Sunday, May 26, 2024

Difficulty With Disease 

Watch for these fungal problems as the season progresses.

⋅ BY AMANDA HUBER ⋅

Like insect problems, the primary diseases that will appear in peanut fields this season will depend on weather conditions. Although leaf spot can be found in nearly every field yearly, other diseases are more dependent on whether conditions are hot and dry or cooler and more moist. Scouting fields is the best way to stay ahead of fungal problems that could defoliate the crop and limit yield. To do this, correct identification is a must, and it may not always be straightforward. Some diseases can mimic other disease problems, insect damage or pesticide injury. Accurate identification is critical to plan for management and record for field history.  

Watch For White Mold Growth

White Mold

White mold was generally less severe in 2022 possibly because of cooler conditions and incessant rains, says Bob Kemerait, University of Georgia Extension plant pathologist. However, with the potential change from a La Niña weather pattern to the El Niño, hot, dry conditions may prevail in 2023. 

Heat, humidity, reduced rainfall and growth of the peanut plants all put the crop at risk for diseases, especially white mold and leaf spot. Once a dense canopy of foliage develops in the field, moisture and humidity can become trapped. Leaf wetness is prolonged, increasing the risk of disease. The dense canopy also makes it more difficult for fungicides applied to the leaves to reach the crown of the plant for protection against white mold.

Kemerait says it is nearly impossible to have “perfect” white mold control, but it is possible to stop white mold from spreading down the row. 

“An initial infection of white mold is likely to occur from individual plants being infected by sclerotia in the soil close to the plant. From this initial infection, the disease can spread and burn along a row, resulting in significant yield loss. 

“A good fungicide program will stop the initial hits of white mold from burning down the row,” he says. “If your program is not stopping the burn, then we need to figure out why. It could be the fungicide is not reaching the intended target, or the fungicide is applied too late or at the wrong rate. It could also be that a better fungicide could be used.” 

Overall, Kemerait says, incidence of disease should be lower in well-rotated fields. 

White Mold Or ‘False’ White Mold

False White Mold

Correct identification is the foundation of all pest control, including disease management. Although the cottony white strands of white mold may seem unmistakable, there is a similar mold that may inhabit peanut fields, especially those in conservation tillage. False white mold is caused by the fungus Phanerochaete and initially looks like real white mold but later becomes more yellowed and has a “toothed” appearance. 

False white mold may blanket peanut stems, but it does not cause injury, Kemerait says. Real white mold produces small, round sclerotia, which false white mold does not.

Leaf Scorch

Leaf Scorch

Another relatively harmless fungal disease is leaf scorch. It causes V-shaped lesions on leaves, which are caused by the fungal pathogen Leptosphaerulina crassiasca. Leaf scorch is almost never associated with any significant injury to the peanut crop or yield loss. However, it may be confused with leafhopper burn or Thimet injury, Kemerait says. 

Leaf scorch typically affects younger plants but can occur anytime in the season. It is considered cosmetic and often controlled with a leaf spot management program.

CBR Or TSWV

Peanut seed with CBR speckles
CBR on stems

Yet another disease identification mistake can happen between plants affected by Cylindrocladium black rot and Tomato Spotted Wilt Virus. 

Kemerait says both diseases cause plants to be yellowed, wilted and with a necrotic taproot.

“The roots of plants affected by TSWV may also be infected by the fungus Neocosmospora vasinfectum, which produces fruiting structures similar to the CBR pathogen.”

The difference in the two diseases is often found in distribution in the field. Plants affected by CBR are usually clustered in specific areas of the field, whereas those affected by TSWV are likely more scattered across the field.

CBR tends to be most severe when cooler and wetter conditions occur at planting as this facilitates infection by the fungus. The disease is also often more severe in fields where the peanut root-knot nematode is also a problem.

Few Management Options For CBR

Kemerait says management of CBR requires rotation away from peanut and soybean crops, which is also a host of the disease. 

“Chemical management of CBR begins with fumigation of the soil with metam sodium or in-furrow applications of a product like Proline (prothioconazole). Though of limited benefit, some fungicides applied for management of white mold may also have efficacy against CBR,” he says.

Because the fungus that causes CBR can be seedborne, it is important to not save seed from infested fields. Seed affected by CBR may have small, red microsclerotia “speckles.” Brick red-orange fruiting structures may also be found on pegs and stems at the soil line or on pods. 

As the season progresses, remain vigilant against diseases that defoliate the crop and limit yield. PG

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