Progress Report for 2005 California Pepper Commission Project
Fungicide Sensitivity of Phytophthora capsici Collected from Fields in California
James P. Prince
Dept. of Biology
Fresno CA 93740
Goals of the project:
- To check the effects on mycelial growth and sporulation of Phytophthora capsici using additional (higher) concentrations of famoxate and cyazofamid
- To check the effect of other fungicides
Materials and Methods:
Fungicides used were: azoxystrobin (Abound; same active ingredient at Quadris), cyazofamid (Ranman 400 SC), famoxadone (Famoxate), and Pristine.
Mycelial growth tests: Selected isolates of P. capsici were grown on V8 agar for 10 days. Agar disks, 5 mm in diameter, were taken from the margins of the actively growing fungus and deposited on V8 agar appropriately amended with various concentrations of fungicides. Three to five replicates of each concentration were evaluated for each isolate. The plates were incubated 7–10 days at 23 +/– 2?C. Colony diameter was measured in two directions, averaged, and compared with control plates without fungicide.
Sporulation assay: Plates for the spore count assay are prepared as above. For each isolate, 15 agar V–8 juice plugs were placed in a sterile Petri dish, submerged in sterile water and incubated at 25oC for 48 to 72 hours, after which the water was decanted out of the dishes, replaced with a sterile soil suspension and allowed to incubate for another 48 to 72 hours. Dishes were then incubated at 10oC for 90 min, then at 25oC for 30 min. The zoospore-containing soil water was decanted into a flask, and the number of zoospores was counted with a hemacytometer.
Results and Discussion:
Abound was ineffective at controlling mycelial growth in any of the isolates tested, out to a concentration of 2000 ug/ml (Fig. 1).
Fig. 1. Growth of P. capsici isolates with various concentrations of Abound (azoxystrobin) in the growth medium.
Abound was, however, highly effective at suppressing zoospore production (Fig. 2). Even at the lowest concentration used, zoospore production was under 4% of the control (no fungicide) samples.
Fig. 2. Zoospore production in P. capsici isolates with various concentrations of Abound (azoxystrobin) in the growth medium.
In our previous work with Famoxate, we achieved almost no control of mycelial growth with the concentrations used. In this work, as you can see in Fig. 3, growth slowed down considerably starting at 500 ug/ml, and all five isolates had 0% growth at a Famoxate concentration of 1400 ug/ml.
Fig. 3. Growth of P. capsici isolates with various concentr ations of Famoxate (famoxadone) in the growth medium.
Famoxate also showed very poor control of sporulation in our previous work. However, when we increased the concentration, there was a precipitous drop in zoospore production by 500 ug/ml and no spores by 1000 ug/ml (Fig. 4).
Fig. 4. Zoospore production in P. capsici isolates with various concentr ations of Famoxate (famoxadone) in the growth medium.
In our previous work with cyazofamid, we also achieved almost no control of mycelial growth with the concentrations used. Even out to a concentration of 1600 ug/ml, we have achieved very poor control of mycelial growth with this fungicide (Fig. 5).
Fig. 5. Growth of P. capsici isolates with various concentr ations of cyazofamid (Ranman 400 SC) in the growth medium.
In our previous work, good control of zoospore production was achieved with cyazofamid. We have confirmed those results, with zoospore production being under 8% with any concentration of cyazofamid used, and in many cases no sporulation has occurred at all (Fig. 6).
Fig. 6. Zoospore production in P. capsici isolates with various concentrations of cyazofamid (Ranman 400 SC) in the growth medium.
4. Pristine: Experiments with pristine are being repeated due to difficulties with reproducibility.
|Fungicide||Effect on mycelial growth||Effect on zoospore production|
|Abound||Negligible||Good control at 50 ug/ml|
|Famoxate||Good control by 1000 ug/ml; full control by 1400 ug/ml||Almost full control by 500 ug/ml; full control by 1000 ug/ml|
|Cyazofamid||Negligible||Good control by 50 ug/ml in all isolates; full control by 50ug/ml in 2 isolates|
|Pristine||In progress||In progress|
Any one of Abound, Famoxate, or cyazofamid could be useful for control of sporulation of P. capsici. Abound and cyazofamid achieve that control at a tenfold lower concentration than does Famoxate. Of these three, only Famoxate achieved control of mycelial growth. The other two had negligible effects.