A Clemson University scientist’s ongoing research on Southern blight will soon result in a management strategy for vegetable growers that is designed to be effective, economical and environmentally safe.
Anthony (Tony) Keinath, vegetable pathologist at Clemson’s Coastal Research and Education Center in Charleston, is continuing to conduct field trials and laboratory research to determine the sensitivity of Southern blight to several commercially available and legally registered fungicides.
“The ultimate goal of this research is to reduce the number of plants that are killed by Southern blight,” said Keinath, who will release his recommendations to growers after completing his research later this year. “The main focus has been using fungicides to replace soil fumigants— which is analogous to using a rifle instead of a shotgun.”
Incidences of Southern blight have become more frequent in recent years because one of its most effective combatants was basically taken off the market. Methyl bromide — a fumigant once widely used to control pests in agriculture — was restricted by the U.S. Environmental Protection Agency in 2009 after it was determined that the odorless gas was playing a role in the depletion of the ozone layer. Without methyl bromide at their disposal, growers have had to turn to fungicides to protect their crops against Southern blight.
Keinath’s research — which began in 2014 and will continue throughout 2016 — has focused primarily on the disease’s effect on tomatoes. In South Carolina, there are more than 3,000 planted acres of tomatoes with an economic impact of about $44 million. Other widely planted crops that could be damaged by Southern blight include peppers, eggplants and green beans.
“Growers still use fumigants with tomatoes,” said Keinath, who spends 80 percent of his time on research and 20 percent working with Clemson Cooperative Extension. “They just use different kinds of fumigants than methyl bromide, and though these other fumigants work well against many diseases, some of them don’t work well against Southern blight.”
Keinath has focused on three fungicides — Fontelis, Priaxor and Cabrio — that are safer to use and more environmentally friendly than many of their competitors. Except for Cabrio, the fungicides are relatively new on the market. Until Keinath began his trials, Priaxor had not been tested for its effectiveness against Southern blight.
“Several years ago, some large tomato growers in South Carolina contacted me saying they were having problems with Southern blight and they asked if there was something I could do to help them,” said Keinath, whose research was funded in 2014 by two of the growers and in 2015-16 by a S.C. Department of Agriculture Specialty Crop Block Grant. “What I’ve determined thus far is that Fontelis and Priaxor applied twice per season after planting have been the most effective fungicides against Southern blight on tomatoes. Cabrio and Blocker are somewhat effective, but not to the same degree as the other two. I also tested a biological fungicide and found that it was not effective.”
Southern blight is caused by a soil-borne fungus called Athelia rolfsii, which is nearly impossible to eradicate even though it exists in relatively low levels. The fungus infects the lower stem of the plant near the surface of the soil. It is called Southern blight because it cannot survive for long stretches in frozen soil and therefore only thrives in warm climates. In the United States, Southern blight is found in the Southeast and other southern portions of the country, including Florida, Texas and large swaths of California.
The fungicides Keinath is testing are designed to be sprayed on the lower stems of the plants, halting the ability of the fungus to sprout from the soil and attach branching filaments to the stems. This prevents the fungus from girdling and/or encircling the stem, which eventually kills the entire plant.
“One of the things I’ve looked at with different fungicides is whether they harm the fungus or simply just protect the plant from the fungus,” said Keinath, who serves as vegetable pathologist for South Carolina growers. “I’ve found that some of these fungicides seem to have both effects. We did some experiments in the lab where we sprayed fungicide directly on the sclerotia (compact mass) of the fungus and got reduced germination or reduced growth after germination. We did additional experiments where we sprayed tomato stems with the fungicides and then put sclerotia on the stem to see what happened. With at least one of the fungicides, this also worked very well.”
One of more surprising results of Keinath’s research, thus far, has been that his tomato plots at Coastal REC did not show a significant decrease in yield despite losing some of their plants to Southern blight.
“Tomatoes are usually planted 24 inches apart, which really is not that much space for a mature plant,” Keinath said. “It became apparent that when a plant died, its neighbors benefitted from a larger share of fertilizer, water and sunlight, and the survivors ended up producing a few more tomatoes than they otherwise would have. For large commercial growers, any risk of significant loss is a reason to use the fungicides as protection. But it’s possible that one of my recommendations for smaller growers might ironically turn out to be that they should not treat for Southern blight, because they might not be able to increase yield enough to recoup the cost of the fungicide.”
(Written by Jim Melvin, Clemson University Public Service Activities.)