Time-tested bactericides still found to be effective

  A.R. ChaseThe uproar over Ralstonia on geraniums during the early 2000s led to a new focus on ornamental bacterial diseases. About two years ago the Interregional Research Project No. 4 (IR-4 program), which assists in registrations for minor use crops including ornamentals, identified bacterial diseases as a researchable target.
 
University of Florida plant pathologist David Norman recently reported on his research findings at this year’s Society of American Florists’ Pest Management Conference. Norman’s trials were conducted on phalaenopsis orchids with Erwinia blight and Xanthomonas leaf spot on geranium. Best results were seen with copper products such as Phyton 27 and carbamates such as Protect DF. Antibiotics, including Firewall (streptomycin sulfate), Flameout (oxytetracycline) and Kasumin (kasugamycin) were somewhat variable. Biologicals (Cease, Companion, Taegro and Actinovate) had limited effectiveness (e.g. Cease on both bacterial diseases). Remember to check all product labels as not all of those reported in research trials are labeled for ornamentals in every state.

Table 1. Effect of bactericides on Erwinia cutting rot on poinsettia and Xanthomonas leaf spot on zonal geranium.

Chase Horticultural Research, 2008. Numbers in the same column followed by different letters were significantly different using Student Newman Keuls method.Trials show variable results
At Chase Horticultural Research we have been working with some bactericides through the manufacturers and IR-4. Two trials were conducted on Xanthomonas blight on geranium, one trial on Erwinia cutting rot on poinsettia and one trial on Pseudomonas blight on lavender. The first two trials were preventive and products were applied two or three times weekly. Best control was seen in both trials with the industry standard, Phyton 27. Kasumin gave very good control of Xanthomonas on geranium at 35 parts per million but failed at 100 ppm perhaps due to phytotoxicity. Taegro worked well on Erwinia but did not give any control of Xanthomonas on geranium (Table 1).

IR-4 trials
IR-4 research looked at some of the same products and additional products, some not registered on ornamental plants (Table 2).
 
Products were applied as sprays except for Taegro, which was applied alternately as a drench and spray.
 
Table 2. Products used in 2009 IR-4 trials on Xanthomonas leaf spot on zonal geranium and Pseudomonas blight on lavender.In the Xanthomonas trial, plants were treated twice before being inoculated with Xanthomonas campestris pv. pelargonii and then treated once afterward on a weekly interval. Three weeks after inoculation, the lowest numbers of spots were found on the noninoculated controls and plants treated with Actigard, the combination of Tanos and Kocide and Phyton 27 (Table 3). The copper products worked well and although Actigard is severely phytotoxic to geranium at the rate tested (0.75 ounces per 100 gallons), it was 100 percent effective. Citrex provided about 68 percent reduction in number of spots and the MOI-106 was also somewhat effective at 45 percent reduction in spots. Products that did not work at all in this trial included Kasumin, SP-2015 and Taegro.
 
A second trial was performed on lavender infected with Pseudomonas leaf spot. When the trial was started, no symptoms were apparent but within a few days it was obvious that the cuttings were infected with Pseudomonas sp. Plants were sprayed four times on a weekly interval (Table 3).
 
Most of the products that provided some control in the first trial were infective in trying to stop an active infection. Only Taegro and Tanos-Kocide gave any control of Pseudomonas leaf spot on lavender. Kasumin was phytotoxic at the rate (45 ounces per 100 gallons) tested and looked worse than the water treated control.

Older products still effectiveTable 3. Percent control achieved with experimental bactericides funded through IR-4.
Study results indicate the older products (especially coppers) were more consistent than newer products. Kasumin provided some control in several trials but appears to have issues with phytotoxicity on some ornamentals. Actigard was also phytotoxic and only worked when used before infection took place as all previous research has shown. Taegro did give some control in these trials.

Ann Chase is president, Chase Horticultural Research, (530) 620-1624; archase@chaseresearch.net; www.chasehorticulturalresearch.com.