PEANUT RX

Risk in a field to tomato spotted wilt, leaf spot and white mold can be estimated based upon a number of factors to include: the variety planted, planting date, crop rotation, tillage, plant population, use of in-furrow insecticides, and field history. When using Peanut Rx, growers can modify their production practices to reduce risk in the field. For fungal diseases, growers have the opportunity to use “prescription” fungicide programs appropriate for a given risk level. Fields found to be “low risk” can be effectively treated with a reduced fungicide program as compared to a “moderate risk” or “high risk” fields without compromising yield. The risk points in Peanut Rx are updated yearly.

Tomato Spotted Wilt

Symptoms

Plants affected by tomato spotted wilt are often stunted and leaves show characteristic rings and mottled patterns. Plants that develop symptoms later in the season may have less-dramatic leaf symptoms and the plants often have a yellowed and wilted appearance. The taproot of these affected plants is often rotted and necrotic.

Causal Organism

Tomato spotted wilt virus (TSWV) which is spread (vectored) by infected thrips, typically the tobacco thrips and the western flower thrips.

Management tactics

Growers can reduce their risk to losses from tomato spotted wilt by planting resistant varieties. Growers can further reduce their risk to spotted wilt by considering the impact of planting date, seeding rate, tillage, single-rows versus double-rows, choice of at-plant insecticide and use of Classic herbicide.

Typical symptoms of tomato spotted wilt early in the season, plants may be stunted and leaves with ring-spots and mottled appearance.  Later in the season, plants affected by tomato spotted wilt often show yellowing, wilt, and a necrotic taproot.
Figure 1. Typical symptoms of tomato spotted wilt early in the season, plants may be stunted and leaves with ring-spots and mottled appearance. Later in the season, plants affected by tomato spotted wilt often show yellowing, wilt, and a necrotic taproot.

Early leaf spot

Diagnostic Symptoms

Tan-to-dark brown spots developing first in the interior of the canopy. Spores, sometimes difficult to see with the naked eye, are found on the top of the leaf.

Causal Organism

Passalora arachidicola (formerly known as Cercospora arachidicola)

Effect on yield

If not controlled, early leaf spot can cause significant yield loss. Premature defoliation because of early leaf spot can affect not only yield potential but also the strength of the pegs, increasing risk to digging losses.

Factors influencing increased risk

Peanut planted too often in the same field, frequent rain events and high humidity. The fungus survives in the crop residue and debris that remains in the field after harvest. Crop rotation and also burying the crop debris can help to reduce the potential for disease. See Peanut Rx for specific information practices that affect risk to leaf spot diseases.

Management tactics

Early leaf spot can be managed with crop rotation, planting resistant varieties, judicious use of fungicides.

Early leaf spot, note sporulation on top of leaf.  The yellow halos encircling the spots are commonly observed; however they are not always present.
Figure 2. Early leaf spot, note sporulation on top of leaf. The yellow halos encircling the spots are commonly observed; however they are not always present.
Early leaf spot, note sporulation on top of leaf.
Figure 3. Early leaf spot, note sporulation on top of leaf.

Late leaf spot

Diagnostic Symptoms

Dark-brown-to-black spots developing first in the interior of the canopy. Many dark spores are typically visible on the underside of the leaf. The spores often profusely cover the underside of the spots. Premature defoliation can reduce the strength of the pegs and increase losses at harvest.

Causal Organism

Nothopassalora persoanta (Formerly knonw as Cercospordium personatum)

Factors influencing increased risk

Peanut planted too often in the same field, frequent rain events and high humidity all increase risk to late leaf spot. As with the pathogen that causes early leaf spot, the fungus causing late leaf spot survives in the crop residue and debris left in the field. See Peanut Rx for further management options.

Management tactics

As for early leaf spot, crop rotation, use of resistant varieties, judicious use of fungicides, and other factors can be integrated to manage late leaf spot and to protect yield.

Late leaf spot, note dark sporulation on the underside of the leaflets.
Figure 4. Late leaf spot, note dark sporulation on the underside of the leaflets.
Late leaf spot, note the yellow halos around the dark spots.  Though yellow halos are more commonly observed on spots caused by early leaf spot, they can also occur with late lead spot.
Figure 5. Late leaf spot, note the yellow halos around the dark spots. Though yellow halos are more commonly observed on spots caused by early leaf spot, they can also occur with late lead spot.

Southern Stem Rot (White Mold)

Diagnostic Symptoms

There are several symptoms commonly associated with white mold. Among these are wilt of plants and, often, the presence of white fungal growth that causes significant lesions on the crown, limbs and pegs of the plants. Also, small BB-sized “sclerotia” are often present as well. Sclerotia are like fungal “seeds” that survive in the soil after the peanut crop is harvested.

“Underground” white mold is caused by the same disease but is not observed until the peanut plants are inverted at harvest. When underground white mold occurs, there not be any above-ground symptoms. However, significant damage may occur to the pegs and to the pods and they may be covered with white fungal growth.

Causal Organism

Sclerotium rolfsii

Factors influencing increased risk

White mold is most often problematic in fields where peanut is planted in short rotation, that is, where peanuts are planted more often than once every three years in the same field. White mold, especially underground white mold, can be especially severe when the growing season is warmer than normal. White mold is favored by the high humidity within the canopy of the peanut plants. Early outbreaks of white mold occur when warmer-than-normal temperatures occur early in the season.

White mold is less of a problem in well-rotated fields and when cooler temperatures prevail either later in the season or during periods of prolonged cloudy weather and rainfall.

Management tactics

White mold is best managed with crop rotation (plant peanuts in a field no more than once out of three years), judicious use of fungicides and use of resistant varieties. For more detailed information, growers should consult Peanut Rx.

Damage from southern stem rot or “white mold”.  Note the complete loss of pods where the disease is severe.
Figure 6. Damage from southern stem rot or “white mold”. Note the complete loss of pods where the disease is severe.
Southern stem rot or white mold Note the white fungal growth and stem lesions associated with this disease.
Figure 7. Southern stem rot or white mold Note the white fungal growth and stem lesions associated with this disease.