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Rural Industries Research & Development Corporation
Impact of Climate Change
on Important Plant Diseases in Australia
by Sukumar Chakraborty, Gordon Murray, Neil White April 2002
RIRDC Publication No W02/010 RIRDC Project No CST-4A
Atmospheric CO2 concentration has increased by 30% since pre-industrial times. If carbon emissions were maintained at near-1994 levels of 6 Gt per year, CO2 concentration would reach about 500 ppm by the end of the year 2100, according to the Intergovernmental Panel on Climate Change. CO2 fertilization of crops is expected to increase net primary production from an enhanced rate of photosynthesis and water use efficiency but this increase will be modulated by damage from disease, weed competition and herbivory by insect pests.
From what little is known about the influence of elevated CO2 on host-pathogen interactions, higher carbohydrate concentration within host tissue at high CO2 promotes the development of some biotrophic pathogens such as rusts. This suggests that significant impacts of elevated CO2 will be manifested through changes in host physiology. In addition, elevated CO2 can increase the amount of usable host tissue due to increased biomass that influences the growth, sporulation, and spread of leaf infecting fungi. Increases in canopy size and density can modify microclimate and disease development. But realistic assessment of the impact of elevated CO2 on plant diseases cannot be made at present due to a paucity of knowledge on disease epidemiology.
The objective of this project was, therefore, to identify opportunities for reducing losses due to plant diseases in agricultural and environmental sectors by quantifying impacts of climate change on diseases of economic significance to Australia. Because of the large geographical areas under major crops in Australia and the number of major diseases affecting agricultural crops, two contrasting diseases were selected as model systems: the anthracnose disease of the tropical pasture legume Stylosanthes and stripe rust of wheat.
The work undertaken here combines two approaches: detailed experimental work carried out in the field and in controlled environments; and modelling studies that utilise this work and the available literature. Two significant issues were addressed through the experimental approach: (a) contrasting the effect of ambient and elevated CO2 on disease severity and pathogen inoculum and (b) the longterm effect of elevated CO2 on pathogen adaptation for increased aggressiveness. The modelling study concentrated on two contrasting case studies using two different tools: (a) climate matching algorithms which does not require detailed knowledge of host and pathogen life stages; and (b) linked process-based models where detailed knowledge is available.
Summary of research findings
Stylosanthes plants grown under 700 ppm CO2 concentration in the CEF were taller and had more leaves, branches and biomass compared to plants grown at 350 ppm. The growth of germtube of C.
gloeosporioides conidia and appressoria production were delayed at 700 ppm. Although similar levels of germtube and appressoria production were reached at both CO2 concentrations after 24 h, further symptom development was delayed at 700 ppm CO2. Percentage germination of conidia was also reduced on leaves of plants grown at 700 ppm CO2 and the incubation period was extended in both cultivars and it was significantly longer in the resistant Seca at 700 ppm. The latent period, on the other hand, remained unchanged for both cultivars at 700 ppm. This suggests that once the pathogen has established itself at the end of the incubation period, it develops rapidly to complete sporulation.
Anthracnose severity was reduced in the CEF at 700 ppm CO2, although this reduction was not significant in Seca. Accumulated and daily spore production was significantly higher at 700 ppm than at 350 ppm, but there was considerable variation in daily spore production within each treatment.
When CEF-grown plants were exposed to natural inoculum in the field, disease severity and number of lesions were consistently higher on plants grown at 700 ppm than at 350 ppm CO2. Therefore, field results did not show an overall reduction in severity at 700 ppm, seen in the CEF experiments.
Apart from a possible increase in the production and accumulation of ethylene in the CEF under high CO2 other important differences between the CEF and field experiments influenced results. These include, a rapid change in CO2 concentration from 700 ppm to ambient at exposure influencing host susceptibility, a more conducive microclimate in the field for the high CO2 plants and trapping of more spores by the enlarged canopy of 700 ppm CO2 plants in the field. Anthracnose severity in the field was associated with rainfall and relative humidity greater than 80% and infection was only recorded once in the absence of rain.
The experimental study on the long-term effects of elevated CO2 on the evolution of the pathogen, demonstrated a consistent trend in pathogen aggressiveness for both isolate-host combinations.
Aggressiveness, designated by the total number of lesions, increased over the 25 cycles at ambient CO2, but not at elevated CO2.
Two modelling tools were investigated to compare and contrast the approaches: climate matching with GIS vs. detailed process-based modelling. The climate matching-GIS approach, applied to the Stylosanthes anthracnose system, was the quickest and simplest to implement and was suitable for interpretations at a regional or larger level in the absence of detailed information on the host or the pathogen. The areas suitable for Stylosanthes are also suitable for the anthracnose pathogen and a change in climate is likely to shift the suitability for both host and the pathogen in a southward direction and towards the coast. The Dymex modelling tool could successfully be used to model complex host-pathogen systems. The extra detail incorporated in the process-based model has the advantage of being able to consider finer resolutions, but has a much greater overhead in terms of development time and data sets.
The process-based modelling tool, Dymex was used to link wheat growth to stripe rust life cycle through damage functions. The model outputs were validated using data collected at selected wheatgrowing regions in NSW and further extended to study the potential impact of a changed climate scenario and climatic fluctuations due to ENSO. A published wheat model was adapted for Dymex modelling and a new Dymex model for stripe rust was developed in this work. The stripe rust model had three stages in its life cycle: spore, infective stage and lesion, each with parameters dealing with their mortality, growth and state transfer functions. The leaf area and lesion area were linked through various feedbacks that limit lesion growth and also reduce the transpiration efficiency of the plant.
A location effect was evident in some areas where diseased leaf area decreased for future climates; however, the response was not always consistent between cultivars. The yield loss is brought about by the interaction between the change in climate, the phenology of the cultivar and the impact of stripe rust. A significant location*year type interaction was evident when the effect of ENSO was taken into account; however, there was a general trend towards lower disease levels in El Niño years.
Hands-on training on Dymex modelling of linked host-pathogen life cycle modelling was provided to plant protection professionals from Australia and New Zealand at a workshop in Wagga.
Implications of research
Using experimental and modelling approaches, this study has significantly improved understanding of host and pathogen interactions and disease epidemiology under changing climate and atmospheric CO2. Further work on host resistance and disease epidemiology building on this pioneering study will lead to the identification of opportunities to minimise crop loss from plant diseases under a changed climate. While confirming a fertilization effect of double-ambient CO2 on plant growth this work has unequivocally established that the benefit from the CO2 fertilization effect will depend on the nature of host resistance to important plant pathogens.
While Stylosanthes plants grown in the CEF showed a reduction in anthracnose severity at elevated CO2, this effect was modified by canopy size, its microclimate and inoculum availability in the field.
Consequently, a higher level of disease was seen on elevated CO2 plants. In northern Australia S.
scabra cultivars are predominantly used to improve the quality and availability of feed resources for cattle. If the documented rise in atmospheric CO2 concentration continues at its current rate, enlarged canopies of both trees and the pasture understorey are likely to provide more favourable microclimates for anthracnose development even without any increase in rainfall. In addition, increase in spore production and amount of susceptible tissue due to growth enhancement under elevated CO2 will provide increased infection opportunities for anthracnose to become more significant.
This is the first study of changing pathogen aggressiveness under elevated CO2 showing a consistent trend for both isolate-host combinations. Total lesions increased over the 25 cycles at ambient CO2, but not at elevated CO2 suggesting that enhanced host resistance at elevated CO2 will modulate pathogen aggressiveness. This study has not considered potential host adaptation through increasing cycles of infection or the effect of microclimate and increased inoculum availability under elevated CO2. A more realistic analysis of pathogen evolution under elevated CO2 needs to come from free atmospheric CO2 enrichment studies. Based on our current results, the rate of pathogen adaptation for increased aggressiveness may be slowed under elevated CO2.
The climate matching work shows that areas suitable for Stylosanthes are also suitable for the anthracnose pathogen and the risk to serious damage may remain the same with changing climate as the pathogen migrates to follow its host.
We have successfully used a linked process based modelling approach to explain stripe rust development and wheat yield reduction at selected sites in Australia and further expanded the climatic-dependency of the model to explore the potential impact of climate change and variability on wheat yield. The yield loss is brought about by the interaction between the change in climate, the phenology of the cultivar and the impact of disease. To make these results more generally applicable, further work is needed to compile important phenological attributes for the current suite of cultivars in the Australian wheat growing regions to extend the linked models. Climate change scenarios are complex and updated regular. It is because of the changing nature of climate change predictions that the weather patterns produced by the ENSO were also investigated. The system used here could easily accommodate other sources of daily meteorological data such as those held by the Queensland Centre for Climate Applications.
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