J. A. Davidson

A new species of Phoma causes ascochyta blight symptoms on field peas (Pisum sativum) in South Australia

Phoma koolunga sp. nov. is described, having been isolated from ascochyta blight lesions on field pea (Pisum sativum) in South Australia. The species is described morphologically and sequences of the internal transcribed spacer region compared with those of the accepted pathogens causing ascochyta blight of field peas. P. koolunga was distinct from Mycosphaerella pinodes (anamorph: Ascochyta pinodes), Phoma medicaginis var. pinodella and Ascochyta pisi. Under controlled conditions the symptoms on pea seedlings caused by P. koolunga were indistinguishable from those caused by M. pinodes, other than a 24 h delay in disease development. Isolates of P. koolunga differed in the severity of disease caused on pea seedlings.

Integrated disease management of ascochyta blight in pulse crops

Ascochyta blight causes significant yield loss in pulse crops worldwide. Integrated disease management is essential to take advantage of cultivars with partial resistance to this disease. The most effective practices, established by decades of research, use a combination of disease-free seed, destruction or avoidance of inoculum sources, manipulation of sowing dates, seed and foliar fungicides, and cultivars with improved resistance. An understanding of the pathosystems and the inter-relationship between host, pathogen and the environment is essential to be able to make correct decisions for disease control without compromising the agronomic or economic ideal. For individual pathosystems, some components of the integrated management principles may need to be given greater consideration than others. For instance, destruction of infested residue may be incompatible with no or minimum tillage practices, or rotation intervals may need to be extended in environments that slow the speed of residue decomposition. For ascochyta-susceptible chickpeas the use of disease-free seed, or seed treatments, is crucial as seed-borne infection is highly effective as primary inoculum and epidemics develop rapidly from foci in favourable conditions. Implemented fungicide strategies differ according to cultivar resistance and the control efficacy of fungicides, and the effectiveness of genetic resistance varies according to seasonal conditions. Studies are being undertaken to develop advanced decision support tools to assist growers in making more informed decisions regarding fungicide and agronomic practices for disease control.

Distribution and survival of ascochyta blight pathogens in field-pea-cropping soils of Australia

Phoma koolunga, Didymella pinodes, and P. medicaginis var. pinodella were detected in DNA extracted from soil following field pea crops across four states in the southeastern and western regions of Australia. P. koolunga was commonly detected in soil from South Australia but rarely in other states whereas D. pinodes plus P. medicaginis var. pinodella were widespread in all regions tested. The quantity of DNA of these pathogens detected in soils prior to growing field pea was positively correlated with ascochyta blight lesions on field pea subsequently grown in infested soil in a pot bioassay and also on field pea in naturally infected field trials. The quantity of DNA of the soilborne pathogens was greatest following a field pea crop and gradually decreased in the following 3 years. The DNA tests were used to quantify the DNA of the pathogens in field pea plants sampled from naturally infected field trials in South Australia over two seasons. The combined results of DNA tests and pathogen isolation from the plants indicated that P. koolunga and D. pinodes were equally responsible for the ascochyta blight symptoms in the diseased trials, while P. medicaginis var. pinodella had a minor role in the disease complex.

G1 Blackspot Manager model predicts the maturity and release of ascospores in relation to ascochyta blight on field pea

A simple model, G1 Blackspot Manager, has been developed to predict the seasonal pattern of release of ascospores in relation to ascochyta blight in field pea. The model considers a combination of two weather factors, daily mean temperature and daily total rainfall, to drive progress of maturity of pseudothecia on infested field pea stubble from past crops. Each day is categorised as suitable or not suitable for continuation of the maturation process. The onset of pseudothecial maturity has been found to take place when approximately ten suitable days have occurred. Following the onset of maturity, ascospore release is triggered when daily rainfall exceeds a threshold. The model was satisfactorily calibrated using three datasets from Western Australia. The calibrated model performed well when independently tested with 21 datasets, 17 from Western Australia and 4 from South Australia. It is concluded that G1 Blackspot Manager model has the potential to be used to formulate sowing guides for field pea in southern Australia that minimise the risk of ascochyta blight.

Management of ascochyta blight in chickpeas in Australia

Ascochyta blight has constrained chickpea production in Australia. Therefore, control strategies are required to prevent major crop losses. Field experiments in 1998 and 1999 showed that all the chickpea varieties grown commercially in Australia at that time were very susceptible to the disease. Fortnightly sprays with the fungicide chlorothalonil could effectively control epidemics but the additional cost significantly reduced profitability. The kabuli variety Kaniva was still profitable to grow but desi varieties were less profitable than alternative crops.Further experiments were conducted throughout Australia in 1999, 2000 and 2001 to compare a range of fungicides and to determine the optimum rates and frequency of fungicide sprays. Chlorothalonil was superior to mancozeb and carbendazim. Fortnightly sprays of chlorothalonil controlled ascochyta blight in all varieties; sprays every 3 weeks did not eliminate yield losses due to ascochyta blight in susceptible varieties under high disease pressure.Lowfungicide rates were less effective than maximum recommended rates when conditions favoured a severe epidemic.Several newvarieties with improved resistance to ascochyta blight have been released and arenowgrown commercially in Australia. Field experiments were established in 2002 and 2005 to compare these new varieties with the older, susceptible varieties. The new varieties had significantly less disease than the older varieties and did not require fortnightly sprays. The best new varieties required fungicide sprays only at the podding stage in order to prevent pod and seed infection.As more varieties with greater resistance become available, growers will need to apply fewer fungicides and the consequences of missing a fungicide spray will be less serious. However, variety specific management strategies still need to be developed to enable growers to tailor their control strategy to each variety’s susceptibility in order to minimise fungicide usage and maximise profits.

G2 Blackspot Manager model to guide field pea sowing for southern Australia in relation to ascochyta blight disease

G2 Blackspot Manager, the second generation (G2) of Blackspot Manager model, predicts disease severity and yield loss in addition to quantified release of seasonal ascospores in relation to ascochyta blight on field pea. The model predicts the disease severity with respect to the expected exposure of field pea crop to ascospores of D. pinodes, with yield loss subsequently related to the disease severity. Both the relationships were developed using published and unpublished data under southern Australian conditions. The model has been used as a decision support tool for developing a field pea sowing guide considering weather-based disease risk and abiotic risk. This paper presents the field pea sowing guide for South Australia, Victoria and Western Australia for the 2010 season and compares it with 2009. The guide is dynamic as the disease severity changes with seasonal weather conditions and is updated weekly starting around mid-April, being delivered principally via the web (http://www.agric.wa.gov.au/cropdisease). The paper also discusses other means of communicating the guide to the stakeholders of southern Australia.

Optimisation of the chemical control of ascochyta blight in chickpea

Ascochyta blight, caused by Didymella rabiei, is the most devastating foliar disease of chickpea in southern Australia. As part of an effort towards developing disease management practices for susceptible cultivars, programs for timing fungicide applications were developed. The efficacy of chlorothalonil and mancozeb in suppressing ascochyta blight was evaluated in five field experiments conducted over 4 years. The results were variable; in some experiments disease was adequately suppressed (control efficacy >89%) whereas in other experiments, control efficacy was insufficient (<32%). Not all of this variability could be explained by differences in the fungicides used or their concentrations. Analysis of the time of spraying in relation to rain events identified possible causes for most of this variability. Ascochyta blight was suppressed when fungicides were applied in time to protect the plants from infections that occurred during rain events, but whenever the plants were not protected during rains, disease suppression was insufficient and control efficacy was low. The coincidence between control efficacy values and the amount of uncontrolled rain was highly significant (P<0.01; R2 =0.937). Data recorded in the field experiments were then used as input into a series of simulations aimed at quantifying how several management approaches could reduce fungicide use. Results were analysed using multiple regression with dummy variables. Compared with continuous protection of the crop throughout the season, which required eight mancozeb or five chlorothalonil applications, using rain forecast to time sprays may enable a reduction in the number of sprays by up to 5.5 and 2.7 per season, respectively, hence, vastly reducing production costs. Initiating sprays after disease onset (based on monitoring) may enable a further reduction of 0.6 sprays per season, on average. Validation of the threshold amount in Australia for local cultivars and implementation of these strategies awaits examination in field experiments.

Advances in winter pulse pathology research in Australia

Pulse crops in Australian broad-acre agriculture are a relatively small but essential component of present-day farming systems. Winter pulses, particularly the five accounted for in this review, dominate pulse area and production in this country. The Australian pulse industry has experienced devastating epidemics of diseases such as lupin anthracnose and chickpea ascochyta blight. In addition, many other diseases have appeared regionally. Research on various aspects was directed towards managing these diseases in individual regions, states and nationally. This review addresses advances in pathology related to bacterial, fungal and viral pathogens in lupins, chickpeas, field peas, lentils and faba beans. In addition to fundamental epidemiological and disease control studies, this paper includes molecular studies and quantitative epidemiology leading to disease modelling and disease forecasting. It also highlights the efforts undertaken recently by pulse pathologists in Australia to strengthen collaborative research nation-wide.

Screening field pea germplasm for resistance to downy mildew (Peronospora viciae) and powdery mildew (Erysiphe pisi).

Downy mildew (caused by Peronospora viciae) and powdery mildew (caused by Erysiphe pisi) cause significant yield losses in field pea crops of southern Australia. The Australian Coordinated Pea Improvement Program (ACPIP) aims to select lines that are resistant to both of these pathogens. A method was developed to allow screening of early generation material for resistance against both diseases, through consecutive testing on single plants. The apical buds of plants were inoculated with conidial suspensions of P. viciae and the same plants were infected with E. pisi via airborne spores in the greenhouse. Of 88 lines tested, 25 had useful downy mildew resistance, 19 lines were resistant to powdery mildew and 14 lines displayed resistance to both pathogens. The results of the controlled environment and greenhouse trials were highly correlated with results of field screening; for downy mildew r = 0.88 (P < 0.001) and for powdery mildew r=0.72 (P < 0.001).

Comparison of cultural growth and in planta quantification of Didymella pinodes, Phoma koolunga and Phoma medicaginis var. pinodella, causal agents of ascochyta blight on field pea (Pisum sativum)

The causal agents of ascochyta blight on field pea in South Australia, Didymella pinodes, Phoma medicaginis var. pinodella and Phoma koolunga, are isolated from a single plant within a crop, suggesting competition for space and nutrients. Interactions among these pathogens were investigated. Diameters of colonies of D. pinodes and of P. medicaginis var. pinodella were significantly reduced on PDA amended with filtrate from broth cultures of P. koolunga as were diameters of colonies of D. pinodes on PDA amended with filtrate from P. medicaginis var. pinodella or D. pinodes. This effect was negated when cultures were transferred to unamended PDA, indicating filtrates were fungistatic instead of fungicidal. The diameter of P. koolunga colonies was not influenced by filtrate from any of the three species. When pathogens were co-inoculated in pairs onto leaves on field pea plants, the quantity of DNA of D. pinodes and of P. medicaginis var. pinodella was significantly reduced if co-inoculated with P. koolunga. The quantity of DNA of P. koolunga was not influenced by co-inoculation. When co-inoculated onto excised leaf disks on sterile water the mean lesion diameter due to D. pinodes and to P. medicaginis var. pinodella was significantly reduced if co-inoculated with P. koolunga isolate DAR78535. Lesions caused by D. pinodes were significantly reduced when inoculum was self-paired. Conversely the diameter of lesions caused by P. koolunga DAR78535 increased when self-paired or when co-inoculated with P. medicaginis var. pinodella. Unlike leaf disks on sterile water, co-inoculation had no influence on lesion size or quantity of pathogen DNA in leaf disks on water agar. Antagonism, including self-antagonism, was detected among these species, leading to reduction in lesion size and quantity of pathogen DNA. The slower growing species, P. koolunga, was not self-antagonistic, and in a few instances the effect of co-inoculation was additive or synergistic.