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Adhesion and Rainfall Wash-off of Phakopsora pachyrhizi
Urediniospores on Soybean Leaves
María Vélez-Climent & Nicholas DufaultPenn State University
National Soybean Rust SymposiumDecember 13, 2007
P. pachyrhizi Urediniospores Urediniospores
– Primary inoculum Dissemination
– Wind– Rain
Ideal germination conditions
– Cool temperatures (18-21º C)
– Prolonged moistureHigh humidityHeavy dews
Adhesion– Direct penetration
Adhesion Questions
When do spores start adhering to the leaf surface?
What is the nature of the adhesion process (dependent or not dependent on metabolic activity)?
Adhesion Objectives:
Determine the timeline of spore adhesion to soybean leaves
Determine if there is a relationship between spore adhesion and spore germination
Adhesion Assay0h – 48h under ideal conditionsSoybean plants cv. Williams (R4-R6)
– 2nd to 4th nodes– Adaxial surface
Moisture chamberDaily harvested spores
– ~1 day-old sporesDilution (~2000 spores/mL)
– Inoculation (100 µL / ~200 spores)
Wash unattached sporesStain
– Lactophenol cotton blueCount and categories
Category N: No-germination
Category S/G: Short Germ Tube
Category L/G: Long Germ Tube
Category S/A: Small Appressoria
Category L/A: Large Appressoria
Category P: Penetration
Results – Florida Isolate
Mean spore count per timepoint
0
50
100
150
200
0h 0.5h 1h 3h 6h 9h 12h 24h 48hTimepoints
Mea
n sp
ore
coun
t PL/AS/AL/GS/GN
Adhesion
Appressoria
Penetration
Adhesion Timeline
Spore adhesion is correlated to germination
Prolonged Rainfall Questions
A majority of the spores in the atmosphere are being washed out within the first 20 minutes of a rain event. (Gregory, 1973)
During prolonged rain events are spores being removed from soybean leaves they were originally deposited on?
Are these spores lost to the soil or deposited on leaves lower in the soybean canopy?
Rainfall Wash-off Objectives
Determine if spores can be rinsed from leaves by continuous rainfall without spores.
Ascertain if spores on the upper leaves of the soybean plant are being re-deposited to lower leaves.
Rain Simulator
Sampling area (2 x 2 m)Varying rain intensity
– 45 mm/hr– 85 mm/hr
Rain droplets – Terminal velocity – Sizes from 0.25 to 3.25
mm
Spores injected into water flow
Rainfall Wash Design
Urediniospores– Germination > 30%
Greenhouse plants– DP7220RR variety– Height 30 cm
Simulated rain – ~1.5 million spores– Treatments
0 min (2 min with spores)1 min (prolonged rain)30 min (prolonged rain)
– 3 sample heightsBioassay
Ground
Top-canopy
Mid-canopy
Wash Results (45 mm/hr)
0
0.5
1
1.5
2
Ground (0 cm ) Mid-Canopy (15.24 cm ) Top Canopy (30.48 cm )
0 min1 min30 min
Pust
ules
per
cm
2
Ground Leaf (0 cm) Mid-canopy Leaf (15 cm) Top-canopy Leaf (30 cm)
Ground highest severity ratings
No significant difference in severity
Significantly reduced severity
0
0.5
1
1.5
2
Ground (0 cm) Mid-Canopy (17.78 cm) Top Canopy (35.56 cm)
0 min1 min30 min
Wash Results (85 mm/hr)Pu
stul
es p
er c
m2
Ground Leaf (0 cm) Mid-canopy Leaf (15 cm) Top-canopy Leaf (30 cm)
Results are the same as other rain intensity
Overall severity observed on the leaves is reduced
This indicates less spores are initially retained
Rainfall Wash-off Conclusions
Majority of spores deposited are not retained by the single plant canopy
Prolonged rainfall (0.5h) decreased the severity observed on the leaves of soybean plants
Spores do not appear to be re-deposited lower in the canopy
Summary
Adhesion– Urediniospores can start adhering to soybean
leaves as soon as 0.5h after inoculation and incubation under ideal conditions
– Penetration occurs between 12 to 24h– Spore adhesion is correlated to germination
Wash-off– Simulated rainfall events of 0.5h significantly
reduced the number of spores on leaves
Acknowledgments
Penn State University– Dr. Scott Isard– Justin Dillon– The Department of Plant
Pathology
Kansas State University– Dr. Erick De Wolf
University of Illinois– Dr. Glen Hartman– Todd Steinlage
USDA-ARS/FDWSRU– Dr. D. Luster– Dr. R. Frederick– Dr. M. Bonde– USDA Staff
NFREC/IFAS– Dr. D. Wright– Dr. J. Marois– Dr. D. Narvaez– Ag. Extension Post Docs
and Staff
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