with lower temperatures under fallen stubble and among standing straw than on top of fallen straw. Most sclerotia (98.1 and 98.4%) survived temperatures under 93°C, ∼43% survived temperatures between 93 and 107°C, and very few (27.5 and 1.8%) survived temperatures above 121°C. These results were then verified in a 90s oven experiment where all sclerotia survived under 93°C, 70% survived between 93 and 107°C and none survived temperatures above 121°C. The ability of sclerotia to germinate myceliogenically and carpogenically was significantly reduced when exposed to oven temperatures between 93 and 107°C. These sclerotia also took longer to germinate myceliogenically but were earlier to germinate carpogenically than sclerotia in the control or sclerotia exposed to temperatures below 93°C. Size did not affect the ability of the sclerotia to survive a stubble fire. Burning of stubble does not appear to be an effective method of control of Sclerotinia stem rot in the field.

Production, germination and infectivity of chlamydospores of Rhynchosporium alismatis

Rhynchosporium alismatis , a pathogen of several Alismataceae species, is being studied in Australia as a potential weed biocontrol agent for Alisma lanceolatum, A. plantago-aquatica and Damasonium minus. Chlamydospores of R. alismatis are described for the first time. Large numbers (5.34 × 10 5 cm −2 ) were produced on potato dextrose agar within 8–15 d and range in size from 4.7 to 14 μm. There was variability between isolates in mean diameter but size was not related to the host of origin. Within 24 h at 30 °C 60% of chlamydospores, 8 d to 3 months old, germinated. Each germinated chlamydospore produced up to four germ-tubes. Chlamydospores were detected in leaf lesions on naturally-infected A. plantago-aquatica. One-month-old chlamydospores were pathogenic to leaf discs cut from mature leaves of A. lanceolatum and Damasonium minus. The relative robustness of chlamydospores is an advantage over conidia for the future production of a biological control agent if they can be produced and harvested in broth culture.

First report of Colletotrichum acutatum causing a leaf spot and hull rot of pistachio

Colletotrichum acutatum J.H. Simmonds has been identified from leaves and the exocarp of fruit of diseased pistachio trees (Pistacia vera L.) growing in New South Wales. When the fungus was reinoculated onto the leaves and nuts of pistachio, it caused symptoms similar to those observed in the field. To our knowledge, C. acutatum has not been previously reported causing a disease of pistachio anywhere in the world.