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