With the pressure on peanut growers to maximize yields and minimize production costs, now is a good time to take some relatively low-cost steps to improve your productive capacity.
One is controlling the soilborne disease Cylindrocladium black rot (CBR), according to Barbara Shew, North Carolina Extension plant pathologist. Caused by a fungus that produces small, hardened clusters of cells called microsclerotia that germinate where peanuts are planted, the key to controlling CBR is reducing the number of microsclerotia in the soil.
That doesn't necessarily have to be expensive, but it does have to be done now: Nearly all CBR control must be performed at planting or before.
The most important control measures are:
Rotations of three years or more to non-host plants deprive the fungus of a food source so that the number of surviving microsclerotia declines over time.
“Cotton, corn, sorghum and small grains are excellent rotation crops to achieve this goal,” says Shew.
But soybeans are not a good choice. The CBR fungus infects soybeans, increasing microsclerotia survival between peanut crops. “Soybeans can be infected and you might never know it since symptoms are not as severe as on peanuts,” she says.
Since some peanuts are now going on land that has never been in peanuts, there is an opportunity to move away from CBR, she says.
“But even the absence of peanuts in a rotation does not rule out the presence of CBR,” she says. “Occasionally, CBR breaks out in fields that have never been in peanuts. The reasons are not always clear, but a history of soybean production usually is suspected.”
Peanuts should not be planted in fields where soybeans have been grown in the past three years, she says. “I think if you have soybeans in a rotation you should consider it as bad as having peanuts. I don't know if it would be any worse, but it is basically equivalent to having peanuts in there.”
To effectively deal with CBR, you need a long-term rotation plan, says Virginia Extension Plant Pathologist Pat Phipps. “A period of two years in cotton or corn can be effective in control of rootknot nematode in peanuts. However, at least two cycles of a three- to five-year rotation of peanuts with cotton or corn is needed to reduce the risk of CBR problems in heavily infested peanuts.”
In a field that has been rotated well, a resistant variety like NC 12C or Perry is often all that is needed to avoid CBR losses, says Shew. In these cases, fumigation probably is not necessary if fewer than 10 percent of the plants had CBR the last time peanuts were grown. But even if you plan to fumigate, highly susceptible cultivars like VA 98R should not be planted in fields with a history of CBR.
When you are not sure about potential CBR problems in new fields, play it safe and plant a resistant variety, Shew adds.
What varieties should you choose? The Virginia varieties NC 12C and Perry are resistant while VA 98R, Champs and NC-V11 are highly susceptible to CBR. Gregory and Wilson are moderately susceptible.
Among the runner types, two varieties, GA-02C and Carver, have been released in the past two years. “They appear to have at least some level of resistance to CBR,” say Georgia Pathologists Tim Brenneman and Al Culbreath.
Georgia Green is moderately susceptible to CBR, they say.
Consider fumigation with metam sodium in fields that had the disease on 10 percent or more of plants the last time peanuts were planted. While not low-cost, fumigation works by killing microsclerotia in soil and also provides some control of rootknot and ring nematodes which can make CBR problems worse.
Always use good quality, commercially treated seed. CBR can be transmitted via seed, though at a very low rate.
Plant on a bed to promote soil warmth and drainage and reduce the chances of infection in the spring.
“The fungus grows best in cool, moist soil, so most root infections occur within a few weeks of planting,” says Shew. “The cooler the soil, the more infection you are going to have. If you plant on a bed, it will warm the soil. That will inhibit infections. It is not a big effect but it will help.”
Don't introduce soil from fields with CBR to fields where it is not yet present. “This can be done best by cleaning equipment and vehicles before traveling between fields,” say Brenneman and Culbreath.
And with future crops in mind, scout for CBR this season. “CBR can be confused with tomato spotted wilt virus, so be sure to obtain a positive diagnosis if there is any doubt,” says Shew. “Make a note of problem fields and attempt to estimate the percentage of plants with symptoms. This information will help with rotation, fumigation and cultivar choices the next time you plant.”