With more tools than ever before now available for controlling soil-borne peanut diseases, and profit margins becoming ever tighter, management may be the grower's greatest input, says Tim Brenneman, University of Georgia plant pathologist.
“Emerging” soil-borne disease such as cylindrocladium black rot (CBR) will continue to present challenges to Georgia producers in the coming season, he adds.
“CBR continues to cause more problems in Georgia,” says Brenneman. “It's a very serious disease. And, once you have it, you'll probably have to deal with it for the life of the time you grow peanuts in an infested field.”
It's important, he says, that growers accurately identify CBR because it can be mistaken for other diseases, particularly tomato spotted wilt virus (TSWV). Leaves and stems infected with CBR turn yellow and wilt. Usually, the central stem is the first to show symptoms. Small, reddish-orange perithecia appear on stems, pegs and pods.
“The real damage caused by the CBR pathogen is below ground, where it causes root rot,” says Brenneman.
A grower's first line of defense for effectively controlling CBR in fields infected with the disease is fumigation, he explains. “We haven't done a tremendous amount of fumigation in Georgia, but more growers probably could take advantage of it.
“There are different ways of fumigating fields. Basically, you want to inject the chemical directly under the row, where you'll come back and plant your peanuts. You must wait two weeks after fumigating before planting,” he says.
Georgia researchers conducted trials in three locations this past year to help determine the efficacy of fumigating for CBR, notes Brenneman. The tests were conducted in Webster, Taylor and Miller counties, with CBR infection rates ranging from 30 to 50 percent.
“These were three very different trials, environmentally speaking,” he says. “Webster County was a dryland situation with severe drought stress. There was no rain and no irrigation, but we did see some response from fumigation.
“Taylor County was extremely dry, but there was enough irrigation to make a crop. Miller County was our best test, with high production and ample irrigation. The best conditions for the crop also are the most favorable for the incidence of CBR. Response to treatment also is highest in these good conditions.”
The Webster County field was a disaster, yielding fewer than 1,000 pounds per acre, says Brenneman. The yield in Taylor County was better than 3,000 pounds per acre, and the response from fumigation was significant. In Miller County, with the highest yield potential, the response to the use of a fumigant was a 900-pound yield increase, he says.
All of these replicated plots were large, he says, and they were harvested and taken to buying points. Researchers looked at the increased value of fumigated peanuts versus non-fumigated peanuts, he adds.
“If you're going into a dryland situation, and you have a lost crop, you're wasting your money by putting out a fumigant before you plant. In Webster County, the fumigant cost about $50 per acre and we saw a return of only $10 per acre.
“In Taylor County, the return was about $115 per acre on that $50 investment. In Miller County, we saw a return of about $270 per acre from that $50 investment. These returns are on quota peanuts. If you're growing additionals, you'll have to compensate for the difference.”
More Georgia growers probably could benefit from fumigation, says Brenneman, but it's important that they know for certain if they have CBR. There are other benefits to fumigation, but they won't pay for the treatment if you don't have CBR, he says.
In the past couple of years, there has been discussion about the effect of fungicides on CBR, says Brenneman. “Folicur has a label for suppression of CBR, but not for control of the disease. We know that Moncut doesn't have activity on CBR. We compared these two products at the Webster and Taylor County sites to see if there was a yield response.
“The response was identical in the two fungicides at both locations. We did not see a degree of control from Folicur, at least not one that could be translated in a yield increase. The history of this has been varied, as responses have been reported in other areas.”
In a Plains, Ga., test, in a field with a severe epidemic of CBR, a response was seen from both Folicur and Abound, he says. “This was a very large and significant response over the non-treated. This was a pure CBR test with no white mold and a small amount of limb rot and other diseases. But, we've also seen tests where we get no response.
“The bottom line is that if you have CBR, there is a chance of getting some benefit from some of these fungicides. Folicur has a label, but Abound is not labeled for that application. Some data suggests Abound will have a response similar to what we get from Folicur.”
Turning to other soil-borne diseases, Brenneman says Georgia growers probably have spent the most money attempting to control white mold.
“There is an ‘impostor’ that looks like white mold. Some growers have seen this, thought they had a severe white mold epidemic and treated excessively with fungicides. But this particular fungus does not respond to fungicides. You can spray every day and you'll still have this fungus in your field, and you will have spent too much money.”
This fungus, he explains, has a white mycelium that grows around the base of the plant. “Although, it also can grow out and away from the base. It generally grows on organic matter. We're seeing more of this fungus because we're doing more no-till or reduced-tillage peanuts. We now have more crop stubble in the soil surface.
“This fungus is just growing on the stubble or debris. It doesn't do anything to the peanut plant. If you look at it closely, it looks like icicles or teeth hanging from the stem. You won't see this on white mold. Also, you can scrape this fungus from the stem and you'll have green tissue underneath. It does not attack the stem and does not cause a lesion.”
White mold has small, white sclerotia that form and turn from brown to black. That's a key, says Brenneman, to distinguishing actual white mold from false white mold.
There are several options, says the plant pathologist, for managing white mold and limb rot in peanuts. “Folicur has been labeled since 1994 and has good activity on white mold, leafspot, limb rot and, possibly, activity on CBR.”
Research in recent years has looked at how peanut fungicides perform in terms of their post-infection activity and their longevity, says Brenneman.
“Folicur appears to have the best activity when we spray it on a plant that's already infected. If you have a disease epidemic, and you see white mold in the field, Folicur probably will have the best activity as far as moving into that plant and stopping an epidemic that's already occurring.
“On the other hand, Abound seems to exceed in terms of longevity. If you want to put out a material with an extended period of protection, Abound seems to last longer in terms of the protection period that it offers in the field. Abound also is very effective against white mold and leafspot. And, it's probably the best material available for limb rot or rhizoctonia.”
Moncut and Montero both have the same active ingredient that's effective in controlling white mold, says Brenneman. They aren't active on leafspot or CBR, although Montero contains Tilt, which gives leafspot control, he says.
In reviewing new chemistry, Brenneman says Stratego, which was developed by Novartis, will be marketed by Bayer. “This is a twin-pack product. Part of the combination is Tilt and the other is a new chemical material — trifloxystrobin — that's in the same class of compounds as Abound, but it has a different range of activity.
“In fact, trifloxystrobin is labeled for leafspot, rust and web blotch. These are all foliar diseases. It doesn't have activity on white mold or stem rot.”
There's an increased interest among some growers, he says, for using in-furrow fungicides in peanuts. “Traditionally, we haven't done this in peanuts, but Abound has an in-furrow label as does Terraclor. We've looked at a number of trials where we get about a 200-pound per acre difference from using an in-furrow fungicide.
“We've seen increases as high as 1,000 pounds. And, in other trials, we've seen no response. There are a number of benefits, including stand insurance. There's evidence of controlling soil-borne diseases, particularly aspergillus crown rot — our Number One seedling disease.
“There's also evidence of early season white mold control. An in-furrow treatment could offer suppression until you can get back in the field with a postemergence spray. Basically, it's insurance, and it'll cost about $10 per acre.”
CYLINDROCLADIUM BLACK ROT (CBR) disease is becoming an increasingly serious problem for Georgia peanut producers.