Ming Pei You

Changes in microbial populations of an avocado plantation mulch suppressive of Phytophthora cinnamomi

Abstract The microorganisms inhabiting organic mulch applied in avocado orchards in Western Australia were studied to determine their role in the suppression of Phytophthora cinnamomi . Populations of aerobic bacteria, fungi and actinomycetes within the organic mulch changed with time. Temperature and moisture levels of the mulch were positively correlated with the microbial populations. Bacterial and actinomycete numbers tended to increase following incubation of the mulch infested with P. cinnamomi . The infectivity of P. cinnamomi after 3 weeks of incubation in the mulch was negatively correlated with populations of fungi and actinomycetes.

Characterization of Phytophthora clandestina races on Trifolium subterraneum in Western Australia

Phytophthora clandestina is a causal agent of root rot disease of subterranean clover in Western Australia (W.A). As a significant number of isolates of P. clandestina from W.A. could not previously be designated using existing differentials, a comprehensive set of subterranean clover (Trifolium subterraneum) cultivars was used as differentials to delineate a broader range of races of the pathogen. One hundred and one isolates of the pathogen collected from W.A. were screened on nine subterranean clover cultivars, of which seven were found to be useful as host differentials. A total of 10 races (in contrast to the five recognized previously) were defined and differentiated using octal nomenclature, presenting a clearer picture of the racial distribution of P. clandestina among W.A. isolates. Differences were found in the race populations between Australian states and are therefore important to the selection/breeding of cultivars for specific regions of Australia to counter the predominant race populations and for enforcing quarantine measures in relation to seed movements within and outside Australia. The octal nomenclature used provides a sound basis for follow-up studies and future race designations. Races 173 and 177 in this study were widely distributed and were the most common races in W.A., and together constitute 80% of the isolates characterized. While six of the seven host differentials were resistant to isolates belonging to race 001 and all were resistant to 000, it is of concern that only one differential was resistant to 157 and 173 and that none of the host differentials were resistant to 177. Our approach to P. clandestina race delineation is clearly conservative and is different from previous studies. The octal nomenclature we applied in this study is not only scientifically sound but also will facilitate rapid recognition and characterization of the races.

Salinity drives host reaction in Phaseolus vulgaris (common bean) to Macrophomina phaseolina

Productivity of Phaseolus vulgaris L. (common bean) is often limited by diseases such as seedling blight and root and stem rot caused by the fungus Macrophomina phaseolina and by abiotic stresses such as salinity. This paper reports controlled environment studies examining the interaction of biotic (M. phaseolina) and abiotic (NaCl) stresses. Studies were conducted at 32°C. On potato dextrose agar, the growth of two isolates of M. phaseolina (M1, M2) was differentially stimulated by 40mM NaCl with 1mM CaSO4. M. phaseolina was applied as either soil-borne inoculum or directly injected into P. vulgaris hypocotyls. For direct hypocotyl inoculation experiments, there was no difference in disease severity resulting from the two isolates. However, when soil inoculation was undertaken, isolate M2 caused more disease than M1. Addition of 40mM NaCl to the soil increased disease development and severity (evident 4 days after inoculation), particularly as demonstrated in the hypocotyl inoculation tests, suggesting that salinity stress predisposes plants to infection by this pathogen. Plants infested by M. phaseolina showed increased tissue concentrations of Na+ and Cl- but decreased K+ concentration. Hypocotyls generally contained higher Na+ concentrations than shoots. Inoculated plants had higher Na+ and lower K+ concentrations than uninoculated plants. Our studies indicate that M. phaseolina will be a more severe disease threat where P. vulgaris is cultivated in areas affected by soil salinity.

The association of necrotrophic fungal pathogens and plant parasitic nematodes with the loss of productivity of annual medic-based pastures in Australia and options for their management

Annual medics (annual Medicago spp.) are often an integral component of dry-land farming and cropping systems worldwide, including significant areas of the winter rainfall regions of southern Australia. Necrotrophic fungal pathogens frequently dominate such areas because of the ease of survival of these trash-and soilborne pathogens on infested residues over the relatively dry summer period. Necrotrophic fungal pathogens, in general, are favoured by the nutrient impoverished soils across many parts of these regions, which provide little microbial buffering against these pathogens. This review first, outlines the major and most widespread diseases caused by necrotrophic fungal pathogens and plant parasitic nematodes; second, defines the association of these pathogens with the loss of productivity of annual medic-based pastures; and finally, investigates the spectrum of control options for their management. Important necrotrophic foliar fungal pathogens include Phoma medicaginis, Colletotrichum trifolii, Leptosphaerulina trifolii, Pseudopeziza medicaginis, Stemphylium botryosum, S. vesicarium and Stagonospora meliloti. Necrotrophic foliar fungal pathogens, for example Phoma medicaginis, are also known to stimulate production of phyto-oestrogenic compounds to high levels that can adversely affect ovulation rates in sheep. There are also necrotrophic fungal root pathogens in Australia that have been associated with significant decline in productivity and pose such a serious threat to annual medic pastures such that reseeding is required. In particular, Rhizoctonia solani and various Fusarium species such as F. avenaceum, F. acuminatum, F. culmorum, F. graminearum and F. lateritium, and various Pythium species, in particular P. irregulare, P. ultimum and P. spinosum, are of concern. Another important soilborne and root-attacking necrotrophic pathogen is Phytophthora clandestina. The association of Fusarium spp. with annual medic roots, crowns and burrs in Australia is cause for additional concern as some of them are responsible for the production of deleterious mycotoxins. Plant parasitic nematodes are important as a potential yield limiting factor in annual medics. Approaches to disease management include strategies that have been utilised to varying degrees for the control of necrotrophic foliar and root pathogens in annual medic pastures. This paper reviews the current literature on the topic and provides an assessment of options available for their management. In particular, host resistance offers the most cost-effective, longterm control, particularly as some useful resistance to several of these pathogens has been identified. One or more cultural control strategies, including grazing, fertiliser application, rotations and seed health, offer further opportunities for restricting losses in annual medics from diseases caused by necrotrophic fungi and/or plant parasitic nematodes, especially if applied as an integrated management strategy with only minor reliance on fungicides. As many of these foliar and soilborne fungal pathogens and parasitic nematodes can also threaten crops grown in rotation, these pathogens pose a wider threat to the farming systems employed across southern Australia and reduce the potential for providing a ‘disease break’. In addition, the role played by these pathogens is likely far wider and of greater adverse impact than previously considered as a consequence of their potential for causing mycotoxin and/or phyto-oestrogen production. The full array of losses, not just herbage and seed yields, needs to be considered for the more important fungal and nematode pathogens. Genetic resistance to individual pathogens provides the best option for management. The success and outcome with sourcing resistance in other annual pasture legumes such as Trifolium spp. highlights the value of seeking out new sources of host resistance from the Mediterranean centre of origin, even if the particular diseases of interest do not occur there, in the same way that has been shown for herbicide resistances and for Kabatiella on subterranean clover. pasture legumes such as Trifolium spp. highlights the value of seeking out new sources of host resistance from the Mediterranean centre of origin, even if the particular diseases of interest do not occur there, in the same way that has been shown for herbicide resistances and for Kabatiella on subterranean clover.

Occurrence of Phytophthora clandestina races across rainfall zones in south-west Western Australia

Cultivars of subterranean clover (Trifolium subterraneum), Meteora and Denmark, sown in high rainfall zones in Western Australia were unique in their sensitivity to races of Phytophthora clandestina. The majority and most diverse of P. clandestina races occurred in the 700–1000 mm rainfall zone. Race 173 occurred across all rainfall zones ≥300 mm and race 177 in all zones ≥400 mm. Cultivar Denmark was highly resistant to races 001, 101, 141, 151 and 143, and moderately resistant to race 121, whereas cv. Meteora was highly resistant to race 151, moderately to highly resistant to races 001 and 101 and moderately resistant to races 121, 141 and 143. These differences are important in relation to the selection and breeding of cultivars for specific regions of Western Australia where particular races occur.

New Trifolium subterraneum genotypes identified with resistance to race 2 of Kabatiella caulivora and cross-resistance to fungal root rot pathogens

One hundred subterranean clover genotypes including 72 advanced breeding lines from Trifolium subterraneum ssp. subterraneum and Trifolium subterraneum ssp. yanninicum and 28 Trifolium subterraneum commercial cultivars were screened in the field for resistance to race 2 of Kabatiella caulivora, and the resistances found were related to known resistance to major root pathogens in the region. Race 2 of K. caulivora causes severe damage on subterranean clover in the south-eastern coastal region of Western Australia and 72 of the 100 genotypes tested were resistant to this race, with levels similar to those shown by the cultivar Denmark. The unique importance of this study was that, for 12 genotypes of subterranean clover, these resistances were related to those shown to major root pathogens, viz. one or more of Phytophthora clandestina, Pythium irregulare, and Fusarium avenaceum. Availability of genotypes with such resistances to multiple pathogens is expected to be particularly valuable for the breeding/selection of subterranean clover in relation to the development of new cultivars with effective resistance to a range of pathogens that commonly occur in southern Australian annual legume pastures.

New sources of resistance in Trifolium subterraneum L. to root rot caused by two races of Phytophthora clandestina Taylor, Pascoe and Greenhalgh

Eighty-four genotypes, comprising 71 ssp. subterraneum and ssp. yanninicum breeding lines of Trifolium subterraneum and 13 cultivars commonly used at the time of commencement of the experiment, were screened in the glasshouse for resistance to root rot caused by 2 races of Phytophthora clandestina that occur most widely in Australia. Resistance to race coded 001 was identified in 7 mid-season genotypes of ssp. subterraneum, including the new cultivar, Coolamon, and one genotype also showed resistance to race coded 373. Of the late flowering ssp. subterraneum genotypes tested, 13 showed resistance to race coded 001 and 4 of them also showed resistance to race coded 373. In the late flowering ssp. yanninicum group, 12 of 13 genotypes tested, including the new cultivar, Napier, showed resistance to both races. Of the mid-season ssp. yanninicum genotypes, all but 2 of 19 tested showed resistance to both races. The resistance observed in the majority of ssp. yanninicum and in some ssp. suberraneum genotypes, indicates that these are useful sources of resistance that can be exploited, either directly as new cultivars to minimise damage from this disease, or as parents in breeding programs to develop cultivars with improved resistance to P. clandestina. This study established the availability of 51 advanced lines and 11 cultivars as sources of resistance against P. clandestina race coded 001 and 36 lines and 4 cultivars for race coded 373, among which 36 lines and 4 cultivars were resistant against both races.

Relationship of rainfall, cultural practices, soil and plant nutrients, and seedling survival with root disease and parasitic nematode numbers in annual Medicago spp. pastures

Surveys were conducted for annual Medicago spp. (medic) pastures in the grain belt of south-west Western Australia during spring 1996 and winternspring 1997 to determine the relationship of rainfall, cultural prac- tices, soil and plant nutrients, and seedling survival with severity of root disease and numbers of parasitic nema- todes. Medic pasture was sampled on 116 farms. Most pastures consisted of a single medic variety, viz. Serena, Santiago, Cyprus, or Caliph, whereas about 33% of sites had mixed varieties. Regression analyses showed that high rainfall and application of phosphorus fertilisers were correlated with increased severity of rot in medic tap roots. Crop history and medic variety were not related to the level of root rot. Numbers of Pratylenchus in medic roots were not correlated with the level of tap or lateral root rot, medic variety, rainfall, or with the application of insec- ticide, fertilisers, or herbicides. Soil with relatively high levels of P, NO3 , or Fe was associated with an increased level of tap root rot. Soils with high pH were associated with reduced tap root rot. Soils with relatively high K were related to severe lateral root rot, wheras relatively high levels of P in soil were associated with reduced lateral root rot. Plants with high levels of tap root rot showed low levels of Mg, whilst low levels of Ca and NO3 n in tissues were related to high levels of lateral root rot. High levels of tap root rot were associated with relatively high levels of total N, K, and S, Cu, Zn, Mn, and NO3 n in plant tissues. Plants with relatively high levels of lateral root rot had rela- tively high levels of Cu in shoots. Of the 116 annual Medicago pastures sampled, only 1% had adequate Mg content and only 19% had adequate Ca content. However, 83% had higher than adequate levels of Cu, 70% had higher than adequate levels of Mn, and all samples showed more than adequate levels of chloride. Experimental sites of M. poly- morpha cv. Serena at 6 farms showed that the percentage survival rate of seedlings was negatively correlated with the severity of tap and lateral root rot in the previous year. These results indicate that in the farms surveyed there is a serious threat to annual medic pastures from root rot fungi. The severity of the disease was partly determined by soil conditions and cultural practices.

Calcium oxalate crystals: An integral component of the Sclerotinia sclerotiorum/Brassica carinata pathosystem

Oxalic acid is an important virulence factor for disease caused by the fungal necrotrophic pathogen Sclerotinia sclerotiorum, yet calcium oxalate (CaOx) crystals have not been widely reported. B. carinata stems were infected with S. sclerotiorum and observed using light microscopy. Six hours post inoculation (hpi), CaOx crystals were evident on 46% of stem sections and by 72 hpi on 100%, demonstrating that the secretion of oxalic acid by S. sclerotiorum commences before hyphal penetration. This is the first time CaOx crystals have been reported on B. carinata infected with S. sclerotiorum. The shape of crystals varied as infection progressed. Long tetragonal rods were dominant 12 hpi (68% of crystal-containing samples), but by 72 hpi, 50% of stems displayed bipyramidal crystals, and only 23% had long rods. Scanning electron microscopy from 24 hpi revealed CaOx crystals in all samples, ranging from tiny irregular crystals (< 0.5 μm) to large (up to 40 μm) highly organized arrangements. Crystal morphology encompassed various forms, including tetragonal prisms, oval plates, crystal sand, and druses. Large conglomerates of CaOx crystals were observed in the hyphal mass 72 hpi and these are proposed as a strategy of the fungus to hold and detoxify Ca2+ions. The range of crystal morphologies suggests that S. sclerotiorum growth and infection controls the form taken by CaOx crystals.

High level resistance to Pseudocercosporella capsellae offers new opportunities to deploy host resistance to effectively manage white leaf spot disease across major cruciferous crops

Field and controlled environment studies were undertaken to define the range and extent of available host resistances to Pseudocercosporella capsellae (white leaf spot) across diverse oilseed, forage and vegetable crucifers, including some wild and/or weedy species, and also within and/or derived from Brassica carinata. In each experiment, there was a wide range in host response from high resistance to high susceptibility as assessed by four disease parameters, viz. in the field for: (i) Area Under Disease Progress Curve (AUDPC) for percent leaves diseased with values ranging from 0 to 375.5; (ii) Percent Leaf Collapse Index (%LCI) for leaf collapse due to disease with values ranging from 0 to 23.0; and (iii), Percent Pod Area Disease Index (%PADI) for pod area affected with values ranging from 0 to 52.1; and (iv) under controlled environmental conditions for Percent Cotyledon Disease Index (%CDI) for cotyledon lesion size with values ranging from 0 to 27.5. At the Crawley field site, B. carinata ATC 94129 was the most resistant genotype with AUDPC = 1.2, followed by Crambe abyssinica (AUDPC 8.7), Eruca sativa Eruc-01 (AUDPC 19.3) and E. vesicaria Yellow rocket (AUDPC 19.4). B. carinata ATC 94129 and B. oleracea var. capitata had the least leaf collapse, with %LCI = 0.2. At the Shenton Park field site, 21 genotypes of B. carinata and B. oleracea var. acephala Tuscan kale showed total resistance, all with AUDPC values of 0. Of the B. napus genotypes carrying one or more B. carinata B genome introgressions, genotypes NC8 (AUDPC 23.0) and NC9-1 (AUDPC 26.2) were the most resistant. Genotypes as assessed on these disease criteria as having high level resistance generally showed no pod infection; in contrast to %PADI values up to 52 on the most susceptible genotypes. Under controlled environmental conditions, the most resistant genotype was B. carinata ATC 94129 with %CDI values of 0 and 0.2, respectively, across two experiments, along with B. napus genotypes Zhongyou 821 and Hyola 42, with a %CDI value of 0 in one of the two experiments. There was a high degree of correlation both within individual experiments across the different disease parameters and also between field and controlled environment experiments. Within both B. napus and B. juncea genotypes tested, the most resistant genotypes were from China, the most susceptible from India, with those from Australia intermediate.