Introduction to Plant Pathology
Objectives:
• Discuss what makes a biotic interaction pathogenic.
• Define different classes of plant pathogens and strategies.
• Identify disease-causing agents.
-----------------------------------------------------------------------------------------------------------------
-------
Context
Diseases are important to humans because they cause damage to plants and
plant products, commonly with an associated economic effect, either positive or
negative. Negative economic effects include crop failure, incremental loss from
lower quality or failure to meet market standards, elimination of crop options
because of disease propagule buildup, or the costs of control methods. Plant
diseases are also responsible for the creation of new industries to develop control
methodsWe heavily rely on a handful of crop species. Today, 75 % of the world’s
food is generated from only 12 plants and 5 animal species.
Only 4 % of the 250000 to 300000 known edible plant species are used by
humans – i.e. 150 to 200 species. Only three - rice, maize and wheat - contribute
nearly 60 % of calories and proteins obtained by humans from plants.
Food security is heavily reliant on a firm understanding of plant-
pathogen interactions.
Planning future security
Food security depends strongly on the yield of the three main cereals. As of
2005-07, the most important crop as food calories is: wheat in Asia, Europe,
North America (Canada and the US), Australia, South America, and some parts of
Northern Africa; rice in Southeast Asia, Maize and others.
However, plant pathogens cause significant yield losses…
Savary et al. (2019) The global burden of pathogens and pests on major food
crops, Nature Ecology and Evolution (online 4th Feb. 2019) estimate global yield
loss as:
• wheat (21.5% (10.1–28.1%))
• rice (30.0% (24.6–40.9%))
• maize (22.5% (19.5–41.1%))
• potato (17.2% (8.1–21.0%))
• soybean (21.4% (11.0–32.4%))
Climate changes also means change in phytopathogen and pest range: Crop
pests and pathogens move polewards in a warming world | Nature Climate
Change
,Bebber et al (2013): Since the advent of crop domestication 10,000 years ago,
farmers have struggled with various pests and pathogens, leading to famines
and societal disruptions. Notable instances include the Irish potato famine and
the Great Bengal Famine. Today, between 10 and 16% of crop production is lost
to pests, with fungi and oomycetes alone capable of feeding 8.5% of the global
population. The diversity of crop pests continues to grow through evolution and
dissemination of new strains.
Recent strains of rust pathogens and the rapid spread of Phytophthora infestans
highlight the ongoing threat. The interconnectedness of the global food chain
facilitates the spread of pests through both natural and human-mediated means.
Weather plays a significant role in crop disease dynamics, with warming trends
potentially exacerbating pest pressures. Studies speculate about the impact of
climate change on global food security, considering factors such as increased
atmospheric CO2 concentrations, changing climatic patterns, and altered
weather extremes.
Analyzing latitudinal shifts of crop pests reveals significant trends towards higher
latitudes since 1960, particularly in the Northern Hemisphere. While there are
variations among pest groups, overall, there's a noticeable northward movement
in pest distributions. This shift aligns with observations of wild species and is
consistent with expectations under climate change scenarios.
Factors like land use change and agricultural practices can influence pest
distributions, but analyses suggest that climate signals dominate the observed
trends. The findings underscore the threat climate change poses to global food
security, especially for vulnerable populations in poorer regions.
Efforts to monitor and manage emerging pests and diseases are crucial,
particularly in regions most susceptible to climate-driven shifts in pest
distributions. Despite advancements in pest detection and management, the
threat to food security remains a significant concern as climate change continues
to reshape pest dynamics.
Singh et al (2023): Climate change impacts on plant pathogens, food security
and paths forward.
Increasing Incidence and Severity of Plant Disease Outbreaks: Plant diseases are
becoming more prevalent and severe, posing significant risks to global food
security and biodiversity. Pathogens and pests cause annual crop yield losses
estimated at US$220 billion, directly impacting food security and regional
economies. Climate change exacerbates this by increasing disease risk and
altering disease pressure, threatening food supply and natural biodiversity.
Diverse Range of Pathogens and Their Interactions: Plants face various
pathogens including bacteria, fungi, viruses, and nematodes, each with different
infection strategies and target tissues. Understanding their interactions with
multiple disease drivers and climate change is crucial. Climate change can
facilitate plant infections by altering pathogen evolution, host-pathogen
interactions, and vector physiology.
Impact of Climate Change on Pathogen Dynamics: Climate warming affects
pathogen population dynamics, increasing their overwintering, survival, and
, reproduction rates. It shortens incubation periods and enhances disease severity,
impacting crops like coffee, potato, and pine trees. Warming also promotes the
emergence of novel, more virulent pathogen strains.
Effects of Elevated Carbon Dioxide: Increased CO2 levels influence plant-
pathogen interactions, sometimes enhancing disease severity and other times
reducing susceptibility. Elevated CO2 alters plant immune responses and
hormone levels, affecting pathogen infectivity and disease outcomes.
Climate Change-induced Variability in Water Availability: Changes in humidity
and soil moisture influence pathogen abundance and infectivity. High humidity
promotes pathogen virulence, while drought can increase severity for some
diseases but reduce it for others.
Interaction of Multiple Environmental Factors: The combined effects of multiple
environmental factors on plant-pathogen interactions are more pronounced than
individual effects. Abnormal pre-harvest seasons due to climate change can
trigger disease outbreaks. Predicted future climates may shift pathogen
distributions globally, impacting food security.
Impact of Climate Change on Pathogen Evolution: Extreme weather events can
spread pathogens to new locations, and pathogens can evolve to infect new
hosts or become more virulent. These shifts in pathogen biology, host specificity,
and environmental favorability pose challenges for disease management and
control methods.
The interaction between climate change, the plant microbiome, and disease is
complex and multi-faceted. Climate change can impact the plant microbiome
directly and indirectly, affecting disease incidence. Changes in climate variables
like elevated CO2, warming, and drought can alter root exudation and attract
beneficial microorganisms, supporting plant growth but potentially facilitating
pathogen invasion. Pathogens can manipulate the plant microbiome by inducing
changes in plant biology and physiology, modulating plant defense responses,
and exploiting microbial interactions. Plant immunity plays a crucial role in
maintaining microbiome homeostasis and limiting pathogen colonization, but
climate-induced alterations in plant immune systems can disrupt this balance. As
climate change continues, plant-microbiome-pathogen interactions are likely to
be altered, but our understanding of these complex dynamics is still limited.
Future research should consider environmental and host microbiomes explicitly
to improve disease management strategies, potentially incorporating eco-
evolutionary frameworks to predict pathogen invasion and disease outcomes
more accurately.
1. What is a plant pathogen?
