Climate change and Plant Diseases

Climate change is a long-term change in the statistical distribution of weather patterns over periods of time that range from decades to millions of years. Climate change may be limited to a specific region, or may occur across the whole Earth. IMPACT ON PLANT PATHOSYSTEMS

It includes following heads...

PHYSIOLOGY OF HOST PATHOGEN INTERACTION

ELEVATED CO2 –

          Increases in leaf area and duration, leaf thickness, branching, tillering, stem and root length and dry weight are well-known effects of increased CO2 on many plants elevated CO2 would increase canopy size and density, resulting in a greater biomass of high nutritional quality.

          When combined with increased canopy humidity, this is likely to promote foliar diseases such as rusts, powdery mildews, leaf spots, and blights. The decomposition of plant litter is an important factor in nutrient cycling and in the saprophytic survival of many pathogens.

• Increased C: N ratio of litter is a consequence of plant growth under elevated CO₂. Evidence from pot and field studies indicates that decomposition of high-CO2 litter occurs at a slower rate.
• Increased plant biomass, slower decomposition of litter and higher winter temperature could increase pathogen survival on overwintering crop residues and increase the amount of initial inoculum available to infect subsequent crops.

ELEVATED TEMPERATURE

          Increases in temperature can modify host physiology and resistance. Considerable information is available on heat-induced susceptibility and temperature-sensitive genes.

• For example, a rise in temperature above 20±C can inactivate temperature-sensitive resistance to stem rust in oat cultivars with Pg3 and Pg4 genes.
• In contrast, lignifications of cell walls increased in forage species at higher temperatures (165) to enhance resistance to fungal pathogens. Impacts would, therefore, depend on the nature of the host-pathogen interactions and the mechanism of resistance.

ELEVATED MOISTURE

          Moisture can impact both host plants and pathogen organisms in various ways. Some pathogens such as apple scab, late blight, and several vegetable root pathogens are more likely to infect plants with increased moisture – forecast models for these diseases are based on leaf wetness, relative humidity and precipitation measurements.

• Other pathogens like the powdery mildew species tend to thrive in conditions with lower (but not low) moisture.
• More frequent and extreme precipitation events that are predicted by some climate change models could result in more and longer periods with favourable pathogen environments.
• Host crops with canopy size limited by lack of moisture might no longer be so limited and may produce canopies that hold moisture in the form of leaf wetness or high canopy relative humidity for longer periods, thus increasing the risk from pathogen infection.

IMPACTS ON MAJOR GROUPS OF PLANT PATHOGENS

FUNGAL DISEASES

          Soil borne fungi survive in soil by producing structures such as sclerotia or thick-walled spores (e.g., chlamydospores or oospores).  Soil borne fungi include species of Botrytis, Fusarium, Phytophthora, Pythium, Rhizoctonia, Sclerotinia, Sclerotium, and Verticillium.

          Survival structures produced by soil borne fungi are persistent and can survive for years in soil; therefore, the milder winters and reduced soil moisture with climate change are not expected to significantly affect their survival.

BACTERIAL DISEASES

          Most bacterial diseases are considered polycyclic. Bacteria are spread to their host plants mainly by water, usually in the form of rain splash, and insects. Moisture is the most important factor in the development of bacterial diseases. Abundant moisture increases multiplication, oozing, and spread of bacteria. In humid, wet conditions, infected plant tissues can exude masses of bacteria that are spread from host to host by rain splash and insects. Therefore, the warmer drier summers expected with climate change should limit bacterial diseases. However, bacteria often enter their plant hosts through wounds and the expected increase in frequency and intensity of summer storms with high winds, rain, and hail will increase wounding of plants and provide moisture for the spread of bacteria.

VIRAL DISEASES

          Vectors are important in spread of viral diseases. Insects such as aphids are expected to have increased survival with milder winter temperatures, and higher spring and summer temperatures will increase their development and reproductive rates and lead to more severe disease.  Milder winters are also expected to increase survival of alternate weed hosts of viruses.

          Increases in frequency and intensity of summer storms with high winds, rain, and hail will increase wounding of plants and result in increased transmission of viruses by mechanical means. Therefore, with predicted changes in climate, viral diseases of plants are expected to increase in importance.

Conclusions: 

• Plant disease has a major impact on agricultural and natural systems

• Current strategies for management need to be maintained and improved, even if the climate did not change.

• Climate change will increase some disease risks and decrease others.

• The effects of climate change will be most important when thresholds and interactions occur to produce unanticipated large responses

• Systems may change more rapidly than in the past, requiring more research and policy attention

Referances 

M. Pautasso & T. F. Döring & M. Garbelotto & L. Pellis & M. J. JegerEur, 2012. Impacts of climate change on plant diseases—opinions and trends, J. Plant Pathol DOI 10.1007/s10658-012-9936-1.

H. R. Gautam*, M. L. Bhardwaj and Rohitashw Kumar CURRENT SCIENCE, 2013. Climate change and its impact on plant diseases, 105(12) 1685-1691.

Article compiled by Mr. Amol Vijay Shitole (Ph.D. Scholar)

Dr. Panjabrao Deshmukh Krishi Vidyapeeth, Akola (M.S.)