QUARRYVILLE, Pa. — Angular leaf spot is a growing challenge in U.S. tobacco fields, and it can be tricky to beat.
Growers learned to combat that and many other diseases during Tuesday’s tobacco grower meetings at the Solanco Fairgrounds.
The sessions were organized by GAP Connections, the tobacco industry’s good agricultural practices organization.
Angular leaf spot creates lesions with pointy, irregular shapes and yellow halos. It’s caused by bacteria that live in crop debris.
“What happens in rain storms is that those bacteria get blown up onto the leaves,” said Chuck Johnson, a Virginia Tech plant pathologist.
Fortunately, the microbes can only get in when the leaves are soaked. Improving air movement in the field may lessen the risk of infection.
Streptomycin sulfate is the most important weapon growers have against angular leaf spot.
But because the antibiotic doesn’t last long, timing is everything.
To be effective, a preventive spray needs to be applied in the days just before the storm.
“If you make it even as much as a week ahead of time, it’s probably not going to help you,” Johnson said.
Streptomycin might also help keep the disease in check after spots start to appear, but it needs to be sprayed in the one to three days after the storm.
Black root rot may be less common than angular leaf spot, but growers are just as limited in their control strategies.
Black root rot is caused by a fungus that thrives when the soil pH is close to neutral. The ideal pH for tobacco is between 5.5 and 6.
The fungus is only active at low temperatures, so it usually shows up early in the season, in the first few weeks after planting.
The first symptom of the disease is uneven growth within a row, but that could indicate other problems too.
“One reason you want to know if it’s black root rot is because soil fertility, other things, those are relatively easy to do something to help,” Johnson said. “With black root rot, that’s not so much the case.”
As its name suggests, the most telling symptom of black root rot is the dark patches it makes on the roots.
There are no chemical controls for the disease, though most burley varieties are resistant.
“That’s one of the reasons we don’t see it that much,” Johnson said.
Still, KY 14xL8 burley and PA 41 wrapper tobacco are susceptible to the disease.
Insect-transmitted viruses pose a similar problem to black root rot. There’s nothing growers can do once the plants are infected.
Insects generally bring the viruses into the tobacco when an adjacent crop like alfalfa is cut.
To minimize this risk when cutting forage, growers should start next to the tobacco and work away to drive the insects in the opposite direction of the tobacco.
“And certainly, if there’s anything you can do to improve your insect control in those other crops alongside the tobacco, that’s going to tend to help avoid these virus problems,” Johnson said.
Many burley varieties, such as TN 90, are resistant to viruses, and some plants will recover on their own.
Generally, though, plants will be stuck with the virus for the rest of their lives, becoming stunted and misshapen.
Of course, growers face disease and management challenges even when the seedlings are started in hydroponic float trays.
Growers should aim to turn at least 85 percent of the seeds into usable transplants, said T. David Reed, a Virginia Tech Extension agronomist.
Uniform germination helps a lot with this goal. If some of the plants sprout more than two days after the majority, the little guys may be crowded out and be minimally productive.
“It may not die, but it might be a small plant,” Reed said.
Heating the greenhouse improves the germination rate, but if it gets too hot, too much water can evaporate. That can cause fertilizer salt injury to the plants.
Tiny plants are most susceptible to salt injury because they are growing at the top of the cell where the salts accumulate. Once the roots reach down to the water, salt injury is unlikely, Reed said.
Sometimes, the plants in the middle of a tray look good, but the ones on the outside are small. Bleach injury is the likely cause.
Reed suspects that the affected trays are the ones that ended up on the bottom of the stack when the growers dipped the trays in bleach to disinfect them.
As with salt injury, plants with bleach injury do better once their roots hit the water, he said.
Growers can reduce the amount of pathogens they harbor in their trays by ditching the standard foam trays for the black plastic trays that have been on the market for a couple years.
The porous foam trays offer microbes many places to hide. The plastic trays, though more expensive, can be more effectively sanitized.
The original version of the plastic tray had pinhole leaks, so it didn’t float as well as hoped.
That problem has been corrected even while the manufacturer has twice reduced the amount of plastic used in the trays.
That reduction in raw materials is good for growers, Reed said. Less plastic should equal a lower cost.