Managing Southern Blight on Tomato and Pepper After Detection

From Clemson Plant Pathologist Tony Keinath.

As reported in the June 6 edition of SC Grower, southern blight is showing up in crops of tomato and pepper. At this late date in the growing season, the best fungicide option for conventional growers is Priaxor, two applications made 2 weeks apart at a cost of $34 (not including labor) per application. It is too late to apply Fontelis.

Signs of the southern blight fungus on a diseased pepper stem: thick white fungal strands and young white sclerotia (the balls).

Before growers run out and spray Priaxor, consider the following points:

  • Priaxor, and all other fungicides used to manage southern blight, works best when applied as a preventative. It is very likely that more plants, in addition to those showing symptoms now, are infected already.  Priaxor will not do anything to stop southern blight on those plants.
  • There is no correlation between the percentage of diseased plants and yield of fresh market tomatoes, based on three experiments at Coastal REC (2015, 2016, and 2021) when fruit were harvested five times. Southern blight on tomato is one of those diseases that looks worse than it is.
    • Plants affected with southern blight still may yield at the first or second pickings, depending on when plants were infected and how many fruit were already set before infection.
    • Tomatoes will compensate for missing plants, that is, the neighboring plants will grow larger.
    • The effects of southern blight will be greater the longer the crop is harvested after disease appears.
    • Whether southern blight affects the flavor of tomato fruit has not been tested. Direct market growers may want to check the quality of fruit harvested from diseased plants before selling it.
  • The economics of fungicide use on pepper have not been tested.

Once disease appears, there are no organic options to manage it in the current crop. Growers and home gardeners should remove diseased plants from the field and burn them. Do not add diseased plants to compost piles, because the sclerotia (survival structures of the fungus) may survive composting. Ideally, diseased plants should be removed before sclerotia form or when the sclerotia are still white (as in the photo above), so sclerotia do not fall off diseased stems and remain in the soil despite the work of removing the plants.

Growers who are concerned about southern blight this year should read up on the disease before planting a susceptible crop in the same field again. Research at Coastal REC and University of Georgia in 2021 showed the rootstock ‘Maxifort’ was resistant to southern blight and increased yields in infested fields.

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