M. J. Ryley

Changes in the Racial Composition of Phytophthora sojae in Australia Between 1979 and 1996

Surveys of commercial soybean fields, disease nurseries, and trial plots of soybean were conducted throughout eastern Australia between 1979 and 1996, and 694 isolates of Phytophthora sojae were collected and classified into races. Fourteen races, 1, 2, 4, 10, 15, and 25, and eight new races, 46 to 53, were identified, but only races 1, 4, 15, 25, 46, and 53 were found in commercial fields. Races 1 and 15 were the only races found in commercial fields in the soybean-growing areas of Australia up until 1989, with race 1 being the dominant race. Race 4 was found in central New South Wales in 1989 on cultivars with the Rps1a gene, and it is now the dominant race in central and southern New South Wales. Races 46 and 53 have only been found once, in southern New South Wales, and race 25 was identified in the same region in 1994 on a cultivar with the Rps1k gene. Only races 1 and 15 have been found in the northern soybean-growing regions, with the latter dominating, which coincides with the widespread use of cultivars with the Rps2 gene. Changes in the race structure of the P. sojae population from commercial fields in Australia follow the deployment of specific resistance genes.

Identity and genetic diversity of the sorghum ergot pathogen in Australia

Sorghum ergot was first discovered in Australia in 1996. It affects seed production and grain usage in stock feed due to concerns of animal toxicity. Three species of Claviceps are known to cause ergot of sorghum with different epidemiological, animal toxicity, and management implications. Claviceps africana was identified as the causal agent but morphological differences between isolates raised the possibility of more than one species being involved. The major aim of this study was to identify the Claviceps species causing sorghum ergot and to determine the genetic diversity among isolates of the ergot pathogen from Australia and overseas. Symptom development, sequencing of the ITS1 region, and radiolabelled DNA amplification fingerprints (RAF) were used to confirm that ergot of sorghum in Australia is caused by C. africana. The morphology of sphacelia, microconidia, macroconidia, and secondary conidia of all 36 Australian isolates studied matched the description for C. africana and the DNA sequence of the ITS1 region of 2 selected Australian isolates was identical to that of C. africana. Based on RAF analysis of 110 Australian and overseas isolates of Claviceps spp., C. africana isolates could be clearly distinguished (<40% similarity) from C. pusilla, C. sorghicola, C. sorghi, and a Claviceps sp. isolated from Panicum maximum. The C. africana isolates formed 2 distinct clusters. Cluster 1 contained 72 Australian isolates and all 21 overseas isolates of C. africana. The 13 isolates in Cluster 2 were all from Australia and more diverse than those in Cluster 1. The high level of genetic diversity of C. africana isolates in Australia is unexpected given that ergot has only been reported recently. The most likely source of this diversity points to introductions from countries such as India.

On the relation between weather variables and sorghum ergot infection

Sorghum ergot (Claviceps africana) has had a significant impact on seed production and breeders’ nurseries in Australia since it was first found in 1996. In this paper, 3 distinct key development stages of sorghum that are related to ergot infection were identified: flag leaf stage, pollen starch accumulation stage, and flowering period. Relationships between weather variables during these 3 stages and ergot severity as well as pollen viability were analysed using observed data from 2 field trials, a serial planting trial and a genotype trial, conducted at Gatton, Queensland. The duration of the flag leaf stage and of the flowering period was estimated from thermal time. An infection factor was introduced and calculated based on hourly temperature during the flowering period. This infection factor and the mean relative humidity at 0900 hours during the flowering period were the main factors influencing ergot infection. Mean daily minimum temperature during flag leaf stage also had a significant effect on ergot severity, although no significant relation was found between this mean daily minimum temperature and pollen viability. A linear regression model using the above 3 factors accounted for 94% of the environmentally caused variation in ergot severity observed in the genotype trial.

Survival of conidia of sorghum ergot (caused by Claviceps africana) on panicles, seed and soil in Australia

Macroconidia of the sorghum ergot pathogen, Claviceps africana Frederickson, Mantle & de Milliano, survived in dried honeydew on soil for 13–14 weeks in a glasshouse at ambient temperatures, but for less than half that time on seed stored in a shadehouse over summer. Those on seeds stored at 4°C, however, survived for over a year (58–62 weeks). During summer, conidia on ergot-infected panicles buried in soil, or on the soil surface, survived for 7.5–12 weeks, whereas over winter the survival times were 4 weeks and 19–27 weeks, respectively. Macroconidia on infected panicles held above the soil surface survived for >38 weeks (8 calendar months) over winter, suggesting that they may play a role in the perennation of C. africana in Australia.

Efficacy, timing and method of application of fungicides for management of sorghum ergot caused by Claviceps africana

Trials conducted in Queensland, Australia between 1997 and 2002 demonstrated that fungicides belonging to the triazole group were the most effective in minimising the severity of infection of sorghum by Claviceps africana, the causal agent of sorghum ergot. Triadimenol (as Bayfidan 250EC) at 0.125 kg a.i./ha was the most effective fungicide. A combination of the systemic activated resistance compound acibenzolar-S-methyl (as Bion 50WG ) at 0.05 kg a. i./ha and mancozeb (as Penncozeb 750DF) at 1.5 kg a.i./ha has the potential to provide protection against the pathogen, should triazole-resistant isolates be detected. Timing and method of fungicide application are important. Our results suggest that the triazole fungicides have no systemic activity in sorghum panicles, necessitating the need for multiple applications from first anthesis to the end of flowering, whereas acibenzolar-S-methyl is most effective when applied 4 days before flowering. The flat fan nozzles tested in the trials provided higher levels of protection against C. africana and greater droplet deposition on panicles than the tested hollow cone nozzles. Application of triadimenol by a fixed wing aircraft was as efficacious as application through a tractor-mounted boom spray.

Fungal diseases of temperate annual pasture legumes in southern Queensland

Diseases of temperate annual pasture legumes in subtropical southern Queensland were surveyed during 1992 and 1993. The following pathogenic organisms were recorded: Colletotrichum trifolii, Stemphylium vesicarium, Oidium sp., Uromyces anthyllidis, Uromyces striatus and Pseudopeziza medicaginis from annual Medicago spp.; Rhizoctonia solani and Colletotrichum destructivum from Ornithopus spp.; and Oidium sp. from Trifolium subterraneum. Three of these disease interactions had not been previously recorded in Queensland and 5 were new reports for Australia. Rust was the most frequently observed and widespread disease on annual medics (44% of M. polymorpha samples). All other diseases of annual medics were found infrequently (2-18% of samples). In contrast, both serradella and subterranean clover were relatively free from any diseases. The years during which the survey was conducted were dry (as low as 31% of mean March-October rainfall) and the expression of disease may have been restricted. Nevertheless, this improved knowledge of diseases of temperate annual legumes in southern Queensland will assist in the future selection and breeding of suitable cultivars for use in the subtropics.

Factors influencing the germination of macroconidia and secondary conidia of Claviceps africana

The influences of temperature, time, and moisture on the germination of macroconidia and secondary conidia of Australian isolates of Claviceps africana were studied in vitro. The optimum temperature for germination of both macroconidia and secondary conidia of C. africana was 20degreesC. Although germination of macroconidia ceased near 31degreesC, approximately 30% of secondary conidia germinated at 37degreesC after 48 and 72 h of incubation. Sorghum flower extract agar stimulated macroconidium and secondary conidium germination, irrespective of temperature. Germination of macroconidia and secondary conidia on water agar started after 4 h of incubation at 20degreesC, reaching a maximum after 16-24 h and 14 h, respectively. Maximum germination of both macroconidia and secondary conidia was at greater than or equal to-5 bars at 20degreesC. Germination of secondary conidia ceased at -35 bars, whereas macroconidia germinated at water potentials as low as -55 bars at 20degreesC.

Rhizoctonia crown and root rot of the pasture legume, sulla (Hedysarum coronarium)

Rhizoctonia solani AG-2-2 was isolated from wilting and dying plants of sulla (Hedysarum coronarium), which is currently being assessed in eastern and southern Australia for its potential as a pasture and forage legume. Infected plants in the field had extensive rotting of the taproot, lateral roots and crown. Koch’s postulates were fulfilled using three inoculation methods. The disease may pose a considerable threat to the potential use of H. coronarium in the dryland, grazing farming systems of Australia, with resistance offering the most viable option for minimising its impact.

Event frequency and severity of sorghum ergot in Australia

The temporal and regional distribution of the severity and potential number of events of sorghum ergot on grain sorghum in Australia were analysed using daily climatic data from 1957 to 1998. This analysis was conducted using both a rule-based method and a regression model. Between December and March, the main flowering period for most commercial grain sorghum crops, we found a likely increase of ergot events in eastern Australia from south to north as well as from west to east. When crops flowered in April or May the number of potential monthly events increased, particularly in the southern areas. The smallest number of events occurred when flowering occurred between September and December. The temporal and geographic distribution of the number of events and severity of sorghum ergot is closely related to relative humidity during the flowering period. The analysis indicates that grain sorghum crops flowering between early December and February are unlikely to be severely infected with sorghum ergot. Late flowering sorghum has increased risk to severe infection, especially in the coastal regions.

Prediction of sorghum downy mildew risk in Australia using daily weather data

The temporal and regional distributions of the potential number of infection events of the exotic disease sorghum downy mildew in Australia were predicted for the years 1957–1998. A set of rules based on overseas research on the influence of environmental factors on infection and sporulation was used to interrogate daily climatic data from 1957 to 1998 for 53 selected localities. Between November and February, sorghum was at arelatively low risk of infection from downy mildew, with the risk gradually increasing during the summer, reaching the greatest risk in May for most localities. During March to May there were geographical south-north, as well as west-east, increasing gradients of predicted downy mildew events in Queensland and New South Wales. In winter and early spring, the analysis indicated that disease would occur only in the coastal regions of Queensland, with serious implications for breeders’ nurseries in these areas. The temporal and regional distributions of events were closely related to those of relative humidity, night temperature and night length. Sorghum downy mildew was predicated to be a minor problem in most part of New South Wales.