Checking peanuts? Take your shovel! Unlikely southern blight* expression found in south Delta.

Yesterday we found our first southern blight* damage to peanuts for 2014. The timing is not unusual, we generally start finding the disease when the crop canopy closes. What is unusual is where on the plants the disease was found.

Jason and I were in a large field of peanuts between Tchula and Greenwood. An occasional plant had a top leaf or two “flipped” (Figure 1), but you had to look for those plants. They were uncommon. Parting the canopy and looking at the crowns and surrounding soil showed no ropy white mycelia. There was no decay on the crown, stem, or limbs.

Once the plants were dug we found damaged root systems. The tap root of some plants was rotted (Figure 2), the upper part of the tap root of others was partially decayed (Figure 3). Again, there were no above ground signs (white mycelia…) of the disease!

Curious, we began to dig other plants at random. A disturbing number of them also showed symptoms of stem rot. We also found some Rhizoctonia diseases.

On the way home, we stopped in east Greenwood and checked another field. We found many of same symptoms of southern blight.  Again, no real above ground symptoms. There were also no signs of Rhizoctonia.

Check your peanuts! Take a shovel and dig random plants. Tell us what you see. Send your images to ahenn@ext.msstate.edu.

Management

• Scout your fields.

• Look for Rhizoctonia limb and pod rot.

• Look for southern blight (stem rot).

What you find will help you select appropriate fungicides – which influences costs.

The disease in the fields we looked at was under the soil. For fungicides to penetrate to the location of the fungus will require two significant practices:
1. Apply the fungicide at night. This will minimize the amount of mix intercepted by the canopy. More of the fungicide mix will contact the soil – where the problem is.
2. Use as much water as your pain threshold will allow. Twenty gallons/A is the minimum.

• Remember that most peanut fungicides move upward in the plant!

• Most fungicides will not move downward in the plant.

• The more water you apply, the more likely the fungicide will move downward to the roots.

• The roots will pick it up and move the fungicide upward, protecting much of disease prone area.

* Southern blight is also called stem rot, and incorrectly, white mold. The disease is caused by the fungus Sclerotium rolfsii.

Figure 1. Upper leaves of peanut plant losing water and flagging or “flipped”. This upper plant symptom is often the first one seen when plants are diseased with southern blight.

Figure 2. Tap root decayed by a below ground infection of southern blight. There were no signs of the fungus on the soil or crown of the plant. There was some wilting as in Figure 1, but not as pronounced.

Figure 3. Lesion on upper root below the soil level. No above ground plant symptoms were observed. While this lesion is not sufficient to cause plant death, it can progress, and even in the current condition, if the weather were to turn dry, the plant would probably wilt, and not produce.

Figure 4. Roots of a non-symptomatic plant. Note the mycelial strand near the lesion.