Soybean Rust Life Cycle, Spore Germination and Survival - Plant ...

Soybean Rust
Life Cycle, Spore Germination
and Survival
Dario Narváez.
NFREC-IFAS
- University of Florida.

Why was the disease widespread?
Three factors:
Uniform susceptibility
in the host plants
The
introduction
of a pathogen
An environment
conducive to
disease

TAXONOMY
Phylum:
Class:
Order:
Family:
Full Name:
Basidiomycota
Urediniomycetes
Uredinales
Melampsoraceae
Phakopsora pachyrhizi H. Sydow & Sydow
Synonyms: Phakopsora sojae Fujikuro
Malupa sojae (P. Hennings) Ono, Buritica, & Hennen comb. nov. (Anamorph)
Phakopsora calothea H. Sydow
Uredo sojae P. Hennings
(For additional synonyms, see the following: Green, 1984; Ono et al., 1992

Reproductive Stages
Alexopoulos, Mims, and Blackwell 1996
Stage 0:
Stage I:
Stage II:
Stage III:
Stage IV:
Spermatia not observed
Aeciospores not observed
Urediniospores common
Teliospores observed but not
common
Basidiospores identified
Alternate host is not known

Phakopsora pachyrhizi
Obligate parasite
Urediniospores
Causes polycyclic disease
Teliospores observed
Unknown alternate host
C. STONE
Survives only in parasitic
phase
Wide host range; legumes

First rust symptoms often along major
leaf veins

Sporulating pustule
under a hand lens

Uredinia and Urediniospores

Infected Soybean Leaves

Infected Kudzu Leaves

Look-Alike Diseases
• One of the challenges of identifying Asian soybean rust is that the
early stage of the disease can look like other leaf diseases of
soybean.
Brown Spot
Bacterial Blight
Downy Mildew
Bacterial Pustule

Look-Alike Diseases…continuation
Cercospora leaf blight
Frogeye leaf spot

Disease Cycle
B. Leite
Infection
development
Free water on leaf surface
Minimum: 6 h
Optimun:12 - 14 h
Penetration
3-6 hr
Direct &
appressorium
Symptoms show
up 4-5 days after
inoculation
Reproduction
Deposition
Dry and wet
M. Iamauti
Germination
(optimum 18 0 C – 26 0 C)
Penetration
G.N.Agrios
W.M.Paiva
7-10 days occurs
spore liberation
Dissemination: wind
Dissemination
Survival ?
J.T.Yorinori
J.T.Yorinori
One pustule
releases spores
for 3 weeks
T. Mueller
Teliospore
Source: Modified from Embrapa

Soybean Rust- continued
Once introduced to a new county, soybean rust could
spread rapidly as the generation time
for new inoculum can be as little as 9 days . . .
Proportion of Diseased Plants
1
0.8
0.6
0.4
0.2
0
Disease Increase Over Time
2005
0 20 40 60 80
Days After Planting
. . . and in the U.S.,
there is little variation
in cultivars.
2006?
This chart depicts the exponential
spread of a soybean rust epidemic
in a field.
Source: SASPP.org 2002

The frequency and amount of rainfall is a
major driving force for the incidence and
severity of soybean rust

Environment and Soybean Rust
‣ Temperatures between 13 o C and 26 o C
• Maximum infection at 20 to 23 0 C
• No lesions >28 o C
‣ Frequency of rain events
• 12-15 rain days per month
• Late vegetative to late reproductive growth stages
‣ Duration of leaf wetness
• 6-12 continuous hrs
‣ Relative Humidity between 75-80 % required for spore
germination and infection

Spore Germination

Spore germination pattern
% de germinação
% Germination
100
80
60
40
20
0
0 5 10 15 20
dias após a coleta
Days after collecting spores

Spore germination
Polystyrene coated slide
B. Leite

Percentage of Phakopsora pachyrhizi germ tube inhibition after exposure to four fungicide
formulations: Chlorothalonil technical, Echo 720, Tebuconazole technical and
Tebuconazole 3.6 F.
Poster #47 Breno Leite,
Jim Marois and D.
Wright.
In vitro and greenhouse
trials of the fungicides
chlorothalonil and
tebuconazole.
Control line represents the % germ tube formation in deionized water. w
Germ tubes recorded in all fungicide
treatments were in average 1/10 of the germ tube length observed in the control. None of the test
substances provided 100 % inhibition (or 0% inhibition), possibly y because urediniospores were placed in a
moist chamber for 48 hours prior to the experiment, therefore some spores may have germinated before
they were exposed to test substances. Echo 720 and Chlorothalonil technical were the most inhibitory
fungicide preparations.

Number of Phakopsora pachyrhizi appressoria on hydrophobic polystyrene coated slides
after exposure to four fungicide formulations: Chlorothalonil technical, Echo 720,
Tebuconazole technical and Tebuconazole 3.6 F.
Poster #47 Breno Leite,
Jim Marois and D. Wright.
In vitro and greenhouse
trials of the fungicides
chlorothalonil and
tebuconazole.
Control line (……) represents the number of appressoria formed in deionized water. Undiluted fungicide
solutions (1) were difficult to count. On polystyrene coated slides the absorption of excess of particles is
not possible. In this graph zero appressoria formation was recorded. It is important to emphasize that an
appressorium is an advanced stage in the pathogenicity process, compared to the formation of germ tubes.
Echo 720 and Chlorothalonil technical again exhibited superior inhibitory capacity compared to
Tebuconazole.

Do urediniospores survive
long distance travel?

Dispersal
Mechanism:
• Soybean rust spreads primarily by wind-borne spores across
regions depending upon prevailing winds and other environmental
conditions conducive to disease development.
• The disease is likely to be restricted to parts of Florida and southern
Texas during the winter in the frost-free areas or areas where the
fungus could overcome short periods of below-freezing
temperatures.
• Occurrence of rust epidemics within the U. S. soybean belt would
depend on south-to-north dispersal of urediniospores (Pivonia and
Yang, 2004).

B. Leite
Soybean Rust
R Schneider
R Schneider
Urediniospores (urediospores, uredospore, uredinospore) from infected host:
Size: Uredinospores are 18-34 to 15-24 microns (Ono et al., 1992).
Shape: Urediniospores are sessile, obovoid to broadly ellipsoid, and minutely and
densely echinulate, and the walls are about 1 micron thick. The color of the
urediniospores range from pale yellowish-brown to colorless (Ono et al., 1992).

B. Leite
B. Leite B. Leite

B. Leite
Figure 1 – A ) Typical urediniospore of Phakopsora pachyrhizi
germinating on the surface of a PDA plate. In PDA urediniospores will
rather form long germ tubes than form appressoria. B) Urediniospore
forming an appressorium on the surface of a polystyrene covered slide
(PCS). On PCS spore will preferentially form appressoria.

B. Leite
200 - 300 spores in this clump
Phakopsora pachyrhizi urediniospore clumps
Arrow points to an individual urediniospore
“Pop corn and honey”
Weak adhesive material
Weak self-inhibition

Clumping Hypothesis
Clumps will protect core cells against
desiccation, UV light and other environmental harms
Long
distances
High humidity
Spores
germinating
Leaf surface
pustules
B. Leite

Germination of clumped urediniospores
B. Leite

Solar Irradiance
• Proportion of P.
pachyrhizi urediniospores
decreased with
increasing exposure to
solar radiation
Exp ≥27.3 MJ/m 2 =no germ
• Urediniospores are
sensitive to exposure to
solar radiation, reducing
viability
Pustules (#/trifoliate)
Pustules (#/trifoliate)
200
160
120
80
40
0
200
160
120
80
40
0
Experiment #1
Water
Experiment #2
Water
a
Control
(0 MJ/m 2 )
Moderate
(10 MJ/m 2 )
Solar Irradiance
a
Control
(0 MJ/m 2 )
a
b
Moderate
(10 MJ/m 2 )
Solar Irradiance
b
High
(23 MJ/m 2 )
c
High
(23 MJ/m 2 )
Isard et all. 2006. Plant Dis. 90:941-945

Wet Deposition: Rainfall Washout
Simulation
• Preliminary Results indicate that spores deposited by
short durations of simulated rainfall are distributed
evenly throughout a soybean canopy.
A Portable Rainfall Simulator for Conducting Wet Deposition Field Studies of
Phakopsora pachyrhizi UrediniosporesA Portable Rainfall Simulator for Conducting Wet
Deposition Field Studies of Phakopsora pachyrhizi Urediniospores

Survival of Phakospora pachyrhizi in
USA
‣ Description: P. pachyrhizi is still known as an obligate parasite.
Urediniospores survive (in resting or dormant state) less than 2 days
under natural outdoor conditions. They do not survive in dried or
decayed tissues or in the soil (Ilag, 1977).
‣ Survival Information: When soybean plants are not present in the
field the fungus is viable only when transferred to another living host
(Ilag, 1977). One widespread host in the United States is kudzu,
Pueraria montana var. lobata, that could serve as a reservoir for
soybean rust. There are a variety of other leguminous crops or
weeds that have shown varying degrees of susceptibility to both
species of soybean rust.

Reservoir-- continued
Further, over 30 legume species
may serve as alternate hosts for
soybean rust, including
Lima bean Butter bean Green bean
Cowpea Lupine Sweet Clover
. . . and Kudzu
Kudzu is a common weed in the southern U.S. and is
great concern that it could serve as a continual source
of P. pachyrhizi inocula. Kudzu begins growth in the
spring before soybeans are planted and thus, the
weed could act as a "disease bridge" by allowing
build-up of the fungus followed by movement into
soybean.

Summary:
• Phakopsora pachyrhizi will preferentially form
appressoria on polystyrene coated slides and germtubes
on PDA and agar plates
• Viability of isolated urediniospores is frequently lost in a
few days after being release from the pustule
• Germination of P. pachyrhizi urediniospores may occur
in clumps.
• Clumps may protect urediniospores against desiccation,
UV radiation and other environmental factors.
• Clumped spores are more likely to travel long distances
and reach new areas.

Summary… continuation
• Information about clumping may help to properly design
disease management programs.
• Proportion of P. pachyrhizi urediniospores decreased
with increasing exposure to solar radiation
• Urediniospores are sensitive to exposure to solar
radiation, reducing viability
• Rain simulator is an effective tool for examining the
vertical distribution of spores in a soybean canopy
• Spores deposited by rainfall are distributed evenly
throughout the soybean canopy
– 20 inches high
– 36 inch row spacing

Acknowledgments
• Dr. Jim Marois
• Dr. David Wright
• Dr. Scott Isard
• Dr. Breno Leite
• Mr. Tristan Mueller
• Mr. Kelly O’Brian
• Mr. Nick Dufault
University of Florida
University of Florida
Penn State University
University of Florida
University of Illinois
University of Florida
Penn State University

Sources
• Soybean Rusts Phakopsora meibomiae & Phakopsora pachyrhizi
– http://www.google.com/search?hl=en&q=phakopsora+pachyrhizi+spore+deposition
– www.plantpath.wisc.edu/soyhealth/powerpoint/Soybean_Rust_technical.ppt
Additional alternative hosts of Phakopsora pachyrhizi , causal agent of soybean rust.
Rytter, JL | Dowler, WM | Bromfield, KR
Plant Disease [PLANT DIS.]. Vol. 68, no. 9, pp. 818-819. 1984.
Effects of duration, frequency, and temperature of leaf wetness periods on soybean
rust.
Melching, JS | Dowler, WM | Koogle, DL | Royer, MH
Plant Disease [PLANT DIS.]. Vol. 73, no. 2, pp. 117-122. 1989.
Strategic plan to minimize the impact of the introduction and establishment of soybean
rust on soybean production in the United States [
http://www.aphis.usda.gov/ppq/ep/soybean_rust/sbrplan12-03.pdfl ].
Status of Scientific Evidence on Risks Associated with the introduction into the
Continental United States of Phakopsora pachyrhizi With Impported Soybean
Grain, Seed and Meal [
http://www.aphis.usda.gov/ppq/ep/soybean_rust/sbr_riskevidoc2_23_04.pdf ].
NPAG data:: Phakopsora pachyrhizi: Australasian soybean rust
[ http://www.aphis.usda.gov/ppq/ep/soybean_rust/UreMelPp502.pdf ].

(Poppler, 2004)