Aeriobiology

1. Aerobiology of Plant Pathogens:
Mechanisms, Gradients and Spatial Patterns
Pravin Kumar Bagaria
L-2018-A-58-D

2. • Aerobiology (Greek: aēr = “air” + bios = “life” + logia = “to study”)
- a branch of biology that studies organic particles, such as bacteria, fungal
spores, very small insects, pollen grains and viruses, which are passively
transported by the air (Anonymous, 2017).
- the study of airborne biological particles and their movement and impact on
human, animal and plant health.
- a scientific multidisciplinary biological science, which deals with the source,
release, dispersion, and deposition of different micro-organisms found in the air
and their impact on the ecosystem or life of plants, animals and human beings
(Edmonds, 1979).

3. • 1935 - Term "Aerobiology" by F.C. Meier (USA)
• 1952 - Term 'Air Spora' by P.H. Gregory published in Nature which describe the
airborne pollen grains and fungal spores as:
“The population of air-borne particles of plant or animal origin, which will
here be called the air ‘spora’ (taking the Greek σπορ′α as a word of similar
usage to ‘flora’ and ‘fauna’), contains spores and pollens of various shapes
ranging in size from 100 μm in diameter for some tree pollens to 3-5 μm
with some of the smallest fungus spores” (Gregory, 1952).

4. Spores collected from air of Calcutta
by Cunningham in 1873.
(Figure collected from the book “The Air Spora” by Maureen E. Lacey and
Jonathan S. West, Springer Pub.2006)
• 1873 - In India, first aerobiological study by D.D.
Cunningham (British physician at Calcutta). The
book entitled, ‘Microscopic examination of air’.
• 1933-52 - Prof. K. C. Mehta collected
uredospores of three rusts of wheat and barley.
Reported the presence of teleutospores, smut
spores, Alternaria and different species of
Puccinia.

5. Aerobiological Triangle
https://shodhganga.inflibnet.ac.in

6. Isard et al 2005

7. • According to geometry, inoculum source can be classified as
Point source
Individuals or small groups of infected plants;
A diameter of less than 1% of the length of the gradient
Line source
Hedges containing infected alternative host plants or
Strips of a susceptible cultivar
Area source
Infected fields
Although such area sources may become point sources if the distance over which
the gradient is measured is kilometres rather than metres
• According to vertical position, inoculum source may be
Above ground level
Ground level sources
McCartney et al 2006
Source of Inoculum

8. Spore liberation/ discharge (Take off)
(process of detachment of spores from the spore-bearing structure)
I. Active Spore Discharge in Fungi: Spores may be violently discharged by the
mechanisms like:
(i) Bursting of spore-producing structures
(ii) Sudden changes in shape of turgid spores or of turgid structures associated with
the spores;
(iii) Rapid twisting movements produced as a result of drying in filamentous
sporangiophores or by hygroscopic movement;
(iv) Sudden breaking of tensile water in conidia or conidiophores, distorted on drying,
which are thereby permitted to return to their original form; and
(v) Impaction

9. II. Passive Spore Liberation:
(i) Liberation of Dry Spores - Through rain drop and winds (Mechanical momentum
and Blowing away:- Rust uredospore)
(ii) Liberation of Slime Spores- Through rain drop
• Spores may also be removed from the leaf surface by puffing action of raindrops
• Occurs when the first raindrops fall
• Release of puffball spores from the mature fruiting bodies has a similar action
• Strong gusts of wind or contact with animals can also result in spore puffs

10. I. Active Spore Discharge in Fungi:
A-C - Trigger device in Sphaerobolus sp.
D-H – Syringe device, D-E in Sphaerotheca sp., F-J in
Pilobolus sp.
Drop-excretion mechanism
A - Basidiospore discharge.
B-E – Successive stage in the fluid drop excretion
and basidiospore discharge

11. I. Active Spore Discharge in Fungi:
A-B – Change in condition of turgid structures in Sclerospora sp.
C-F – Hygroscopic movements of sporangiophores in
Peronospora tabacina.
G-K – Gas bubble mechanism in Deightoniella torulosa
Spore liberation mechanism by impact of rain
A-F – In Niduriales
G-J – Lycoperdon sp.

12. Dispersal of Plant Pathogen
• Transport of spores or infectious bodies, acting as inoculum, from one host to
another host at various distances resulting in the spread of the disease
OR
• Displacement of a plant pathogen from its place of production or origin to a
suitable place where it can grow/ established
• Fundamental step for repeated cycles of infection and multiplication
• Dependent on the method of discharge by individual taxa as well as
environmental factors, such as temperature, humidity, and wind speed
• Strong gusts disperse spores even when the average wind speeds are too low
• Wind dispersal is promoted by warm and dry weather
• Peak during afternoon hours, when humidity is low and wind speeds are
increased
McCartney et al 2006; Levetin, 2016

13. Mechanism of Spore Dispersion
• Active dispersion
• Passive dispersion:-
Dispersion by Wind
High temperatures & low relative humidity
Long range dissemination – b/w fields or regions
Dispersion by Rain splash
Also provides moisture required for germination
Short range dissemination – within plant or nearby
plants

14. Movement in the Atmosphere
• Wind is the major factor, with gusts and lulls affecting
take off, transport, and deposition
• In a crop, spores have to pass from the laminar layer
close to the leaf surface into the turbulent layer
within the crop
• Gusts and turbulence enhance spore removal from
leaves by sweeping away the layer of slow-moving air
next to the leaf surface
• Spores must pass through the crop boundary layer
surrounding the crop
Levetin 2016
The ground is black.
Above ground is thin layer of laminar air (L)
and on top of this turbulent air (T).
Discharged spores are disseminated by wind.

15. • Carried by wind, spores are transported both horizontally and vertically
• Horizontally, spores carried for thousands of kilometers, dispersing pathogens into new
areas
• Vertically, carried upward by convective activity or thermals and have been recovered
from altitudes higher than 5,000 m
• Rain splash can also propel spores into the atmosphere, and it is second to wind in
importance as a means of pathogen dispersal
• Splash-borne spores or bacteria are usually produced in mucilage which inhibits their
direct removal by wind
• The first raindrops dissolve the mucilage and leave a spore suspension available for splash
dispersal by additional raindrops
Levetin 2016

16. Survival in the Atmosphere
• Fungal spores are more resistant to environmental stress compared to parent hyphae.
• Exposure to harmful radiation, extremes of temperature and humidity can decrease the
viability.
• Changes in relative humidity, often caused by changing wind speeds, may affect survival,
especially for thin-walled spores, which may easily plasmolyze.
• Desiccation risks are more during the daytime and close to the ground.
• At night and at high altitudes, conditions are less stressful.
• Thin-walled colorless spores may be more vulnerable compared to pigmented spores.
• Low temperatures in the upper atmosphere may be preservative and protect spores
from UV damage
Levetin 2016

17. Scale of spore/ disease spread
Microscale,
Mesoscale, or
Synoptic scale Levetin, 2016
Scales of Pathogen Dispersal Scales of air turbulence
Mahaffee and Stoll, 2016

18. Microscale spread
• Limited to less than a few hundred meters within one field and occurs within one
growing season
• Roughly corresponds to a zero-order epidemic
• The focus begins with a single successful propagule causing an infection and creating a
lesion
• After several generations of localized pathogen spread for polycyclic diseases, which
produce many generations of inocula and many cycles of infection during a single growing
season, the focus may reach a detectable size
• In an annual crop, these are about 1 m in diameter around the initial source of infection
• If growing conditions are unsuitable for the pathogen, the focus may stop expanding or
even disappear with new growth in the canopy. Levetin, 2016

19. Mesoscale spread
• However, when conditions are favorable for the pathogen, the disease will spread
• As the primary focus continues to expand, secondary foci and later tertiary foci appear.
• This continued focal spread over a larger area is considered mesoscale spread and
corresponds to a first-order epidemic
• This may be restricted to one field but may spread over many fields or up to an area
several hundred square kilometers or even over part of a continent during a single
growing season
Levetin, 2016
Synoptic or Macroscale spread
• Synoptic or macroscale spread occurs when the epidemic progresses for several years and
spreads over an area of several thousand square kilometers
• This is also referred to as a second-order epidemic.
• This pandemic may cover a whole continent after a certain number of years

20. Spore Landing (Deposition)
• Sedimentation
• Impaction
• Inpingement
• Filtration
• Boundary layer exchange & turbulence
• Rain deposition
• While airborne – spores touch wet surfaces – get
trapped
Spore movement is influenced by
• Gravity
• Brownian motion
• Electric charge
• Temperature
• Inertial precipitation and impaction
• Periodicity of wind
Diurnal
Nocturnal
• Turbulence in air
Levetin 2016

21. Dispersal by wind/ rain-splash and spore deposition gradients
• Individual spores released from the same source under the same wind conditions follow
different paths and travel different distances.
• With disperse downwind, spore concentrations in the air decrease referred to as
‘concentration gradients’.
• The turbulent nature of wind causes a dilution in the concentration of a spore plume.
• Consequently, dispersal gradients for splash-dispersed spores are generally much shorter
than those for wind-dispersed spores.
• Crop canopy structure affects the deposition of splashed droplets and the potential for
spread by secondary splash.
• Thus, duration of exposure to rain and rain intensity may modify ‘primary splash’
gradients.
• Primary dispersal is dominant at the beginning of a rain shower.
• However, as rain duration continues, secondary spread may begin to be important.
• If the rain persists for sufficient time to deplete the source, inoculum deposited may be
lost by wash-off.
McCartney et al 2006

22. SPORE DEPOSITION AND DISEASE GRADIENTS
• For both wind and splash-dispersed plant pathogen inoculum, deposition rates decrease with
distance away from the inoculum source
• The disease pattern that develops will also show a decrease in disease with increasing distance
away from the source, i.e. a disease gradient
• Disease gradients can also result from gradients in host or environmental factors
• Background inoculum from a large number of distant sources produces a uniform distribution of
disease with distance across a crop
• Vertical disease gradients can also be observed when inoculum sources are at ground level
• Disease gradients produced by splash-dispersed inoculum are usually steeper than those
produced by wind-dispersed inoculum
• Secondary spore dispersal can flatten primary spore dispersal and disease gradients with time
McCartney et al 2006

23. • Monocyclic diseases produce only primary disease gradients
• Over long periods of time the disease gradients gradually become less steep
• Disease gradients with polycyclic diseases are first observed in a crop as primary disease
foci resulting from a single lesion
• Initially disease gradients are steep but spores which escape from the crop canopy soon
establish secondary foci
• Primary disease gradients become more shallow as foci expand and, with the expansion
of secondary foci
• Initial horizontal gradients are caused by wind-dispersed primary inoculum but
subsequent horizontal spread and vertical spread up the crop canopy is achieved by
splash-dispersed secondary inoculum
• Gradients from sources above ground level are generally less steep than those from
ground level sources
• The same spore dispersal mechanism could account for steep gradients close to a
source and shallow gradients farther away McCartney et al 2006

24. Measurement of gradients (spore dispersal or disease gradient)
Spore numbers per m3 (spore concentration gradient) or
Spore numbers per m2 (spore deposition gradient) (C) or
Disease incidence or severity (Y ) at different distances (x)
• Spore numbers can be estimated with artificial samplers
• Spore deposition gradients can be measured by passive samplers (horizontal slides under
rain-shields for wind-dispersed spores) or beakers for splash-dispersed spores
• Concentration gradients can be measured with volumetric samplers
• The disease component of disease gradients has been measured as numbers of lesions,
numbers of infected leaves, numbers of infected plants, the percentage leaf area affected or
the percentage of the population of plants which is affected
McCartney et al 2006

25. Spatial patterns
• Spatial patterns of disease may be quite different from the spore dispersal
patterns
• Because spore dispersal is a short term phenomenon compared to most other
stages of disease development
• Spatial patterns are the result of many individual dispersal events from many
sources over periods of days or even weeks
• Disease patterns and epidemic size are strongly influenced by dispersal patterns
• Spatial patterns foci are often circular but strongly affected by wind, may become
comet- or V-shaped.
• Foci generally have a constant radial expansion, with the rate varying with the
scale of the infection from a few centimeters per day for a localized infection to
hundreds of kilometers per year for a pandemic
McCartney et al 2006

26. • Trajectory analysis - a standard tool in the study of air movement and it tracks the
movement of air parcels using information on wind fields and atmospheric temperature
structures
• Potential long distance aerial transport uses air parcel trajectory analysis to establish links
between source and receptor regions
• Backward trajectory analysis is frequently used to trace the previous movement of a
spore-laden parcel of air and locate the inoculum source
• Once the source is identified, forward trajectories are used to indicate further potential
areas of fallout.
• Various dispersion models have been used to trace the movement of spores from
dissemination at a source to deposition at a sink by calculating trajectories based on upper
air winds, temperature, and other parameters.
McCartney et al 2006; Levetin 2016

27. NASA’S Balloon Missions
Exposing Microorganisms in the Stratosphere 1 (E-MIST 1) - 2014
• For studying bacteria in Earth’s stratosphere (about 10 to 31 miles)
at NASA’s Kennedy Space Center, Florida
• A radiation-tolerant strain of bacteria (Bacillus pumilus) was
carried inside the E-MIST payload (mounted on large balloon, New
Mexico)
• Purpose: To expose bacterium to harsh conditions of the
stratosphere (5 hours)
• Result: Data could be collected within the stratosphere
Exposing Microorganisms in the Stratosphere 2 (E-MIST 2) – 2015 October
• Exposed bacterium to Mars surface-like conditions (extremely cold, dry air, harsh ultraviolet
radiation and low air pressure) to test how well they could survive
• Result: After 8 hours of exposure, 99.999% of the bacteria were dead, damaged, or destroyed
beyond the point of being able to regrow (19 miles above sea level)

28. Microbes in Atmosphere for Radiation, Survival and Biological Outcomes Experiment (MARSBOx) –
2019, Sept
• This aerobiology research experiment was flown on a NASA scientific balloon mission launched from
Fort Sumner, New Mexico (6.5 hours and altitude of 110,000 feet)
• Purpose: To measure effect of ionizing radiation conditions in the stratosphere
Carried 9 different types of microorganisms (bacteria & fungi) in dormant state
• Results: most of bacteria died, but fungal spores were able to better withstand the harsh
environment at >20 miles up
Aircraft Bioaerosol Collector (ABC) - (installed NASA’s C-20A aircraft)
• ABC - an instrument, custom built at NASA’s Armstrong Flight Research Center
• Purpose:
To capture and seal up bioaerosol samples in troposphere and in lower stratosphere
(as high as 8.5 miles)
To tackle difficult challenge of sampling and studying microorganisms at extreme
altitudes during ascent, descent and sustained cruises
To discover airborne bacterial diversity at different levels
• Result: a similar distribution of bacteria in the atmosphere at all altitudes

29. Periodicity of airborne concentration of
Botrytis cinerea conidia above a
strawberry field monitored using rotating
arm sampler and a qPCR assay for
quantification of conidia
Carisse 2016

30. Dynamics of B. cinerea airborne conidia monitored in raspberry
(a), strawberry (b), and grape (c) plantings in Canada (2010)
Progress of Botrytis fruit rot in strawberry plantings with various
cultivars at the Agriculture and Agri-Food Canada experimental
farm in 2010 Carisse, 2016

31. Fungal spores were identified and quantified in the air of Bratislava during the 1-year
period (2016) using a Burkard 7-day volumetric aerospore trap.

32. Spore calendar for Bratislava Exponential classes (spores/m3):
a 1–5, b 6–10, c 11–25, d 26–50, e 51–100, f 101–500, g 501–1000, h
> 1000
Relative contributions (% of total spore
concentration) of the major spore types in
the air of Bratislava
Annual total spore count (spores/m3)-
836,418 fungal spores belonging to 53
spore types in Bratislava during 2016
Scevkova and Kovac 2019

33. Monthly variations in the spore concentrations of the major
fungal taxa and total fungal spores in the air of Bratislava
Prevailing weather conditions (mean, maximum and
minimum air temperature, absolute air humidity and
rainfall)
Scevkova and Kovac, 2019

34. Airborne spores of Cladosporium spp. were sampled on the roof, 21 m above sea level in
Viborg during 115 days, 31 May–22 September 2015, on the 48 × 14 mm slides using a
Hirst-type spore trap

35. Olsen et al 2019a
Daily average concentrations of Cladosporium spp. at
Viborg station during 31 May- 22 September 2015 (n =
115), mean daily average concentration over the period:
1897 spores m-3. Red vertical lines confine the longest
period of high concentrations. Peak daily average
concentration occurred on 16 August (13,553 Spores m-3)
Cladosporium spp. diurnal distribution at Viborg
station on the days with daily average
concentrations: above 3000 Spores m-3 (green line,
n = 21), above 3000 Spores m-3 without considering
14 August and 16 August (red line, n = 19), below
3000 Spores m-3 (blue line, n = 94)

36. The episode of 13–25 August 2015:
a 3-h time series of Cladosporium spp. concentrations at Viborg (blue) and Copenhagen (red) stations, daily
precipitation at Foulum station (green) ;
b Lines represent 48-h back trajectories for the period of 13–25 August: green on 13 August and 25 August, red
on the day with maximal concentration, i.e. on 16 August, black on the other days within the period;
Olsen et al 2019a

37. Burkard volumetric spore sampler.
Compared the concentrations of airborne Alternaria spores and the patterns of air mass
transport using HYSPLIT model between Copenhagen and Viborg with the main focus on
the days with daily average spore concentrations >100 s m-3 (high concentration days).

38. Monthly spore integrals of airborne Alternaria spores
(2012–2015) for all days and for high concentration days
(with daily average concentration > 100 s m-3); CPH
Copenhagen, VIB Viborg
Daily time-series of airborne Alternaria spp. At
Copenhagen and Viborg stations (2012–2015)
Olsen et al 2019b

39. Clusters and cluster means of 48-h back-trajectories for the Copenhagen station on the days with daily average
concentration:
a > 100 s m-3 (high days) and b <100 s m-3 (low days)
Olsen et al 2019b

40. Clusters and cluster means of 48-h back-trajectories for the Viborg station on the days with daily average
concentration:
a > 100 s m-3 (high days) and b <100 s m-3 (low days)
Olsen et al 2019b

41. Air samples were collected using settle plate method. Petri plates containing potato
dextrose agar (PDA), Martins rose bengal agar (MRBA) and Czapek’s Dox agar medium
supplemented with chloramphenicol (250 mg/ml) were used for collecting the air samples.
The rhizosphere, air and phylloplane were dominated by Rhizopus stolonifer.
Pestalotiopsis disseminata is one of the major pathogens of Som and was found highest in
aerosphere followed by phyllosphere.

42. Fungal diversity in air in the Som plantation area
Ray et al 2019
Common population among the four environments:
rhizosphere, non-rhizosphere, air and phylloplane

43. Two qPCR TaqMan assays were developed to detect pathogen DNA: the first used a generic
probe to detect Phytophthora spp., and the second was based on a specific probe for
detecting P. ramorum and P. lateralis.
All samples tested positive for the genus Phytophthora, although P. ramorum and P.
lateralis were not detected.

44. Migliorini et al 2019
Seasonal variation in DNA quantities
(pg/µl) of Phytophthora species
(black line) shown with
meteorological variables, i.e.
rainfall (mm, a);
relative humidity (%, b);
maximum, minimum and mean air
temperature (°C, c); and
extremes of air temperature (°C, d)

45. Conclusion
• A wide variety of plant pathogens, including viruses, bacteria, oomycetes, and fungi, are
dispersed through the atmosphere
• When conidia are produced on a source near the ground or in the lower canopy, they are
exposed to slow wind speeds, low turbulence, and rapid rates of sedimentation,
conditions that are conducive to short-distance transport
• When they are deposited on a susceptible host, infection can
occur, and when environmental conditions are favorable, the resulting disease spread
may lead to widespread crop loss
• Measurement of disease or spore gradients can be extremely important for identifying
sources of disease, for identifying inoculum dispersal mechanisms, for assessing the
effectiveness of some disease control strategies and for interpreting the results of field
experiments
• Long range dispersal favours more widespread epidemics and increases the likelihood of
disease persistence
• A thorough understanding of the role of the aerobiological pathway in pathogen
dispersal is necessary for the management and control of disease
• Knowledge of aerobiology can help researchers and farmers to assess, predict and
decrease the effects of epidemic pathogens