Southern Bacterial Wilt Update for Georgia County Agents
by Dr. Jean L. Williams-Woodward, Extension Plant Pathologist,
University of Georgia
February 21, 2003
Greenhouse growers in your county may be receiving visits from Georgia Department of Agriculture inspectors concerning a recent geranium disease outbreak. The disease of concern is southern bacterial wilt caused by Ralstonia solanacearum race 3 biovar 2.
Recent detections of the pathogen in several states, including Georgia, have been traced back to rooting and distribution facilities in Michigan (Glass Corner Greenhouses) and New Hampshire (Pleasant View Gardens), which received R. solanaceaum race 3 biovar 2 infected geranium cuttings from a Goldsmith Plants, Inc. greenhouse facility in Kenya. In Georgia, 27 greenhouses have received suspect geranium shipments. Below is a summary of the disease to help answer potential questions.
Potential impact and importance: The reason for all the concern about this disease outbreak is that although southern bacterial wilt commonly occurs within Georgia on solanaceous garden plants (tomato, eggplant, pepper, and potato) this particular race of the pathogen is included on the USDA APHIS "select list" of plant pathogens. This list is often referred to as the "bio-security"or "bio-terrorism" list of plant pathogens because the pathogens are not established in the U.S.A., and if introduced, they can threaten important agriculture crops. Geranium plants found to be positive for the disease must be destroyed. Obviously, geranium producers are distressed by this disease outbreak and the potential destruction of their crop.
Current regulatory action: Growers with suspect plants shipped from the Michigan or New Hampshire facility have been notified by Goldsmith Plants and the GA Department of Agriculture. Not all geranium stock is infected. Several geranium cultivars including Dark Red, Bright Red, Pink II, and Violet in the Americana Series from Goldsmith were known to be infected. Other cultivars including some Oglevee cultivars have tested positive for bacterial wilt, and are believed to have been infected within the greenhouse through worker activity. Department of Agriculture inspectors are collecting samples from suspect nurseries and bringing them to the Extension Plant Pathology Plant Disease Clinic in Athens for testing and initial pathogen identification. Growers have been told to isolate suspect plants from other crops in the greenhouse to prevent disease spread. Plants showing wilting symptoms should be removed from the greenhouse and enclosed in double plastic bags. Plants should not be discarded in dump or compost piles. Plants most likely will be destroyed by incineration under the supervision of USDA APHIS PPQ agents. At this time it is not known whether entire shipments will be destroyed or only those showing symptoms and the plants within a 1 meter (39 inches) area surrounding symptomatic plants.
Symptoms: Disease symptoms do not develop in cool temperatures. Soil temperatures above 21EC are required for symptom development in greenhouse tests. Disease is favored by high temperatures (30-35EC) and high soil moisture levels. Initial symptoms are wilting and yellowing of lower leaves. Whole stems will wilt. Dark vascular discoloration is seen in the lower stem that rots the stem from the inside out, until the stem turns black and compressed. Disease is different from Pythium black leg in that the rot starts above the soil-line rather than from the root system upward. Leaves may also show necrosis or scorching around the margins followed by darkening of the affected leaves particularly down the petiole.
Survival and Spread: The pathogen can survive in soil for years, even in the absence of a host. It is typically spread through infected cuttings, divisions, cutting knives, hands, insects, in soil, and in water. Initial infection of plants normally occurs through roots. All cuttings taken from infected plants are potentially also infected. The pathogen does not cause leaf spots as bacterial blight caused by Xanthomonas campestris pv. pelargonii, therefore it typically is not spread through splashing water. Since the bacterium is confined to the vascular system, anything that injures or exposes the vascular tissues can aid in spreading the disease. If stems are cut, bacterial ooze will develop on the cut end. This exposed ooze can be spread by water-splash. Typically the bacterium is released from broken root tissues as the root grows through the potting medium or soil. Once the bacterium is in the soil medium it can be washed out of pots during irrigation. It is commonly spread through shared water sources such as puddles on concrete floors or solid benches, sub-irrigation systems such as ebb and flow, or from water dripping from infected hanging baskets over a bench crop.
Control: Control relies on minimizing the risk of contaminating a greenhouse by not introducing infected plants and sanitation. All infected (symptomatic) plants need to be removed from the greenhouse. For the time being, infected plants should be enclosed in unbroken plastic bags to prevent the dispersal of bacterium from soil medium or rotting plants. Infected plants may remain symptomless for a period of time, especially during colder environmental conditions, but all infected plants will eventually die. Greenhouse benches, tools, drip tubes, and floors need to be disinfested with quaternary ammonia products. If benches are wooden, then bleach is the only effective disinfectant. Wooden benches need to be kept wet with a 10% bleach solution for 10 minutes to be effective. Foot baths and hand washing stations need to be present and used frequently by employees and inspection personnel to reduce disease spread. All sub-irrigation needs to be stopped. Hanging baskets need to be removed above and below bench crops to prevent contamination through dripping water. No chemical control is known.
Other potential hosts: Not all hosts of southern bacterial wilt are known. Host lists include plants from 50 families. Regulated hosts of R. solanacearum are geranium, tomato, pepper, eggplant, potato, bean, beet, and tobacco. However, the disease may affect other greenhouse ornamentals including marigold, zinnia, petunia, dahlia, nasturtium, geranium, browalia, Catharanthus (annual vinca), cyclamen, fuchsia, gerbera daisy, hydrangea, impatiens, and lantana. Weeds also can be infected, such as nightshade, horsenettle, jimson weed, purslane, mustards, lambsquarters, and bittergourd.