Cylindrocladium black rot is not a new disease to peanut growers, but it continues to slowly spread across the Southeastern peanut production belt and is a particularly big threat to Virginia type peanuts.
CBR was first found on peanuts in 1965 in Terrell County, in southwest Georgia. Though typically a slow-moving soilborne disease, CBR has gradually spread across the entire Southeastern peanut production area.
Yield losses from CBR can be severe — in some cases in excess of 50 percent yield loss. On Virginia-type peanuts, in particular, it can have a devastating impact on peanut quality.
In a year, like 2009, in which peanut contract prices are expected to be down, high yields and high quality are essential to make a profit.
Speaking at a recent South Carolina peanut meeting, University of Georgia Plant Pathologist Tim Brenneman says one reason for increasing management problems with CBR is that the disease is frequently miss-diagnosed by growers as tomato spotted wilt virus.
“If you don’t have the classic foliar symptoms of TSWV, it is easy to confuse the two diseases. With TSWV you get classic root rot, yellow and wilting of the peanut plant that can be very similar to CBR,” the Georgia scientist says.
Peanut plants infected with the CBR fungus may initially start with browning of stems, roots and pods. As the disease progresses distinctive black discoloration, especially around the crown of the plant is a good indicator of CBR, not TSWV.
Like many other soilborne diseases, CBR does not occur in a random pattern in fields. Rather, CBR occurs in non-random patches. After the disease becomes distributed within a field across many years, the patchy occurrence becomes less obvious. Often the more severely affected areas in the field are in the wetter sites.
Though caused by different organisms, the above-ground symptoms can be very similar. If a grower chooses a variety with resistance to TSWV, he may be planting a variety that is highly susceptible to CBR.
In addition, Brenneman stresses that management of the two diseases is very different. TSWV is vectored by thrips and can spread rapidly through a peanut field. CBR in contrast spreads much more slowly. Though often confused, the two diseases are dramatically different.
North Carolina State University Peanut Specialist David Jordan notes that varietal response to TSWV is often erratic. The already erratic nature of CBR resistant varieties can be made worse by reducing seeding rates, he says.
Gregory and NC 12C, for example, are large, and peanut emergence may be less uniform than smaller seeded-varieties. Because seed of these varieties is large and more expensive at high seeding rates, growers are tempted to reduce seeding rates below recommended levels and low planting rates may negate any benefits of partial resistance to TSWV.
Perry is a large-seeded peanut that has some resistance to CBR. However, it is moderately susceptible to TSWV. Knowing which disease is more prevalent in a field is critical to knowing whether to plant Perry or Gregory.
Rotation and fumigation (Vapam 10 gallons per acre) can be used to control CBR. There is also increasing evidence that Folicur and Abound provide some suppression of CBR. The combination of Proline and Provost offers some benefit for CBR, according to Brenneman.
North Carolina State’s Jordan says tests in North Carolina indicate peanut yields tend to drop when peanuts are planted behind either peanuts or soybeans, compared to cotton in the rotation. This can be made worse by planting CBR susceptible varieties.
The North Carolina specialist says at one in-state research site, CBR increased dramatically when the susceptible variety Gregory was planted, and this increase in disease resulted in lower peanut yields. In contrast, planting the CBR tolerant variety Perry resulted in consistent yields regardless of rotation length.
Brenneman says in Georgia some the better varieties for CBR resistance have been Carver, GA 01C and GA 01R runner varieties. Among the Virginia varieties NC 12C and Perry showed some resistance to CBR, but Perry in particular was devastated by TSWV in the Georgia tests, Brenneman adds.
Recent increases in soybean acreage in the upper Southeast may not bode well for CBR management, according to North Carolina State Plant Pathologist Barbara Shew. Soybeans in a peanut rotation is not good — may be as bad as following peanuts with peanuts, she says.
“The CBR fungus infects soybeans, increasing microsclerotia survival between peanut crops. Soybeans are rarely treated for CBR and in most cases growers don’t know beans are infected because symptoms are not as severe as in peanuts,” she says.
With cotton acreage expected to shrink to its lowest level in the past 15-20 years in North Carolina, some peanuts may go on land that has never been planted to the crop. While planting on new ground reduces the risk of CBR, it doesn’t eliminate the risk.
Brenneman says, the disease-causing organism can survive in the soil for a long, long time. It also can survive and spread on beggarweed and other weeds common to the Southeast. So, just because peanuts haven’t been planted on the land doesn’t necessarily mean there won’t be CBR problems.
“We don’t really know how widespread CBR is in the Southeastern Peanut Belt. In Georgia, a recent grower survey gave us some information about how widespread the disease is in our state. However, Terrell County, where CBR was first detected anywhere in the world was perceived by growers in that county to be CBR-free. We know it’s spreading, but we don’t know how many acres are affected,” Brenneman says.
“We do know that CBR is spread primarily by seed. Most of the work on seed spread of CBR was done in North Carolina. In that study, researchers looked at 162 commercial seedlots, and they found some CBR in every seed block. They found that growers could get up to 93 diseased plants per acre from commercially treated seed going into a field with no history of CBR.
“In clean fields planted with CBR-infected seed, growers would likely see a spread of 30-feet or so. Each subsequent peanut crop would spread the disease further throughout the field,” Brenneman says.
In addition to a 3-4 year rotation, Brenneman says good sanitation is a practice growers sometimes overlook in managing CBR. “If you have an infested field and you move peanut hay out of that field, for example, don’t move it in or around a field in which you plan to plant peanuts,” he stresses.
The consensus advice among Southeastern peanut disease specialists is the best way to manage CBR is to not get it. While that may not always be possible because of naturally occurring host plants, good sanitation, a 3-4 year rotation, close observation of seed, targeted use of fungicides and planting less susceptible varieties are all good management tools to slow down the progress of CBR in Southeastern peanuts.