Dr. Kari Peter, associate research professor, tree fruit pathology at Penn State, wants growers to be aware of a relatively new orchard problem.
“Southern blight (SB) is a lethal, soil-borne disease,” said Peter. “It produces sclerotia, which are tiny round structures that resemble seeds. That’s a very diagnostic sign of the pathogen. It’s extremely persistent because of the sclerotia, and it’s difficult to control. This is one of the most destructive diseases on crops in the southeastern U.S.”
In the eastern U.S., there are two causative SB species: Sclerotium rolfsii, with over 500 host species, and S. delphinii. Although both thrive in hot, humid conditions, sclerotia can survive cold temperatures.
S. rolfsii is believed to be confined to the southern U.S. S. delphinii has slightly larger sclerotia, a smaller host range, is more cold-tolerant and is found in the Midwest.
As growing regions experience above-average temperatures, SB is being identified in more northern states. “Moisture is staying relatively stable,” said Peter, explaining variations in weather patterns. “We aren’t getting good sterilization events in winter to kill pathogens that normally wouldn’t survive winter. Southern blight might be one of those pathogens.”
SB was first observed in Pennsylvania in 2018. It’s been a continuous problem in Adams County, which is ground zero for the problem. It’s been identified every year since except for 2023, which Peter said was likely due to dry conditions.
She observed SB during the 2024 growing season. “It was definitely here in May, and we saw it in June and July,” she said. “Southern blight has a broad host range, and it infected other species until ideal conditions in 2018-2019 led to apple tree infection.”
SB infection is obvious on fruit trees, with characteristic white mycelium at the soil line on the trunk. “It’s a fluffy, web-like structure,” said Peter. “It only shows up when it’s wet and humid, then it disappears. But even if it seems to come and go, the sclerotia attached to the mycelia are still there. If there are sclerotia, mycelia are there.”
SB appears the same no matter what plant it’s on.
The issue isn’t new. “We’ve known about southern blight in apples since 1935,” said Peter, “but there has been very little research on it. We are starting from the ground up with research.”
SB likely originated in soil and has perpetuated on other hosts. Peter doesn’t believe it came from nurseries because she has seen 10-year-old trees become infected and die. Nurseries would be aware of disease due to the presence of sclerotia on roots.
The white mycelium and sclerotia are not hard to miss. Photo by David B. Langston, University of Georgia, Bugwood.org
Graduate student Cameron Mehalek is conducting SB research, using what’s known about the problem in other crops as a base.
“It isn’t currently known which conditions make orchards susceptible,” said Peter. “Apple rootstock susceptibility is the most important aspect of research, and there are a lot of different rootstock. Generally, all rootstock are susceptible but no one has done any formal testing.”
There are no fungicides labeled for SB on apples, but there are for other crops including carrots, tomatoes, soybeans and peppers. Many fungicides are labeled for apple, but not for soil application.
“We are looking at orchards with a history of southern blight, current issues with southern blight and no history of southern blight,” said Peter. “Cameron is talking with growers and collecting soil samples. He’s looking primarily at pH because we know Sclerotium like more acidic soils. He’s also sieving soil it to look for sclerotia, which aren’t small, so they’re easy to find.”
Their research goal is to test as many rootstock as possible for SB susceptibility and tolerance. They’re starting with 10 rootstock, both dwarfing and semi-dwarfing.
Peter said sclerotia are finicky fungi that require specific conditions to grow, including the right soil temperature and soil moisture. Mehalek’s project in a controlled greenhouse.
“Bud.9 is our positive control because it’s the one I found the most southern blight on in the field,” said Peter. “Cameron is making a wound at the soil line and putting sclerotia in the wound, then wrapping it with Parafilm®. He collects information on the onset of infection, girdling, wilting, necrotic leaves and sclerotia formation. At 20 days he evaluates the trees – pulls them from the pot, sieves soil core samples and counts and weighs sclerotia.”
Preliminary results showed that Bud.9 is susceptible, with all trees in the study showing signs of SB infection. Mehalek observed mycelia at about five days after inoculation. Sclerotia were evident and abundant starting at 12 days. The infective pathogen can move and reproduce quickly.
Mehalek recently evaluated M.7, G.11 and G.935 rootstock. “He was already seeing differences with G.935,” said Peter. “By six days, the progression of the infection didn’t move far in contrast to Bud.9, which is highly susceptible. We might be seeing some tolerance with G.935.”
Additional preliminary results show that G.11 seems tolerant to SB with minimal mycelia progression and healthy roots. In contrast, half of the trees on G.935 were badly infected, while others seemed okay. Trees on M.7 were completely obliterated.
Peter said preliminary results show positive differences with at least two Geneva rootstocks but cautions growers that these results will require more testing.
After the trials, Peter didn’t want to discard the potentially infected soil. She placed the used soil under a black tarp to determine whether solarization might be a management tool, especially for growers who remove entire blocks and want to prevent infection.
She also cautioned growers to be aware of soil-borne pathogens in water. “Irrigation ponds can be a hotbed for soil-borne pathogens,” she said. “We’re in the process of trying to figure out a good method to find pathogens, and we find them, figure out how to prevent what’s in the pond from affecting trees.”
Ongoing research includes testing fungicides labeled for other crops with the goal of obtaining fungicide labels that allow growers to use products legally.
Fumigation works against SB, but not all growers fumigate regularly. “Fungicides are probably one line of defense,” said Peter. “Some are labeled for apples, but none are labeled for soil application. We need to see if any work against SB on apple, and if they do work, will they work in soil as a prevention. Present research is making a big dent, but there’s a lot more to come.”
by Sally Colby
