Carrot Country

WSU Researchers Evaluate Bacterial Blight Control in Carrot Seed Production

Fall 2006
Carrot Country

What chemical controls for bacterial blight (Xanthmonas campestris pv. carotae) are working best in carrot seed crops?

In a 2004 study, Dr. Lindsey du Toit and Mike L. Derie evaluated Actigard, bactericides and compost teas for control of the disease. Their findings were mixed. The researchers are stationed at Washington State University’s Northwest Washington Research and Extension Center in Mount Vernon, Wash.

In May, a field check revealed bacterial blight at low populations in all of the plots except those treated with Kocide 2000 or ManKocide DF. By July, there were no significant differences among treatments.

By August, however, there was significantly less bacterial blight in plots treated with ManKocide DF (66-70 percent of the plants were infected) compared to the control plots (100 percent of the plants were infected) and other treatments, duToit and Derie reported.

The bacterial blight population on the harvested seed was signifi cantly lower for plots chemigated with ManKocide DF, followed by plots treated with boom applications of Kocide 2000 or ManKocide DF, the researchers added. Actigard 50 WG, compost tea and chlorine (Sunny Sol 150) applications did not reduce the infection rate of harvested seed versus the control plots.

“Seed germination did not differ significantly among treatments, indicating no phytotoxic effects,” du Toit and Derie observed. “The greater effi cacy of ManKocide DF applied by chemigation versus boom application probably reflects improved coverage of the canopy by the former method of application.”

Research Particulars
In their study, du Toit and Derie planted stecklings (carrot roots) of a proprietary open-pollinated carrot cultivar (Amsterdam type) grown in California’s Imperial Valley. The roots were then shipped to Washington, where they were vernalized for eight weeks at 34ºF, and then planted at the WSU NWREC on April 12 into a Puget silt loam soil. A randomized complete block design was used with four replications of eight treatments.

Each plot was 25 feet long with eight rows spaced 22 inches apart, du Toit detailed. Within the rows, the roots were spaced 12 inches apart. A 25-foot alley surrounded each plot. Prior to planting, the researchers made a broadcast application of fertilizer (34-0-0 at 100 lb/A, 0-0-21 at 300 lb/A, Mangro 31 at 60 lb/A, Borate at 15 lb/A and ZnSO4 at 30 lb/A). The plots were side-dressed (4-8-4 at 200 lb/A) on April 19 and May 10.

For weed control, the researchers incorporated Trefl an HFP (1 pt/A) on April 9; and Lorox DF was applied on April 16 (1 lb/A) and May 11 (1.5 lb/A).

Using a hand-held sprayer with a 110º nozzle, du Toit and Derie inoculated each plot on May 18. The plots were inoculated again on June 15, July 20 and Aug. 26 at 15 psi using a CO2-pressurized tractormounted sprayer with a six-nozzle boom (8003 fl at fan tips at 20-inch spacing). Treatments were applied four times at 27- to 35-day intervals. R-11 surfactant was used (1.0 pt/100 gal) with all broadcast treatments except compost teas.

Control plots were sprayed with water and R-11, the researchers said. Broadcast applications were made using the CO2-pressurized tractor-mounted sprayer. Chemigation treatments were applied through one rotary sprinkler/plot using a tractor-mounted tank and PTO-driven pump calibrated to apply 0.112 acre inches of water/30 min/plot. ManKocide DF was chemigated with mechanical agitation. Municipal water was aerated in a brewer (Grower Solutions Inc., Sonoma, Calif.) for 24-48 hour to volatilize chlorine. A proprietary vermicompost (Bejo Seeds Inc.) was then added (5 lb compost/25 gal water) and brewed with aeration for 24 to 48 hours.

“This tea was applied (6.3 gal/plot) using the CO2-pressurized tractor-mounted sprayer,” the researchers said. “Magic Earth compost (seaweed extract, humic acid, vermicompost and forest duff) was
brewed aerobically (7 lb compost/25 gal water) for 24 hours and broadcast by Magic Earth Landscapes (Guemes Island, Wash.) using a hose and spray nozzle.”

Compost tea and Actigard 50 WG applications were made three or four days prior to inoculating the plots with bacterial blight, du Toit and Derie said. Sunny Sol 150 (12.5 percent NaOCl, JCI Jones Chemicals Inc., Sarasota, Fla.) was applied one to three days after inoculation by chemigation; the rate was 4 ppm Cl2.

All other treatments were made 24 hours or less from the time of inoculation. Using aseptic techniques, the fi rst sampling (fi ve plants/plot) took place on May 24-27. Similarly, leaves, stems and immature umbels were sampled from the same number of plants per plot on July 5-9. Fifty umbels per plot were sampled on Aug. 9-13.

“Fifty mature umbels per plot were harvested on Sept. 3 and dried with forced air at ambient temperature for two weeks and at 85ºF for one week,” the researchers said. “Seed was threshed, cleaned and sized according to specifications for the cultivar.

Within 48 hours of each sampling, foliage or umbels were chopped, and a subsample was placed in a sterile buffer on a rotary shaker for 60 minutes.”

The concentrate was then assayed for bacterial blight, the plates incubated for six to eight days and the bacterial blight colonies enumerated.

Editor’s note: The results from this study were previously published in Fungicide & Nematicide Tests. Dr. Lindsey duToit can be reached at dutoit@wsu.edu.