PHYTON-27 CONFIRMED AS
POSSIBLE INDICATOR OF XANTHOMONAS INFECTION

For the past few growing seasons, geranium growers throughout the country have reported an interesting phenomenon. Geranium plants infected with Xanthomonas campestris pv. pelargonii (Xcp), grown under certain conditions, turn a bright yellow color when sprayed with Phyton-27 (50 ounces per 100 gallon) prior to the onset of symptoms normally associated with Xcp infection. In other words, Phyton-27 could be used on geraniums as an early indicator of Xcp infection. My immediate reaction was one of skepticism: "These people must be seeing things" I thought. But the reports just kept coming, and growers in Ohio, my home state, kept asking me the same question: "Does this really work?"

In the spring of 1994, I submitted a grant proposal entitled "The use of copper sulfate pentahydrate (Phyton-27) as an early indicator of Xcp infection in geranium" to Bedding Plants Foundation Inc. The proposal was funded in June 1994, and I set out to conduct a series of controlled experiments to test if this observed phenomenon was true. Rooted geranium cuttings were root inoculated by soaking each cutting in a suspension of Xcp and water. After allowing the cutting to soak for five minutes, the inoculated cutting was potted in a 4-inch pot and placed in the greenhouse under 12-hour days and daytime temperatures of 80 degrees F.

Five days after inoculation, the plants were sprayed until run-off with Phyton-27 at a rate of 50 ounces per 100 gallons of water. Plants were observed on a daily basis, and within five days of spraying, the leaves of the plants infected with Xcp Showed the bright, almost fluorescent yellowing like that previously described by the growers. None of this premature yellowing, however, was observed in plants that were infected, but not sprayed with Phyton 27 or in plants that were not inoculated, whether they were sprayed or not. Both of the Xcp-infected groups of plants (50 plants per group) eventually showed the classic symptoms associated with Xcp infection, but it was more than a week after the yellowing showed on the Xcp-infected/Phyton-27 treated geraniums. It is interesting to note that when I conducted this study using a lower rate of Phyton (30 ounces per 100 gallons), the yellowing of Xcp-infected/Phyton-27-treated plants was not observed.

What does this mean to a grower and how can it be used to help prevent the spread of Xcp in the greenhouse? Before I answer those two questions, I must first give a word of caution. The premature yellowing phenomenon associated with Xcp infection and Phyton-27 must not be used as a method of initial diagnosis of Xcp in your geranium crop. If you suspect Xcp infection, you must first have the suspect plants diagnosed by a qualified laboratory associated with a university or in private operation. Only after you have confirmed the presence of the bacterial disease in your crop should the premature yellowing phenomenon be used, and only as a way to rogue out geraniums that have a strong possibility of infection.

Geraniums expressing the yellowing should be removed from the greenhouse or placed in another part of the greenhouse far away from other geraniums. To be assured that these yellowed plants are indeed infected with Xcp, a representative sample or all of them should be tested by a laboratory. If they test positive for Xcp, they should be destroyed. This may seem a bit overboard, but we still aren't sure why this yellowing happens. It may be that the infected plants become stressed more easily. In that case, non-Xcp-infected plants could yellow if subjected to the same stress factors. Hence you would be throwing out non-infected plants if they were not tested. Also, this phenomenon is not seen in large stock plants or with plants that are growing under conditions of cool temperatures and slow growth.

Regardless, in the spring and early summer, if growers know they have a problem with Xcp, they could slow the spread of the disease by removing the yellowed plants on a timely basis. A week is a long time to allow an infected plant to sit in your greenhouse and be a source of bacterial inoculum for your non-infected plants. Besides the early warning that seems to be provided by Phyton-27, there is the added benefit of Phyton-27 protecting the non-infected geraniums from plant-to-plant movement of Xcp in splashing water.