Efficacy of PCNB for the Management of Southern Blight in Fresh Market Tomatoes

Patrick Colyer, Vernon, Philip R.  |  6/14/2005 6:54:50 PM

Figure 1. Complete collapse of tomato plant with southern blight. (Photo by Patrick Colyer)

Figure 2. Fungal growth of Sclerotium rolfsii around the base of an infected tomato plant. (Photo by Patrick Colyer)

Figure 3. Incidence of southern blight over time for tomato plants treated with PCNB or untreated in 1998 and 1999.

Figure 4. Premium and marketable yield of tomato plants treated with PCNB or untreated in 1998 and 1999.

Patrick D. Colyer and Philip R. Vernon

Southern blight, caused by the fungus Sclerotium rolfsii, is a serious disease that attacks many plant species, including most vegetables grown in home gardens. The most obvious symptom of the disease is the sudden wilt or collapse, near or at mid season, of all the above-ground parts of the plant (Figure 1). The mycelium of the fungus is often visible as a white, cottony growth around the base of the stem near the soil line (Figure 2). Reddish-brown sclerotia, which are survival structures of the fungus, are often present in this mass of mycelium.

The disease is difficult to manage, and all control practices are directed at preventing infection. Crop rotation and the application of the fungicide pentachloronitrobenzene (PCNB) to transplants are recommended to reduce the incidence of the disease. There are numerous reports, however, that the application of PCNB has failed to protect the plants from the disease. In 1998 and 1999, a study was conducted to evaluate the effect of the application of the PCNB (Terraclor 75WP) on the incidence of southern blight and yield of tomatoes grown on plastic mulch with drip irrigation. Transplants not treated with the fungicide were compared with transplants that were dipped in a suspension of the fungicide (3 pounds in 100 gallons of water) before planting into the black plastic mulch. There were 10 transplants per plot planted 18 inches apart and four replications of each treatment.

The incidence of southern blight was recorded weekly starting in mid-June through crop termination in mid- to late July by counting the number of infected plants (Figure 3). Infected plants were distinguished by wilted foliage and the presence of mycelium around the base of the plant. Except for the early rating dates when the incidence of disease in all plots in the test was less than 10 percent, the incidence of southern blight was lower in the fungicide-treated plots on every planting date. At the termination of the test each year, more than 60 percent of the plants were killed by southern blight in plots not treated with fungicide compared with less than 25 percent in plots treated with a fungicide at planting. Consequently, although complete control of the disease was not achieved, incidence of the disease was lower in fungicide-treated plots.

Although premium and marketable yields or numbers of fruit were higher in the fungicide-treated plots, the differences between treatments were not significant (Figure 4). The failure to improve yields, despite the differences in plant survival, is related to the number of fruit that had to be culled. The surviving plants in the fungicide-treated plots continued to produce fruit, but many had to be culled because of insect injury and because high temperatures caused cracking.

(This article was published in the winter 2003 issue of Louisiana Agriculture.)

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