Ralstonia solanacearum Race 3 biovar 2 is a bacterium that causes the following diseases: Southern Wilt of Geranium, Brown Rot and Bacterial Wilt of Potato, Bacterial Wilt of Tomato and Other Solanaceous Crops. R. solanacearum Race 3 biovar 2 is considered to be a select agent by the United States under the Agricultural Bioterrorism Protection Act of 2002 because it has the potential to be a severe threat to the potato industry and does not naturally occur in the U.S.
Signs and Symptoms
Geraniums: The lower leaves of plants infected by R. solanacearum Race 3 biovar 2 may show yellowing, wilting, and browning. Another common symptom is vascular discoloration of the stem. Roots may also show a brown discoloration. Bacterial ooze and bacterial streaming both represent signs of the pathogen. Bacterial ooze can be observed in cut stems. If the petioles of infected leaves are removed, cut up, and placed in a water-filled test tube the water will become cloudy as the bacteria stream out of the xylem into the water. Potatoes: Foliage of potatoes infected with R. solanacearum Race 3 biovar 2 may show stunting, yellowing, and wilting. Early in the infection wilting leaves may be limited to the top portion of the plant. The plants can appear to recover at night, but soon wilting is irreversible and death of the plant follows. The stems of young plants may collapse and/or have narrow dark streaking present. Vascular discoloration of the stem appears to be grey or brown and bacterial ooze is present. Again, upon placing cut stem material in a test tube with water, bacterial streaming may occur from the vascular tissue (Elphinstone 2004). Symptomatic potato tubers will show discoloration of the vascular ring. Initially the vascular ring appears yellow to light brown, but as the infection progresses the ring will become browner. Bacterial ooze may also be present; in later stages this ooze may emerge from the eyes and heel (stolon) end to which soil particles will attach. Tomatoes: Tomato plants infected with R. solanacearum Race 3 biovar 2 initially show a reduction in the firmness of young leaves. Upon further development the plant will wilt on one or both sides during conditions favorable to the pathogen which include saturated humidity and warm soil temperatures around 25 °C (77 °F). When the temperature is below 21 °C (70 °F) the environment is less favorable for the pathogen and excess roots may develop along the stem. Brown discoloration of the vascular tissue as well as bacterial ooze may be present.
R. solanacearum Race 3 biovar 2 is a soilborne pathogen that primarily infects hosts through the roots. Plants may be infected through wounds caused by handling or nematode feeding or by openings occurring during normal root development. Crops susceptible to the disease may become infected when grown in contaminated soils or when they are irrigated with contaminated water (Janse 1996). In greenhouse operations, subirrigation systems could be a means for disseminating the pathogen. R. solanacearum Race 3 biovar 2 can also be spread through the use of infected plant material. This was what was witnessed in the U.S. with the geranium industry. The disease was introduced into greenhouses through the use of infected plant cuttings. For potatoes, the disease can be spread by the use of infected potato seed stock. (Lemay et al. 2003)
Currently there are no chemical controls available. Some methods of control include: eradication of weedy hosts, altering soil pH over the growing season and screening plants for disease before planting. A pathway for introduction of R. solanacearum Race 3 biovar 2 into the U.S. has been through the introduction of geranium cuttings obtained from production greenhouses located outside the country. Geraniums imported into the U.S. on several occasions in 1999 and 2000 were positive for R. solanacearum Race 3 biovar 2. (Kim et al 2003; Williamson et al 2002; Kim, 2002). During 2001 and 2002 the U.S. had no reported cases of R. solanacearum Race 3 biovar 2. In February 2003, the bacterium was again identified in geraniums in the US but this time on plants grown from cuttings from a facility in Kenya. The outbreak derived from a breach of sanitation in Kenya that led to the contamination of 7 stock plants. The 2003 sampling program conducted by USDA-APHIS in collaboration with the state depts. of agriculture resulted in the organism being detected in 127 individual greenhouses in 27 states. Work was conducted by State Departments of Agriculture and APHIS personnel to contain, destroy, and eradicate any diseased geraniums (Daughtrey 2003). Geraniums with symptoms of bacterial wilt caused by Race 3 biovar 2 were detected again in one greenhouse in December of 2003. This was believed to be a new introduction and not a continued contamination from the spring of 2003. The greenhouse had not received plants from the Kenya cutting station earlier that year and the diseased cuttings were traced back to Guatemala (O'Hern 2004).