R. B. E. Kimber

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.

SNP discovery and high-density genetic mapping in faba bean (Vicia faba L.) permits identification of QTLs for ascochyta blight resistance

Ascochyta blight, caused by the fungus Ascochyta fabae Speg., is a common and destructive disease of faba bean (Vicia faba L.) on a global basis. Yield losses vary from typical values of 35-40% to 90% under specific environmental conditions. Several sources of resistance have been identified and used in breeding programs. However, introgression of the resistance gene determinants into commercial cultivars as a gene pyramiding approach is reliant on selection of closely linked genetic markers. A total of 14,552 base variants were identified from a faba bean expressed sequence tag (EST) database, and were further quality assessed to obtain a set of 822 high-quality single nucleotide polymorphisms (SNPs). Sub-sets of 336 EST-derived simple sequence repeats (SSRs) and 768 SNPs were further used for high-density genetic mapping of a biparental faba bean mapping population (Icarus×Ascot) that segregates for resistance to ascochyta blight. The linkage map spanned a total length of 1216.8 cM with 12 linkage groups (LGs) and an average marker interval distance of 2.3 cM. Comparison of map structure to the genomes of closely related legume species revealed a high degree of conserved macrosynteny, as well as some rearrangements. Based on glasshouse evaluation of ascochyta blight resistance performed over two years, four genomic regions controlling resistance were identified on Chr-II, Chr-VI and two regions on Chr-I.A. Of these, one (QTL-3) may be identical with quantitative trait loci (QTLs) identified in prior studies, while the others (QTL-1, QTL-2 and QTL-4) may be novel. Markers in close linkage to ascochyta blight resistance genes identified in this study can be further validated and effectively implemented in faba bean breeding programs.

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.

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).

Efficient in-field plant phenomics for row-crops with an autonomous ground vehicle

The scientific areas of plant genomics and phenomics are capable of improving plant productivity, yet they are limited by the manual labor that is currently required to perform in-field measurement, and a lack of technology for measuring the physical performance of crops growing in the field. A variety of sensor technology has the potential to efficiently measure plant characteristics that are related to production. Recent advances have also shown that autonomous airborne and manually driven ground-based sensor platforms provide practical mechanisms for deploying the sensors in the field. This paper advances the state-of-the-art by developing and rigorously testing an efficient system for high throughput in-field agricultural row-crop phenotyping. The system comprises an autonomous unmanned ground-vehicle robot for data acquisition and an efficient data post-processing framework to provide phenotype information over large-scale real-world plant-science trials. Experiments were performed at three trial locations at two different times of year, resulting in a total traversal of 43.8 km to scan 7.24 hectares and 2423 plots (including repeated scans). The height and canopy closure data were found to be highly repeatable (r2 = 1.00 N = 280, r2 = 0.99 N = 280, respectively) and accurate with respect to manually gathered field data (r2 = 0.95 N = 470, r2 = 0.91 N = 361, respectively), yet more objective and less-reliant on human skill and experience. The system was found to be a more labor-efficient mechanism for gathering data, which compares favorably to current standard manual practices.