While many studies have demonstrated the sensitivities of plants and of crop yield to a changing climate, a major challenge for the agricultural research community is to relate these findings to the broader societal concern with food security. This paper reviews the direct effects of climate on both crop growth and yield and on plant pests and pathogens and the interactions that may occur between crops, pests, and pathogens under changed climate. Finally, we consider the contribution that better understanding of the roles of pests and pathogens in crop production systems might make to enhanced food security. Evidence for the measured climate change on crops and their associated pests and pathogens is starting to be documented. Globally atmospheric [CO(2)] has increased, and in northern latitudes mean temperature at many locations has increased by about 1.0-1.4 degrees C with accompanying changes in pest and pathogen incidence and to farming practices. Many pests and pathogens exhibit considerable capacity for generating, recombining, and selecting fit combinations of variants in key pathogenicity, fitness, and aggressiveness traits that there is little doubt that any new opportunities resulting from climate change will be exploited by them. However, the interactions between crops and pests and pathogens are complex and poorly understood in the context of climate change. More mechanistic inclusion of pests and pathogen effects in crop models would lead to more realistic predictions of crop production on a regional scale and thereby assist in the development of more robust regional food security policies.

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Integrating pests and pathogens into the climate change/food security debate

The incidence, severity and yield loss caused by 41 pathogens were assessed from a survey of 18 wheat pathologists covering the wheat-growing areas of Australia. The survey provided data on the frequency of years that each pathogen developed to its maximum extent, the proportion of the crop then affected in each growing area, and the yield loss that resulted in the affected crops with and without current control measures. These data were combined with crop production and quality data to estimate the value of the losses aggregated to the Northern, Southern and Western production regions. Pathogens were estimated to cause a current average loss of $913 × 106/year or 19.5% of the average annual value of the wheat crop in the decade from 1998–99 to 2007–08. Nationally, the three most important pathogens were Pyrenophora tritici-repentis, Puccinia striiformis and Phaeosphaeria nodorum with current average annual losses of $212 × 106, $127 × 106 and $108 × 106, respectively. If current controls were not used, losses would be far higher with potential average annual losses from the three most important pathogens, P. striiformis, P. triticirepentis and Heterodera avenae, being $994 × 106, $676 × 106 and $572 × 106, respectively. The average value of control practices exceeded $100 × 106/year for 12 pathogens. Cultural methods (rotation, paddock preparation) were the only controls used for 10 pathogens and contributed more than 50% of the control for a further eight pathogens. Breeding and the use of resistant cultivars contributed more than 50% of control for seven pathogens and pesticides for three pathogens. The relative importance of pathogens varied between regions and zones.

Estimating disease losses to the Australian wheat industry

Virus Diseases of Peanut

A review of Septoria diseases of wheat and barley

The incidence, severity and yield loss caused by 40 pathogens associated with 41 diseases of barley were assessed from a survey of 15 barley pathologists covering the winter cereal growing areas of Australia. The survey provided data on the frequency of years that each pathogen developed to its maximum extent, the proportion of the crop then affected in each growing area, and the yield loss that resulted in the affected crops with and without current control measures. These data were combined with crop production and grain quality data to estimate the value of the losses aggregated to the Northern, Southern and Western production regions. Pathogens were estimated to cause a current average loss of $252 × 106/year or 19.6% of the average annual value of the barley crop in the decade 1998-99 to 2007–08. Nationally, the three most important pathogens are Pyrenophora teres f. maculata, Blumeria graminis f. sp. hordei and Heterodera avenae with current average annual losses of $43 × 106, $39 × 106 and $26 × 106, respectively. If current controls were not used, losses would be far higher with potential average annual losses from the three most important pathogens, P. teres f. maculata, H. avenae and P. teres f. teres, being $192 × 106, $153 × 106 and $117 × 106, respectively. The average value of control practices exceeded $50 × 106/year for nine pathogens. Cultural methods (rotation, field preparation) were the only controls used for 14 pathogens and contributed more than 50% of the control for a further 13 pathogens. Breeding and the use of resistant cultivars contributed more than 50% of control for five pathogens and pesticides for four pathogens. The relative importance of pathogens varied between regions and zones.

Estimating disease losses to the Australian barley industry

A survey of cereal plant pathologists in each State has shown that stem rust is ranked as potentially the most important foliar disease of wheat in most regions. Other major foliar diseases were stripe rust, flag smut, Septoria tritici blotch and leaf rust. Yellow spot has increased in importance since 1975 with the adoption of stubble retention farming in north-eastern Australia and Western Australia. Root rots of major importance were take-all, crown rot, Rhizoctonia bare patch and cereal cyst nematode. Bunt is potentially the most serious disease of the inflorescence but it is very rare because it is effectively controlled by seed-applied fungicides. Most biotrophic foliar diseases are well controlled by selection for resistance. Cultural practices and crop rotation are the main controls for root diseases, while fungicides are mainly used for control of inflorescence diseases. Regional differences in the severity and incidence are pronounced for several diseases.

The incidence and relative importance of wheat diseases in Australia.

Surveys of petal infestation and stem infection conducted in 1998, 1999 and 2000 indicated that Sclerotinia sclerotiorum poses a threat to the Australian canola industry. Inoculum was present throughout all canola-growing regions of New South Wales and the stem disease was widespread throughout southern New South Wales. Percentage petal infestation increased over the 3 years surveyed with values ranging from 0 to 99.4%. The highest petal infestation values were observed in 2000 (maximum of 99.4%, mean of 82.2%), with lower mean values in 1998 (38.4%) and 1999 (49.6%). Stem infection ranged from 0 to 37.5% and most fields had less than 10% stem infection. Stem rot incidence before harvest did not relate to percentage petal infestation determined during flowering. This indicated that factors other than percentage petal infestation were important in influencing stem rot incidence. While there was no relationship between percentage petal infestation and stem rot incidence, stem infection never occurred without prior petal infestation.

Prevalence of sclerotinia stem rot of canola in New South Wales

a Central Science Laboratory, Sand Hutton, York, YO41 1LZ, UK; b P.O. Box 309, Castlemaine, Victoria 3450, Australia; c Plant Production Systems, Wageningen University, P.O. Box 430, 6700 AK, Wageningen, the Netherlands; d IBIMET CNR, Via G. Caproni 8, 50145 Florence, Italy; e Rural Economy Research Centre, Teagasc, Mellows Campus, Athenry, County Galway, Ireland; f Bioforsk, Norwegian Institute for Agricultural and Environmental Research, Division of Plant Health and Plant Protection, Hogskoleveien 7, N-1432 AAS, Norway; g NSW Department of Primary Industries, Wagga Wagga Agricultural Institute, PMB, Wagga Wagga, NSW 2650, Australia; h CRA Centro di Ricerca per la Patologia Vegetale (CRA-PAV), Via C.G. Bertero 22, I-00156 Rome, Italy; and i Environment, Resources and Technology, Faculty of Life Sciences, The University of Copenhagen, Hoejbakkegaard Alle 9, 2630 Taastrup, Denmark

The new Pest Risk Analysis for Tilletia indica, the cause of Karnal bunt of wheat, continues to support the quarantine status of the pathogen in Europe

Aggregate sheath spot and sheath spot of rice were found in Australia in 2001. A disease survey revealed that both diseases are already well distributed within the Australian rice growing areas and that disease severity can be relatively important in some crops. Epidemiological studies showed that under Australian conditions, both Rhizoctonia oryzae and R. oryzae-sativae could overwinter as mycelium on straw debris, regardless of whether the straw is left on the ground or buried. Mycelium of R. oryzae-sativae present on rice straw was also found to be able to produce sclerotia, as a saprophyte, during the overwintering period. Results also strongly suggest that overwintered hyphal fragments present in the debris supplement the sclerotia as a primary source of inoculum, and also highlight the importance of straw management to reduce the inoculum of both pathogens in rice paddocks. The effect of burning stubble on the survival of laboratory-produced sclerotia of R. oryzae-sativae was investigated and it was shown that the vast majority of the sclerotia present on the soil surface survived stubble burning regardless of whether it was a ‘cold burn’ or a ‘hot burn’. A threshold temperature for sclerotial mortality was found to be between 93 and 12 PC.

Prevalence and survival, with emphasis on stubble burning, of Rhizoctonia spp., causal agents of sheath diseases of rice in Australia

Tilletia indica, the cause of Karnal bunt of wheat, replaces part of the seed with a black powdery mass containing millions of spores and produces a strong unpleasant odour like rotten fish. The fishy smell has serious consequences for the marketability of wheat. The fungus is thus subjected to very strict quarantine regulations in countries not known to have the pathogen. The pathogen is spread through natural or human dissemination of spores. Research had shown that a detectable level of the disease would indicate the pre-existence of the pathogen for several years. Once present, this pathogen is extremely difficult to eradicate. Early detection at the incursion stage is thus very important to prevent disease establishment and the spread of the pathogen in a new area. Early detection in the field is unlikely. The key to its detection is thus the deployment of strategic surveillance and quarantine regulations in the wheat supply chain with the use of very sensitive and accurate diagnostic tools to detect and identify a small number of spores. This paper reviews the development of diagnostic assays of increasing sensitivity and specificity for the identification and differentiation of T. indica and other Tilletia spp. contaminants commonly found in wheat grains.

Gordon M. Murray

Papers

The incidence and relative importance of wheat diseases in Australia.

Prevalence of sclerotinia stem rot of canola in New South Wales

The new Pest Risk Analysis for Tilletia indica, the cause of Karnal bunt of wheat, continues to support the quarantine status of the pathogen in Europe

Prevalence and survival, with emphasis on stubble burning, of Rhizoctonia spp., causal agents of sheath diseases of rice in Australia

A review of the methodology to detect and identify Karnal bunt—a serious biosecurity threat