N.A. Ward, R.W. Schneider LSU AgCenter

Documentation of an Extended Latent Infection Period

with Phakopsora pachyrhizi in Soybean

•Phakopsora pachyrhizi

•Still a major disease of soybean in the southeastern US

•Preventative fungicide applications result in large grower inputs

•Effective timing of fungicides is the most effective means of control and/or prevention of the disease

Overwinters on kudzu

Sporulating pustules are often detected in January and February

Soybean symptoms at seed set (R5)

Hypothesis

Latent infection period can be longer than 10 days, regardless of weather conditions or inoculum load

In laboratory and greenhouse, pustule development 7-10 days after inoculation

Even under cool temperatures and humid conditions, disease symptoms do not occur in the field until mid-reproductive stages

ApproachUse qPCR to quantify Phakopsora pachyrhizi

High specificity

Detects as little as 1 picogramof DNA of P. pachyrhizi in 10ng total DNA

2009 – Baton Rouge, 3 soybean fields (April, May, and July)

2010 – New Iberia, kudzu (April)

2010 – Quincy, FL kudzu (March to October)

Experimental Procedure

2009

2009

Parish Date Growth Stage

St. Martin June 5 R4

Iberia June 24 R5

Acadia June 16 R5

AssumptionNear Baton Rouge

July 27 R7

Patricia Bollich, IPM Pipe

2009April Soybeans

June 4

R1

60+ days

Apr 17

Planting

<43 days

qPCR negative

qPCR positive

Symptoms

2009May Soybeans

Jul 23

R1/R2

Sept 4

R6

43 days

May 20

Planting

64 days

qPCR negative

qPCR positive

Symptoms

2009July Soybeans

Aug 20

V4

Sept 25

R5

33 days

July 21

Planting

30 days

qPCR negative

qPCR positive

Symptoms

2009Summary

June 4

R1

60+ days

Apr 29

Planting

Jul 23

R1/R2

Sept 4

R6

43 days

May 20

Planting

Aug 20

V4

Sept 25

R5

33 days

July 21

Planting

Aug 20

Green Stem

2010

2010

Weather

Compare to 2009

2010

Results New Iberia KudzuCollection by Tomas Rush

Sample Date +/-

Stem April 3 -

April 11 -

April 18 -

April 27 -

Leaf April 3 -

April 11 -

April 18 -

April 27 -

May 11 -

2010

Kudzu

North Florida Research and Education CenterUniversity of FloridaQuincy, FL

Collection by Kelly O’Brian

Month County SymptomsLatent

Infection

MayGadsdenHolmesJackson

- -

JuneGadsdenJackson

- -

July Gadsden - -

AugustHolmesJackson

- -

OctoberGadsdenHolmesJackson

-/+ -/+

2010July Soybeans

Aug 25

R3?

Sept 12

R6?

18 days

July 4

Planting

52 days

July 13

SBR Reported

on nearby kudzu

43 days

2009

Fungicide Time of Infection Study

R.W. Schneider, C. L. Robertson

Field Views of Fungicide Treatments

Nonsprayed control

Topguard, 7 0z, R1

Summary:

2009 – early sources of inoculum

early infection

2010 – late or no inoculum

late or no infection

Infection detected V4 to R2/R3

Disease symptoms R4 to R6

Conclusion:

Latency is a function of plant growth stage.

Infection can take place long before disease symptoms occur.

Fungicides should be applied at onset of latent infection.

Acknowledgements