Australasian Plant Pathology

Leaf scald, caused by Rhynchosporium secalis, has been detected for the first time in Turkey on rabbit barley (Hordeum murinum).

Spot blotch caused by Cochliobolus sativus has been the major yield-reducing factor for barley production during the last decade. In this study, a systematic sequencing of expressed sequence tags (EST) was chosen to obtain a global picture of the assembly of genes involved in pathogenesis. To identify a large number of plant EST, which are induced at different time points, an amplified fragment length polymorphism display of complementary DNA was utilised. Transcriptional changes of 456 EST were observed, of which 22 have no previously described function. On one hand, EST-annotations showed homologies to a number of classical pathogenesis-related or PR genes that play a role in the signal transduction pathway. However, a notable number of EST were derived from hypothetical protein-coding genes with no clear domain sequence identity. Hence, the transcriptomic approach presented here constitutes no prior assumption about the genes activated during the C. sativus-barley interaction.

Black stem, caused by Phoma macdonaldii, is one of the most important diseases of sunflower in the world. Quantitative trait loci (QTLs) implicated in partial resistance to three isolates of P. macdonaldii including MA6, MP6 and MP10 were investigated using F2/F3 population from the cross between sunflower resistant mutant line ‘M6-54-1’ and susceptible inbred line ‘ENSAT-B4’. A genetic linkage map was constructed with 88 amplified fragment length polymorphism (AFLP) and 44 simple sequence repeat (SSR) markers using 101 F2 individuals. The map comprises 17 linkage groups (LGs) with an overall length of 1,490 cM and mean density of one marker per 12.44 cM. Parental lines and their 101 F3 families were evaluated for their resistance to P. macdonalii isolates in controlled conditions in a randomized complete block design with three replications. High genetic variability and transgressive segregation were observed among F3 families for partial resistance to all of three P. macdonaldii isolates. Composite interval mapping analysis revealed 14 putative QTLs, localized on seven linkage groups, with phenotypic variance ranging from 4 to 42 %. The QTL bsrMP6.8.1 was detected as non isolate-specific QTL and the rest of them were ‘isolate-specific’ QTLs. The major QTL on LG8 which was involved in partial resistance to three isolates could be good candidate to introduce resistance to three P. macdonaldii isolates into elite sunflower breeding lines via marker assisted breeding program.

A small survey of the potting mix taken from 15 consignments of nursery grown plants imported into Western Australia from other states in Australia found that Phytophthora spp. were present in 10% of the samples and Pythium spp. were present in 25% of the samples. Plant pathogenic nematodes were isolated from 12 of 13 consignments. Potting mix appears to be an important route by which plant pathogens can be passively introduced into Western Australia.

Cypress canker is a common disease in south-eastern Australia. The taxonomy of the Seiridium species that cause the disease has been debated due to variable morphological characters, but in Australia most workers have followed H.J. Swart and called the pathogen Seiridium unicorne. Using β-tubulin gene intron sequences, five Australian cultures were determined to be S. cupressi, rather than S. unicorne. Morphological identification of a range of herbarium specimens confirmed that S. cupressi is the common cause of cypress canker in south-eastern Australia.

Pycnidia of Phaeomoniella chlamydospora (W. Gams, Crous, M. J. Wingf. & L. Mugnai) Crous & W. Gams (syn. Phaeoacremonium chlamydosporum) were found on Pinot Noir grapevines suffering from the disease known as black goo decline in a commercial vineyard near Geelong, Victoria. To our knowledge, this state of the fungus has not been seen in the field before, although it has been produced under laboratory conditions.

The generation and accumulation of hydrogen peroxide (H2O2) and superoxide anion (O 2 .- ), as well as guaiacol peroxidase, catalase, polyphenol oxidase, β-1,3-glucanase and chitinase activity, were studied in leaves of resistant and susceptible tomato genotypes inoculated with Oidium neolycopersici. Plants of the resistant genotype CNPH 1287 (Solanum habrochaites sin. Lycopersicon hirsutum) and susceptible genotype Santa Cruz Kada (S. lycopersicum sin. Lycopersicon esculentum), with the seven-nine and five-seven leaves completely developed, respectively, were inoculated in the second, third and fourth true leaves. Leaves were collected at the time of inoculation and at 4, 8, 12, 24, 48, 72, 96 and 120 h post inoculation (hpi). The production and accumulation of H2O2 and O 2 .- were evaluated in situ using diaminobenzidine and nitroblue tetrazolium, respectively. Starting at 24–48 hpi, high accumulation of H2O2 and O 2 .- was detected, and epidermal cells demonstrated a hypersensitive response, especially in the inoculated leaves of the resistant plant (S. habrochaites). An increase in guaiacol peroxidase, catalase, polyphenol oxidase, β-1,3-glucanase and chitinase activity was mainly detected by 24 hpi in the resistant plant. An association between the production of reactive oxygen species and the activity of enzymes related to reactive oxygen species metabolism (guaiacol peroxidase, catalase), hydrolytic enzymes (β-1,3-glucanase and chitinase) and phenol metabolism enzymes (polyphenol oxidase), as well as hypersensitive response, was evident during the defence responses of the resistant plants when inoculated with O. neolycopersici.

Chestnut rot is an important disease of chestnut trees in Oceania (Australia, New Zealand), and Europe (Italy, France, Switzerland, United Kingdom). The causal agent has been identified as the fungus Gnomoniopsis smithogilvyi (Gnomoniaceae, Diaporthales). Koch’s Postulates was demonstrated on nuts from three Australian chestnut varieties with G. smithogilvyi from Australia, New Zealand and Italy. The Australian and New Zealand isolates were pathogenic on all three varieties, however the Italian isolate produced smaller lesions on all three, and was only mildly pathogenic on Decoppi Marone. Infection of chestnut floral and vegetative tissues was investigated during the chestnut phenological stages of flowering (sampled twice over two years), immature nuts and burrs, mature nuts and burrs, and tree dormancy (immature nuts and burrs, mature nuts and burrs, and tree dormancy were sampled once in one year). Gnomoniopsis smithogilvyi was present in all the sampled phenological stages, most frequently from female flowers during the flowering period. In the first year, the fungus infected 82% of female flowers, while in the second year only 10% were infected. Ascospore trapping in a laboratory chamber experiment confirmed ascopores are released in to the air from infected burrs. Ascospore trapping in an Australian chestnut orchard during the flowering period also showed ascospores are released in to the air, with peak trapping times at sunset and the hours following sunset (8-11 pm), and the hours following sunrise (7-9 am). These experiments support the hypothesis that ascospores of G. smithogilvyi infect chestnut trees, particularly the flowers. The fungus then exists as a latent pathogen in reproductive and vegetative tissues, leading to the development of the disease during nut maturity.

Spot type net blotch caused by Pyrenophora teres f. maculata (Ptm) has become a prominent disease in Western Australia, as has also occurred elsewhere. The disease has a negative impact on both grain yield and quality resulting from reduced grain size. Lack of resistance and stubble retention are the likely factors in the increased severity of the disease in barley growing areas of Western Australia. Because of the increasing importance of spot type net blotch and the need to improve barley resistance, understanding pathogen virulence is a high priority as this has direct impact on the identification and utilization of resistance genes in breeding programs. Ninety nine isolates of Ptm were collected from geographically dispersed barley fields of Western Australia during 2001 and 2002. Forty nine sporulating isolates of Ptm were classified into seven isolate groups (IGs) on the basis of their infection responses on 26 differential barley lines. The 26 lines were likewise classified into four line groups (LGs) based on their distinguishing response to the spot type net blotch isolates. The varied infection responses among the differential barley lines demonstrated a wide geographic dispersal of IGs as well as previously undetected virulence in Ptm in Western Australia. The commercially grown barley cultivars Baudin and Gairdner are regarded as susceptible to spot type net blotch, but showed a range of reactions to the various Ptm isolates as seedling plants. The variability in the pathogen and the resistance identified in some genotypes used in this study are being investigated further to develop superior, adapted germplasm for use in barley breeding programs in Australia.