Post on 02-Apr-2019
Soybean RustLife Cycle, Spore Germination
and Survival
Dario NarvDario Narvááez. ez. NFRECNFREC--IFAS IFAS -- University of Florida.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: Basidiomycota
Class: Urediniomycetes
Order: Uredinales
Family: Melampsoraceae
Full Name: Phakopsora pachyrhizi H. Sydow & Sydow
Synonyms: Phakopsora sojae Fujikuro
Phakopsora calothea H. Sydow
Malupa sojae (P. Hennings) Ono, Buritica, & Hennen comb. nov. (Anamorph)
Uredo sojae P. Hennings
(For additional synonyms, see the following: Green, 1984; Ono et al., 1992
Reproductive StagesAlexopoulos, Mims, and Blackwell 1996
Stage 0: Spermatia not observed
Stage I: Aeciospores not observed
Stage II: Urediniospores common
Stage III: Teliospores observed but not common
Stage IV: Basidiospores identified
Alternate host is not known
Phakopsora pachyrhizi
Obligate parasite
Urediniospores
Causes polycyclic disease
Teliospores observed
Unknown alternate host
Survives only in parasitic phase
Wide host range; legumes
C. STONE
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
Bacterial PustuleDowny Mildew
Dissemination: wind
Dissemination
7-10 days occurs spore liberation
J.T.Yorinori
Reproduction
Symptoms show up 4-5 days after
inoculation
W.M.Paiva
One pustule releases spores
for 3 weeksJ.T.Yorinori
Germination
(optimum 180C – 260C)
Penetration
development
Infection
Free water on leaf surfaceMinimum: 6 hOptimun:12 - 14 h
Penetration
3-6 hr
Disease Cycle
M. IamautiG.N.Agrios
Survival ?
DepositionDry and wet
Source: Modified from EmbrapaTeliospore
B. Leite
T. Mueller
Direct &appressorium
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 . . .
. . . and in the U.S., there is little variation in cultivars.
0
0.2
0.4
0.6
0.8
1
Pro
porti
on o
f Dis
ease
d P
lant
s
0 20 40 60 80Days After Planting
Disease Increase Over Time
20052006? 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 13o C and 26o CMaximum infection at 20 to 230 CNo lesions >28oC
Frequency of rain events12-15 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 pattern
020406080
100
0 5 10 15 20dias após a coleta
% d
e ge
rmin
ação
% G
erm
inat
ion
Days after collecting spores
Percentage of Phakopsora pachyrhizi germ tube inhibition after exposure to four fungicideformulations: Chlorothalonil technical, Echo 720, Tebuconazole technical and
Tebuconazole 3.6 F.
Poster #47 Poster #47 BrenoBreno Leite, Leite, Jim Jim MaroisMarois and D. and D. Wright. Wright. In vitro and greenhouse In vitro and greenhouse trials of the fungicides trials of the fungicides chlorothalonilchlorothalonil and and tebuconazoletebuconazole..
Control line represents the % germ tube formation in deionized wControl line represents the % germ tube formation in deionized water. Germ tubes recorded in all fungicide ater. Germ tubes recorded in all fungicide treatments were in average 1/10 of the germ tube length observedtreatments were in average 1/10 of the germ tube length observed in the control. None of the test in the control. None of the test substances provided 100 % inhibition (or 0% inhibition), possiblsubstances provided 100 % inhibition (or 0% inhibition), possibly because y because urediniosporesurediniospores were placed in a were placed in a moist chamber for 48 hours prior to the experiment, therefore somoist chamber for 48 hours prior to the experiment, therefore some spores may have germinated before me spores may have germinated before they were exposed to test substances. Echo 720 and they were exposed to test substances. Echo 720 and ChlorothalonilChlorothalonil technical were the most inhibitory technical were the most inhibitory fungicide preparations.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 Poster #47 BrenoBreno Leite, Leite, Jim Jim MaroisMarois and D. Wright. and D. Wright. In vitro and greenhouse In vitro and greenhouse trials of the fungicides trials of the fungicides chlorothalonilchlorothalonil and and tebuconazoletebuconazole..
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.
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 woulddepend on south-to-north dispersal of urediniospores (Pivonia and Yang, 2004).
Soybean Rust
R SchneiderR Schneider
R SchneiderR Schneider
B. Leite
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
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
“Pop corn and honey”Phakopsora pachyrhizi urediniospore clumps
Arrow points to an individual urediniospore
Weak adhesive material
Weak self-inhibition
Clumping Hypothesis
Leaf surface
Long
distances
Spores
germinating
pustules
Clumps will protect core cells against
desiccation, UV light and other environmental harms
High humidity
Leaf surface
Long
distances
Spores
germinating
pustules
Clumps will protect core cells against
desiccation, UV light and other environmental harms
High humidity
B. Leite
Solar Irradiance
0
40
80
120
160
200
Pus
tule
s (#
/trifo
liate
)
Solar Irradiance
Water Control(0 MJ/m2)
Moderate(10 MJ/m2)
High(23 MJ/m2)
Experiment #1
0
40
80
120
160
200
Pus
tule
s (#
/trifo
liate
)
Solar Irradiance
Water Control(0 MJ/m2)
Moderate(10 MJ/m2)
High(23 MJ/m2)
Experiment #2
b
b
c
a a
a
• Proportion of P. pachyrhizi urediniosporesdecreased with increasing exposure to solar radiationExp ≥27.3 MJ/m2=no germ
• Urediniospores are sensitive to exposure to solar radiation, reducing viability
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 ofPhakopsora 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 thefield 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 beanCowpea Lupine Sweet Clover
. . . and KudzuKudzu 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 germtubeson 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 University of Florida
• Dr. David Wright University of Florida
• Dr. Scott Isard Penn State University
• Dr. Breno Leite University of Florida
• Mr. Tristan Mueller University of Illinois
• Mr. Kelly O’Brian University of Florida
• Mr. Nick Dufault 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, KRPlant 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, MHPlant 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 ].