Austin Hagan and John Olive

Southern blight is a damaging hot weather disease on a wide variety of flowering trees, shrubs, and herbaceous ornamentals in both the nursery and landscape. Within the past two years, serious disease outbreaks have been reported in several Alabama nurseries on container-grown butterfly bush, forsythia, hosta, and Prague viburnum. Aucuba and ajuga also are highly sensitive to southern blight. Results of a three-year AAES study show that the fungicides ProStar 50W and Fluazinam 500F, along with selected rates of Lynx 3.6F, completely protected container-grown aucuba from southern blight and gave better disease control than the current fungicide standard Terraclor 75W.

Typical symptoms of southern blight, which is caused by the soil-borne fungus Sclerotium rolfsii, include sudden wilting or flagging on one or more shoots, then a dieback of the damaged shoots, and finally plant death. Usually, the superficial, white, fan-like fungal growth mat of the causal fungus can be found on the soil surface around the base and/or lower stem of the wilted plant as well as on fallen leaves and other organic debris. Clusters of brown, round sclerotia, which are about the size of mustard seeds, are often scattered on the surface of the soil or potting medium as well as on the lower stem or collar of the dying plant. Disease development in both the nursery and landscape can occur from June through September after several days of hot, wet weather.

PCNB, which is available under the trade names Terraclor, Terrazole, and Defend and as a wettable powder, emulsifiable concentrate, and granular formulations, is the only fungicide registered for the control of southern blight on nursery crops. Recently, several newly released fungicides, including Lynx 3.6F (tebuconazole), Fluazinam 500F (fluazinam), and ProStar 50W (flutolanil), have provided better control of southern blight on peanuts than PCNB. These chemicals are not yet labeled for use in horticultural crops, but may be soon. Effectiveness of these and other fungicides for the control of this disease on container-grown nursery crops has not been established. AAES trials were conducted at the Ornamental Horticulture Substation in Mobile to identify fungicides with superior activity against southern blight and determine their optimum drench rate.

Liners of Aucuba japonica Variegata were potted in trade gallon containers in a 3:1 pine bark/peat moss medium amended with 14 pounds of Osmocote 17-7-12, six pounds of dolomitic limestone, two pounds of gypsum, and 1.5 pounds of Micromax per cubic yard. Aucuba was chosen for the study because it is readily availbale and a good host of southern blight in the landscape. Aucuba were grown in an open, plastic-covered greenhouse with open ends and watered daily with overhead irrigation. Initial screening trials were conducted in 1992, 1993, and 1994 to identify those fungicides with activity against southern blight. A second study on aucuba in 1993 and 1994 focused on determining the optimal application rate for those fungicides which proved effective in the initial screening trial. Fungicide treatments in both series trials were applied as a heavy spray/drench using a CO2 pressurized sprayer. A second drench treatment was made approximately two to four weeks after the first.

An isolate of S. rolfsii was obtained from diseased aucuba. The fungus was grown on autoclaved oat seed for seven to 30 days. Approximately 20 to 25 fungus-infected seed were buried in two to three holes around the base of each plant immediately after the first fungicide drench was made. No oat seeds were buried in the potting medium of the noninoculated controls. Plant survival was determined approximately 60 days after inoculation.

Among the six fungicides in the general screening trials, only ProStar 50W and Fluazinam 500F completely protected container-grown aucuba each year from southern blight (see Table 1). In two of three years, both of these fungicides provided consistently better disease control than did Terraclor (PCNB) 75W. Lynx 3.6F was not as effective in controlling southern blight as either ProStar 50W or Fluazinam 500F. Terraguard 50W and Curalan DF demonstrated little to no activity at the rates tested against the causal fungus. Each year, nearly all the Curalan DF-treated aucuba, as well as the inoculated controls, succumbed to southern blight. Conversely, the noninoculated controls in all three screening trials remained symptom-free.

Results of the rate use studies again demonstrated the superior efficacy of ProStar 50W and Fluazinam 500F for the control of southern blight on aucuba (see Table 2). In both years, all rates of ProStar 50W and Fluazinam 500F gave superb disease control. In fact, no symptoms of southern blight or signs of S. rolfsii were seen on any aucuba treated with any rate of either fungicide.

Lynx 3.6F, when applied at rates of four and eight fluid ounces per 100 gallons of water, consistently failed to prevent S. rolfsii from killing aucuba (Table 2). Only at the rate of 16 fluid ounces per 100 gallons of water did Lynx 3.6F prove as effective in controlling southern blight as either ProStar 50W or Fluazinam 500F.

In summary, Terraclor 75W, which was applied at twice the labeled drench rate, failed to provide an acceptable level of southern blight control on aucuba. On the other hand, ProStar 50W and Fluazinam 500F, over a range of application rates, consistently controlled this disease. Further studies need to be done to identify the rate at which either fungicide will no longer control southern blight.