This document summarizes a seminar presentation on the interaction between the environment and plant pathogens in plant diseases. It discusses how climate change impacts the three elements of the disease triangle: the host, pathogen, and environment. It describes how increased carbon dioxide and temperatures can influence pathogen growth and disease development. Specifically, it provides examples of how higher temperatures and carbon dioxide levels can increase the severity of certain diseases in crops like wheat, citrus, and soybean, while decreasing other diseases. The document also reviews historical developments in the field of plant disease epidemiology.

1. MASTER’S SEMINAR
“Environment-Pathogen
Interaction In Plant
Diseases” Rajat Sharma
H-2017-67-M
M.Sc. 2nd Year
on

2. INTRODUCTION
• Climate change is a major environmental challenge worldwide. Green house
gases (GHG) viz., water vapour (H2O), carbon dioxide (CO2), Methane (CH4),
nitrous oxide (N2O), hydrofluorocarbons (HFCs) and Ozone (O3) in the
atmosphere trap reflected radiation to warm the earth surface (Mahato, 2014).
• Human activities are widely involved in increasing global climate changes that
directly influences the ecology (Pachauri and Reisinger, 2007 and Ahanger et al.,
2013).
• According to Inter-govenrmental Panel on Climate Change (IPCC, 2007); the
planet earth is experiencing a climate change and atmospheric CO2 is a major
GHG, which increased by nearly 30% and temperature by 0.3- 0.6°C.

3. • This global climate changes by various factors (Pachauri and Reisinger, 2007 and
Pachauri et al., 2014) and change or influence all the 3 major elements of disease
triangle, viz., host, pathogen and environment (Legreve and Duveiller, 2010).
• Crop growth and production can be significantly affected due to high atmospheric
CO2 concentration, temperature, changes in precipitation patterns and frequency
of extreme weather phenomena and diseases presence will altered under these
condition (Rosenzweig and Tubiello, 2007, Ghini et al., 2008 and Chakraborty,
2011).
• When the host present pathogens with short life cycles, reproduction rates is high
and dispersion mechanisms respond quickly and adapt faster to climate change
(Coakley et al., 1999).

4. • Climate change would affect plant diseases together with anthropogenic
processes such as air, water and soil pollution, long-distance introduction of
exotic species and urbanization (Regniere, 2012).
• These factors contribute to the spread of diseases viz. sudden oak death
(Prospero et al., 2009). Elevated temperature and CO2 concentration have
impact on plant-disease interaction (Lopez et al., 2012) and posing a higher
threat perception of late blight (Phytophthora infestans) of potato and blast
(Magnaporthe grisea) and sheath blight (Rhizoctonia solani) of rice (Kobayashi
et al., 2006).

6. EPIDEMIC Change in disease intensity in a host
population over time and space.
EPIPHYTOTIC
Unger (1833), Whetzel (1920's)
Being a plant disease that tends to recur
sporadically and to affect large numbers of
susceptible plants.
EPIDEMIOLOGY Science of disease in populations.
Vanderplank(1963)
• Study of the spread of diseases, in space
and time, with the objective to trace factors
that are responsible for, or contribute to,
epidemic occurrence.

7. HISTORY (ANCIENT TO MODERN TIMES)
Hippocrates (~400 BC) : First use of "epidemic", widespread disease (human
disease.
Theophrastus (~340 BC) : Plant diseases in fields, Environmental influences
Pliny (~50 AD) : Plant diseases; soil; climate.
Duhamel de Monceau : Disease progress curves.
(1728 AD)
Late 19th Century and forward…
Kuhn (1858) - 1st textbook of plant pathology.
Ward (1901): Book "Diseases in Plants" emphasized ecology
(populations) of disease.
Jones (1913) - Role of the environment.
Gaumann (1946): "Principles of Plant Infection” -Disease spread, -
Conditions leading to an epidemic, -'Infection Chain' (=
disease cycle),-compare with medicine (disease of
humans).

8. Large(1952, andothers)
-Disease progress curves
-Crop losses
-Disease assessment (measurement)
Horsfall & Dimond (1960)- "Plant Pathology,
Volume 3"
-Populations
-Inoculum density:disease relations
-Spore dispersal
-Analysis (mathematics)
-Forecasting, prediction
-Traditional definition ---> Modern
definition
Gregory(1963, 1973)
"The Microbiology of theAtmosphere"
-spore dispersal, disease spread
Aerobiology
Vanderplank (1963) (usedto be van
der Plank)
"Plant Diseases: Epidemics and
Control"
-Populations
-Rates (dynamic processes)
-Analysis, mathematics
-Models, theory
-Link epidemiology and control
-Established the science of plant
disease epidemiology
Other pioneers:
Zadoks (1960-1995), TheNetherlands
Kranz (1968-1995),Germany
Waggoner (1960-mid --1980s),USA
S. Nagaranjan1983-India

9. ELEMENTS OF AN EPIDEMIC
1) Host
2) Pathogen
3) Environment
Interactions of the 3 main
components are described by
the disease triangle.
The Disease Triangle
Disease development is also affected by
4. Time
5. Humans
Disease Tetrahedron
Interactions of the 5
components are described by
the disease pyramid.

12. HOST FACTORS
All plants can be considered hosts.
o Degree of genetic uniformity.
o Age – affects disease development depending on plant-pathogen
interaction.
There are different levels of susceptibility, which include:
o Immune - cannot be infected.
o Susceptible - can be infected.
o Resistant - may or may not be infected.

13. ENVIRONMENTAL FACTORS
i. Moisture
-Rain, dew, high humidity.
-Dominant factor in diseases
caused by oomycetes, bacteria &
nematodes.
ii.Temperature
- Affects disease cycles of
pathogens
Diseasedevelopment is also
affected by
 Time factors
• Season of the year
• Duration & frequency of
favorable temp. & rains
• Appearance of vectors, etc.
 Humans
How HumansAffect Development of
Epidemics
• Site Selection & Preparation.
• Selection of Propagative Material.
• Introduction of Exotic Pathogens.
• Cultural Practices.
• Disease control measures.

14. CLIMATE CHANGE & MICROBIAL INTERACTIONS
i. Nitrogen deposition level, CO2 concentration and temperature are
important factors affecting soil microbial communities (Garret et al.,
2006).
ii. Increased CO2 levels in the atmosphere have major consequences on
carbon cycling and the functioning of various ecosystems.
iii. Short-term and long-term changes in the abiotic conditions not only
affect plant growth and productivity but also the populations of
microorganisms living on plant surfaces.
iv. Any change in phyllosphere microflora, affects plant growth and
plants’ ability to withstand aggressive attack of pathogens.

15. EFFECT OF TEMPERATURE
Certain minimum temperature is required by both plants and
pathogens to grow. Temperature affects the chain of events in
disease cycles such as survival, dispersal, penetration,
development and also reproduction rate for many pathogens.
Generally high moisture and temperature favours and initiate
disease development, as well as germination and
proliferation of fungal spores of diverse pathogens (Agrios,
2005).

16. EFFECT OF TEMPERATURE
• Plants, as well as pathogens, require certain minimum temperatures to
growandcarryout their activities.
• At higher temperatures, pathogens become active and, they can infect
plants andcausedisease.
• Cankerdiseasesof perennial plants causedby:
 fungi Nectria, Leucostoma (Cytospora), the oomycete
Phytophthora
 bybacteriasuchas Pseudomonas,
• Infections beginanddevelop primarily in earlyspring or in thefall.
• During these periods the temperatures are high enough for these fungi
to growwell but aretoo low to allow optimum hostdevelopment.

17. TEMPERATURE AFFECTS:
1. The number of spores formed in aunit plant
area.
2. The number of spores released in a given time
period.

18. • Due to changes in temperature and precipitation, climate change may
alter the growth stage, development rate, pathogenicity of infectious
agents, and the physiology and resistance of the host plant
(Charkraborty and Datta, 2003).
• Sunlight affects pathogens due to the accumulation of phytoalexins or
protective pigments in host tissue.
• Host plants such as wheat and oats become more susceptible to rust
diseases with increased temperature; but some forage species become
more resistant to fungi with increased temperature (Coakley et al.,
1999).

19. • With increasing temperature spore germination of rust fungus Puccinia substriata
increases (Tapsoba and Wilson, 1997).
• In southern Germany, a northward shift of Cercospora beticola, leaf spot of sugar
beeet was due to increasing annual mean temperature by 0.8-1°C (Richerzhagen
et al., 2011).
• Altered temperatures favour over-wintering of sexual propagules which increased
the evolutionary potential of a population (Pfender and Vollmer (1999).
• Conidia of powdery mildew have the ability to germinate even at 0% relative
humidity (RH) (Yarwood, 1978).
• Conidia of Erisiphe cichoracearum germinate at temperature from 7 to 32°C with a
RH of 60 to 80% (Khan and Khan, 1992); and spores of Erysiphe necator germinate
at temperatures from 6 to 23°C with a RH from 33 to 90 % (Bendek et al., 2007).

20. • Phytophthora infestans, late blight of potato and tomato, infects and
reproduces most successfully at high moisture when temperatures
are between 7.2°C and 26.8°C.
• Infection of Eucalyptus sp. by Phytophthora cinnamomi due to
increased soil temperature of 12-30°C (Podger et al., 1990).
• Even the incidence of virus and other vector borne diseases also alter.
Mild and warmer winters make aphids easy to survive thus
spreading Barley yellow dwarf virus (BYDV) and also increase viruses
of potato and sugar beet (Thomas, 1989; Mackerron et al., 1993).

21. RISK ANALYSIS STUDIES ON THE EFFECTS OF CLIMATE CHANGE
STUDY RESULT REFERENCE
• In the UK, the effects of climate
change on Phoma (Leptosphaeria
maculans) were assessed.
Epidemics will not only increase in
severity but also spread northwards by
the 2020s.
Evans et al., 2007
• Future scenarios of downy mildew
on grapevine (Plasmopara viticola)
were simulated from the results of
two climate change models. The
results suggested that the incidence
of disease would increase and the
production of grapes in
northwestern Italy would decrease.
Predicted an increase of the disease
pressure in each decade to
consequence of more favorable
temperature conditions.
Salinari et al., 2006
• A model to assess the severity of
Phytophthora infestans under
climate change was developed
A marked shift of the disease in the
infestation pressure to higher altitude
Kocmankova et al., 2007

22. STUDY RESULT REFERENCE
The effects of elevated levels
of CO2 and temperature on
the incidence of four major
chili pepper diseases
 (Anthracnose
(Colletotrichum acutatum)
 Phytophthora blight
(Phytophthora capsici))
and
• two bacterial diseases
(bacterial wilt (Ralstonia
solanacearum) and
bacterial spot
(Xanthomonas campestris
pv. vesicatoria)) were
determined.
Elevated CO2 and
temperature significantly
increased the incidence of
two bacterial diseases.
Anthracnose decreased and
Phytophthora blight slightly
increased.
Shin and Yun, 2010

23. A summary of the influence of elevated Temperature on some Host and Pathogen Interaction
CROP/HOST DISEASE/PATHOGEN CLIMATE CHANGE CHANGE IN DISEASE
SEVERITY
AUTHOR/REFERENCE
Wheat Stripe rust – Puccinia
striiformis
Elevated average annual
temperature
Decrease Yang et al., 1998
Wheat Dwarf bunt- Tilletia
controversa
Elevated temperatures Increase Boland et al., 2005
Wheat Wheat Stripe rust –
Puccinias triiformis
Higher Temperature Increase Milus et al., 2006
Citrus Anthracnose-
Colletotrichum acutatum
Elevated Temperatures Increase Jesus Junior et. al., 2007
Potato Late blight – Phytophthora
infestans
Elevated temperatures
causing earlier seasons
Increase Hannukkala et al., 2007
Papaya Asperisporium caricae Elevated temperatures and
lower relative humidity
Decrease Jesus Junior et. al., 2007
Pineapple Fusarium subglutinans Elevated temperatures Decrease Matos et al., 2000
Coffee Meloidogyne incognita Elevated temperatures Increase Ghini et al., 2008

24. EFFECT OF CO2
a. Increased size of plant organs, leaf area, leaf thickness, more numbers of leaves,
higher total leaf area/plant, stems and branches with greater diameter are
resulted from increased CO2 levels (Bowes, 1993 and Pritchard et al., 1999).
b. Dense canopy favours the incidence of rust, powdery mildew, Alternaria blight,
Stemphylium blight and anthacnose diseases.
c. Higher CO2 concentrations induce greater fungal spore production. Increased
CO2 also enhances photosynthesis, increased water use efficiency and reduced
damage from ozone (von Tiedmann and Firsching, 2000); and leaf area, plant
height and crop yield are increased at higher doses of CO2 (Eastburn et al., 2011).
d. The physiological changes on the host plant due to increased CO2 can conversely
result in increase host resistance to pathogens (Coakley et al., 1999).

25. EFFECT OF C02
i. Under elevated CO2 conditions, potential of dual mechanism i.e., reduced
stomata opening and altered leaf chemistry results in reduced disease
incidence and severity in many plant pathosystems where the pathogen targets
the stomata (Mcelrone et al., 2005).
ii. In soybean, elevated concentration of CO2 and O3 altered the expression of 3
soybean diseases, downy mildew (Perenospora manshurica), brown spots
(Septoria glycines) and sudden death syndrome (Fusarium virguliforme)
(Eastburn et al., 2010).
iii. Elevated CO2 also leads to production of papillae and accumulation of silicon
by barley plants at the site of appressorial penetration of Erysiphe graminis
and changed leaf chemistry that decrease susceptibility to the powdery
mildew pathogen (Hibberd et al., 1996).

26. EFFECT OF INCREASED CO2 CONCENTRATIONS ON PATHOGENS
STUDY RESULT REFERENCE
• The effect of elevated concentrations
of CO2 on the infection of barley by
Erysiphe graminis was determined.
The percentage of conidia that progressed
to produce colonies was lower in plants
grown in 700 than in 350ppm CO2.
Hibberd et al. (1996)
• Interactive effects of elevated CO2 and
O3 levels on wheat leaves infected
with leaf rust fungus Puccinia triticina
were described.
Elevated CO2 increased the
photosynthetic rates of the diseased
plants by 20 and 42% at the ambient and
elevated ozone concentrations,
respectively.
Tiedemann and Firstching (2000).
• The effects of elevated CO2
concentrations on the development of
Phytophthora parasitica (root rot) in
tomato were evaluated.
The extra CO2 completely
counterbalanced the negative effect of
the pathogenic infection on overall plant
productivity.
Jwa and Walling (2001).
• Pyricularia oryzae Cavara and
Rhizoctonia solani Kühn were
evaluated.
Rice plants grown in an elevated CO2
concentration were more susceptible to
leaf blast than those in ambient CO2.
Kobayashi et al. (2006).

27. STUDY RESULT REFERENCE
• The effects of carbon dioxide (CO2)
and ozone (O3) on three soybean
diseases (downy mildew, Septoria
and sudden death syndrome) were
determined in the field.
Changes in atmospheric composition
altered disease expression. Elevated CO2
reduced downy mildew disease
severity. But increased brown spot
severity and without effect in sudden
death syndrome.
Eastburn et al. (2010).
• The response of tobacco to potato
virus Y was evaluated.
The titre of viral coat-protein was
markedly reduced in leaves under these
conditions at both nitrogen levels. The
accumulation of phenylpropanoids,
may result in an earlier confinement of
the virus at high CO2.
Matros et al. (2006).
• The germination rates of conidia of
C. gloeosporioides were determined.
spore germination was reduced and
extended incubation period was at 700
ppm, and Anthracnose severity was
reduced.
Chakraborty et al. (2002).

28. EFFECT OF MOISTURE
• It influences the initiation and development of infectious plant
diseasesin many interrelatedways.
Moisture is indispensablefor:
 The germination of fungalspores
 Penetration of the host by the germ tube
 Activation of bacterial, fungal, and nematode pathogens
before they caninfect theplant.
 The spread of pathogens on the same plant and from one plant to
another
• Moisture increases the succulence of host plants and thus their
susceptibility to certain pathogens, which affects the extent and
severity ofdisease.

29. EFFECT OF MOISTURE
With increased temperature extreme rainfall events and higher atmospheric
water vapour concentrations take place.
i. These encourage the crops to produce healthier and larger canopies that
retain moisture as leaf wetness and RH for longer periods and results in
condition conducive for pathogens and diseases such as late blights and
vegetable root diseases including powdery mildews (Coakley et al., 1999).
ii. High moisture favours foliar diseases and some soil borne pathogens such
Phytophthora, Pythium, R. solani and Sclerotium rolfsii.
iii. Drought stress affect the incidence and severity of viruses such as Maize
dwarf mosaic virus (MDMV) and Beet yellows virus (BYV) (Olsen et al.,
1990 and Clover et al., 1999).

30. EFFECT OF WIND
Wind influences infectious plantdiseases:
• primarily by increasing the spread of plant pathogens and the
number of wounds on host plants.
• to a smaller extent, by accelerating the drying of wet surfaces of
plants.
• fungi, bacteria, and viruses that are spread either directly by the
wind or indirectly by insect vectors that can themselves be
carried over long distances by thewind.
• wind-blown rain helps release spores and bacteria from infected
tissue and then carries and deposits them on wet surfaces of
plants, which, if susceptible, can be infected.

31. EFFECT OF LIGHT
• The intensity and the duration of light may either
increase or decrease the susceptibility of plants to
infection.
• Low light produces etiolated plants, which
increases the susceptibility of plants to non
obligate parasites.
Tomato plants to Botrytis or to Fusarium.
• Reduced light intensity generally increases the
susceptibility of plants tovirus infections.
• Low light intensities following inoculation tend to mask
the symptoms of somediseases.

32. EFFECT OF SOIL pH & SOIL STRUCTURE
• The pH of the soil is important in the occurrence and
severity of plant diseases caused by certain soil borne
pathogens.
 Clubroot of crucifers (Plasmodiophorabrassicae)
severe -pH 5.7
drops between pH5.7 -6.2 and
completely checked -pH7.8.
 Cotton root rot fungus (Phymatotrichopsis omnivora)
exists only in soils contain relatively high
concentrations of calciumcarbonate.

33. EFFECT OF HOST-PLANT NUTRITION
Nutrition affects the rate of growth and the state of readiness
of plants.
• Nitrogen: abundance results in the production of young,
succulent growth, a prolonged vegetative period, and
delayed maturity of the plant, make the plant more
susceptible topathogens.
• Phosphorus: increase resistance by improving the
balance of nutrients in the plant or by accelerating the
maturity of the crop and to escape infection by
pathogens that prefer younger tissues.

34. Potassium :
• Have a direct effect on pathogen establishment and
development in thehost.
• And an indirect effect on infection by promoting wound
healing.
• Potassium also increases resistance to frost injury.
Calcium: Effects the composition of cell walls and their
resistance to penetrationby pathogens.
Various micronutrients (Fe, Cu,Mn,Mg,Si) showed decreased
infection when levels of nutrients increased.

35. EFFECTOF
HERBICIDES
AND AIR
POLLUTANTS:
i. The direct effects may include
stimulation or retardation of the
growth of the pathogen or in the
susceptibility of the host.
ii. Indirect effects include effect on
activity of soil microflora, elimination
or selection of the pathogen by
certain alternate hosts, or alteration
of the microclimate of the crop plant
canopy.
iii. ozone, can affect a pathogen and
sometimes the disease it causes. The
rate of infection is reduced if the
exposure to ozone is early but is
increased if exposure occurs late.

36. EFFECT OF CLIMATE CHANGE ON VECTOR-
BORNE DISEASES
i. The risk of vector-borne disease at the local and regional level is limited by the
climatic requirements of disease vectors (Malmstrom et al., 2011).
ii. Both host plant and insect-vector populations are affected by climate change
and spread the plant viruses (Jones, 2009).
iii. Climate change influences the primary infection of the host, the spread of the
infection within the host and/or the horizontal transmission of the virus to new
hosts by the vector along with phenology and physiology of the host thereby
affect its virus susceptibility and virus ability to infect.
iv. Climate change has various effects on vectors like modification of vector
phenology, vector’s over-wintering, density, migration and its stability.

37. CASE STUDY-I

40. CASE STUDY-II

41. 1. POWDERY MILDEW

42. 2. LEAF RUST

43. 3. STEM RUST

46. CASE STUDY-III

49. RESEARCH NEEDS
1. The effectiveness of disease management strategies. Assessing current management strategies
and proposing alternatives will prepare us for the challenge of climate change. This will allow
our mitigation measures to be more efficient and adaptable to change.
2. The risk of disease must be analyzed to determine the geographical distribution and
modification of diseases due to climate change.
3. Plant diseases and crops must be modeled. Mathematical models can be used to perform
quantitative analysis of the phytosanitary problems. They provide a very powerful tool to
understand and represent interactions among weather, crop and disease variables (Allman and
Rhodes, 2004).
4. Mathematical models can help to assess the probability of introduction, reproduction and
dispersion of diseases, and the magnitude of their effects on crops yield and quality in current
scenarios and under climate change.
5. Factors limiting the survival of pathogens should be characterized (e.g., temperature, humidity,
CO2, O3 and radiation).

50. CONCLUSION
• Climate change is an important phenomenon that affects agricultural
production. By anticipating the future, we can prepare ourselves for problems
caused by climate change, especially those related to agricultural activities.
• Global warming may modify areas affected by pests and diseases, studies must be
performed to assess pest and disease stages under the effects of climate
change, determine the magnitude of disease and identify measures to minimize
the risk of infection.
• Exposure to altered atmospheric conditions can modify fungal disease expression.
Studies had shown that exposure at elevated CO2 increases disease incidence or
severity in some cases but in other cases decreased.

51. • These highlight how the host–pathogen interactions make it
difficult to devise general principles that govern changes across
fungal pathosystems. So increase or decrease disease will be in
function of the host and pathogen.
• Climate change may affect the actual, spatial and temporal
distribution of diseases; however, the magnitude of these effects
remains unclear.
• Disease risk analysis based on host-pathogen interactions should
be performed, and research on host response and adaptation
should be conducted to understand how an imminent change in the
climate could affect plant diseases.