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Mefenoxam Insensitivity of P. capsici Isolates Collected in South Carolina in 2003

County (Farm) Crop Sensitive Intermediately

sensitive Insensitive Total Lexington (1) Bell pepper 0 10 0 10 Lexington (1) Summer squash 0 7 5 12 Bamberg Summer squash 19 0 0 19 Bamberg Zucchini 17 0 0 17 Lexington (2) Bell pepper 19 29 2 50 Lexington (2) Jalapeno pepper 5 6 1 12 Total 60 52 8 120

Keinath, A.P. 2007. Plant Disease 91:743-748.

General Procedures

4 aliquots of zoospore suspensions counted

1 to 4, varied by assaySubsamples

Mostly linear with log (mg/L)Regression

2 platesReplications

2 to 3Tests

0, 0.03, 0.10, 0.31 [1.0]

3-4 concentrations plus 0;Evenly spaced on log scale at 1 or 0.5 log units apart

Fungicide Concentration (mg/L)

ExampleLevelsFactor

Reference: Keinath, A.P. 2007. Plant Disease 91:743-748.

Sandwich Plate Technique for Producing Zoospores

in the Presence of a Fungicide that Reduces Sporangium Formation

Young and Vjugina, 2002. Phytopathology 92:S89.

V8 agar

Water agar + Fungicide

Chilled SDW

Calculating EC50 Values with Regression Equations

Log zoxamide concentration-1.5 -1.0 -0.5 0.0 0.5

Rel

ativ

e zo

ospo

re c

ount

(%)

0.0

0.2

0.4

0.6

0.8

1.0

Test 1 Test 2 Test 3 Predicted

Isolate RCS4

1.800.255-0.5770.6474LP48

0.21-0.682-0.3150.2845RCS4

EC50log10 EC50

SlopeInter-cept

Isolate

EC50 Values for Zoospore Cyst Germination on Dimethomorph-Amended V8 Agar

0.00

0.02

0.04

0.06

0.08

0.10

0.12

Dim

etho

mor

ph E

C50

(mg/

L)

Isolate of Phytophthora capsici

Lexington 1 Lexington 2Bamberg

0.00

0.02

0.04

0.06

0.08

0.10

0.12

Dim

etho

mor

ph E

C50

(mg/

L)



Mean, Median = 0.07 mg/L

Keinath, A.P. 2007. Plant Disease 91:743-748

EC50 Values for Zoospore Production on Zoxamide-Amended Agar


Zoxa

mid

e EC

50 (m

g/L)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0



Zoxa

mid

e EC

50 (m

g/L)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0


Mean = 0.47 mg/LMedian = 0.29 mg/L

Summary of EC50 Values (mg/L)

Fungicide Mycelial Growth

Zoospore Germination

Zoospore Production

Dimethomorph 0.19 ± 0.02

0.18 0.07 ± 0.02

0.07 0.63 ± 0.42

0.53 **

Zoxamide 0.50 ± 0.52 0.33 **

n.a. 0.47 ± 0.51 0.29 **

Cymoxanil > 50 > 50 <50

Mean ± Std Dev.Median, ** indicates Non-normal Distribution

Keinath, A.P. 2007. Plant Disease 91:743-748

0

0.05

0.1

0.15

0.2

0.25

EC50 Values for growth of P. capsici isolates on Fluopicolide amended V8 agar

Mean=0.161 mg/L

Fluo

pico

lide

(mg/

L)

Range=0.098-0.239 mg/L

NC SC GA FL

Fungicide Mycelial growth Sporulation

P.infestansFluopicolide 0.161 0.059 4.1

0.098-0.24 <0.03-0.062

Mandipropamid 0.022 0.0080.008-0.032 <0.0015-0.032 0.02-2.98

EC50 values (mg/L) for P. capsiciagainst new fungicides

Kousik & Keinath, 2007

Mycelial growth of P. capsici isolates on Cyazofamid amended V8 juice agar

Check, 0 mg/L 300 mg/L

0

50

100

150

200

250

300

EC50 Values for growth of P. capsici isolates on Cyazofamid and SHAM amended agar

Cya

zofa

mid

(PPM

)

Range=<1 to >300 mg/L

NC SC GA FL

100 mg/L Salicylhydroxamic acid

Check Ranman Fungicide X

Effect of Ranman (2.75 fl oz/A) on Fruit Rot of Watermelon caused by P. capsici isolates

Ranman concentrations

0 Check10 100300

Growth of P. capsici Isolates on Ranman treated Cucumbers

Silwett L-77 (2 Fl Oz/A)

Cyazofamid (mg/L)

3 P. capsici isolates6 replications

0 10 100 300 mg/L Cyazofamid

Isolate 1

Isolate 2

Isolate 3

EC50

13.7 mg/L(4)

>300 mg/L(108)

233 mg/L(Spore:68)

Fungicide Mode of Action Resistance Risk

Mefenoxam Inhibits RNA synthesis High

Dimethomorph Phospholipid biosynthesis Low-Med& cell wall synthesis

Mandipropamid Phospholipid biosynthesis Low-Med& cell wall synthesis

Zoxamide β tubulin assembly in mitosis Low-Med

Cyazofamid Respiratory inhibitor, QiI Med-High

Fluopicolide disrupts membrane activity Res. not known

FRAC Code List

FRAC: Fungicide resistance action committee www.frac.info/publications

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