It Pays to Know the Difference By Dr. S.G.P. Nameth Department of Plant Pathology The Ohio State University If you grow geraniums you are well aware of the many diseases and pests that you have to deal with. Two of the most important diseases of geraniums are Bacterial Blight (BB) caused by the bacterium Xanthomonas campestris pv. pelargonii and Southern Bacterial Wilt (SBW) caused by the bacterium Ralstonia solanacearum. These two diseases have the potential to devastate your geranium crop and though both diseases are caused by a bacterium, the way each disease manifests itself symptomatically in the geranium is different. Trying to differentiate the two diseases based on symptoms can be a real challenge and should only be the first step in a sound disease identification process. Visual diagnosis should always be followed-up with a laboratory test, since this is truly the only way these two diseases can be distinguished beyond a doubt. However, in saying this, there are some subtle differences you, the grower can use to distinguish these diseases in infected geraniums in the greenhouse. And, because it will be necessary for you to initiate somewhat different disease management strategies, it pays to know the difference. With most bacterial-induced diseases in greenhouse crops, the bacteria are splashed onto the plant by irrigation water. Once on the plant leaf the bacteria enter the plant through some natural opening in the host plant such as the stomates. Once inside the host the bacteria multiply very quickly and move throughout the plant via the plants own vascular system. For both BB and SBW the bacteria produce enzymes and toxins and toxins that break-down the plant tissue. When tissue break-down occurs in the host's vascular tissue, the tissue becomes "plugged" and the plant has a difficult time moving water from the roots to the rest of the plant. Hence, resulting in the wilting of the affected geranium. With SBW the wilting of the infected geranium tends to be systematic. The wilting starts at the lower leaves and petioles and works it's way up the entire plant, older leaves first and younger leaves last (Figure 1). Eventually the entire plant collapses onto the growing media (Figure 2). With BB the wilting symptom is manifested in the plant in a more random fashion with some leaves at the bottom of the plant wilting at the same time as leaves at the top (Figure 3)and (Figure 4). Eventually, like SBW the entire plants collapses to the media and dies. The end result for both of these diseases is the same, plant death. There is nothing you can apply to the infected plant that will cure them of the disease. For both diseases symptomatic plants should be removed from the production area immediately and samples should be sent for testing at a university or private testing lab. Affected geraniums should be separated from the rest of the crop, however, these plants should not be discarded based solely on symptoms. Only when the plants have been tested and the exact disease determined should the affected plants be discarded. Once the disease has been identified, the proper disease management strategies need to be initiated (Please refer to May 2001 Ohio Floriculture Newsletter ).