Soybean Foliar Disease Identification

Boyd Padgett and Trey Price, LSU AgCenter Plant Pathologists

Cercospora leaf blight/Purple seed stain

Foliar symptoms are usually not evident until soybean is in the mid to late reproductive growth stages. Initial symptoms are small chocolate brown lesions on the petioles near the leaflet. As the disease progresses, foliar symptoms are expressed as a reddish brown to tan discoloration on the upper leaf surface in the upper canopy. Leaves have a leathery appearance. The fungus can sporulate in older lesions (resemble ashes). Advanced disease stages result in premature defoliation and discolored pods, and reduced seed quality. The seed phase is evidenced by purple-stained seed.

Frogeye leaf spot

Symptoms occur predominately on the leaves, but may appear on the petioles, stems, and pods. Initially, small chocolate brown to purplish spots form on leaflets. If the disease continues to develop, mature lesions have light brown to gray centers with a reddish brown to purplish margin. Stem lesions are rare and are elliptical with red centers and dark brown to black margins. Pod lesions are circular to elliptical, sunken, and light gray to brown.Infected seed exhibit grayish to light purple lesions.

Aerial blight

Caused by the same fungus causing sheath blight in rice. Initial symptoms appear as water-soaked greasy blotches on the leaflets (usually in the lower to mid canopy). As the disease progresses, adjacent leaflets adhere together by white fungal mycelium. If favorable conditions persist, the foliage becomes brown, and pods will have reddish-brown lesions. Under high severity pod abortion can occur. The disease is usually evident during the early reproductive stages of growth and later.

Soybean rust

Symptoms begin in the lower canopy as small brown to tan raised pustules (volcano-like) on the lower leaf surface. Young spores produced in the pustules resemble sand and are tan in color. Older spores are darker in color. Mature pustules can coalesce and cause the leaflets to defoliate. Symptoms are usually evident when soybean is in the mid (R3) to late (R6) reproductive growth stages. Pustules can occur on petioles and pods when disease is severe.

Target spot

A fungal disease affecting the leaves, stems, pods, seeds, hypocotyls, and roots. Foliar symptoms usually initiate in the lower canopy and appear as small reddish-brown specks. Mature lesions are round to elongate with brown centers (zonate pattern) and dull green to yellow-green halos. Pod lesions are usually circular with purple-brown centers and brown margins. Dark reddish-brown lesions can form on the hypocotyl, taproot, and lateral roots. These lesions turn dark violet-brown when the fungus produces spores.

Bacterial pustule

Caused by a bacterium and is not a major disease in Louisiana. Symptoms are very similar to soybean rust. Symptoms begin as small pale green, water-soaked spots with elevated centers on the upper and lower leaf surface. Mature lesions are dark brown with elevated volcano-like pustules on the lower leaf surface (easily confused with rust). Pustules are dry in appearance. Pustules can be found on the pods in susceptible varieties.

Soybean Root, Stem and Pod Disease Identification

Boyd Padgett and Trey Price, LSU AgCenter Plant Pathologists

Pod and stem blight

A fungal disease that occurs most frequently on the stems and pods. Infection can occur early in the season; however, signs of the disease are not evident until late season (R7). Pycnidia (fruiting bodies / black specks) occur in linear rows on the stems and pods. If favorable conditions persist, seed quality will be compromised.

Anthracnose

Caused by a fungus. Early infections can result in pre- and postemergence damping-off. Foliar symptoms include petiole cankers, leaf rolling, necrosis of the laminar veins, and premature defoliation. The fungus can produce acervulli (fruiting bodies / black specks) on the stems and pods. If the disease continues to develop on the pods, seed quality will be compromised.

Charcoal rot

A fungal disease. Infected seed may not germinate or seedlings may die soon after emergence. Symptoms from plants with latent infections or mid to late season infections die prematurely during hot, dry weather. Symptoms can be associated with dry spots (sandy areas) in the field. The roots and lower stems are deteriorated, and the epidermal and sub-epidermal tissue will be silvery in color and dotted with black pepper-like sclerotia (survival structures).

Taproot decline

A fungal disease relatively new to soybean. Foliar symptoms have been observed on plants from V6 to R6 and appear as mild interveinal chlorosis on young plants to severe interveinal chlorosis/necrosis on older plants. Roots are blackened and break easily. When stems are split longitudinally, the pith is usually colonized with white mycelium of the fungus.

Tillage practices can have a direct effect on disease establishment and development of some foliar and soil-borne diseases. These practices affect soil moisture, soil temperature, and plant residue. Soils are generally cooler and moisture is higher in reduced tillage systems. This slow seed germination and plant establishment. Debris left in the soil can also harbor plant pathogens and allow them to survive and built up from year to year.

Scouting and Control for Tarnished Plant Bugs and Redbanded Stinkbugs

James Villegas, LSU AgCenter Field Crops Entomologist

Adult tarnished plant bug (left) and redbanded stinkbug (right)

Plant Bugs in Cotton

Plant bugs are destructive insect pests of cotton. In Louisiana, the term "plant bugs" encompasses a group of closely related pests, including the tarnished plant bug, the cotton fleahopper, and the clouded plant bug. Both adult and immature plant bugs possess piercing, sucking mouthparts and feed on squares, flowers, and bolls. The feeding injury inflicted by plant bugs on small squares and bolls lead to reductions in size, quality, and yields and delays in maturity. To minimize economic losses resulting from plant bug infestations, it is crucial to scout and treat both at pre- and post-bloom stages of cotton.

At the pre-bloom stage, the objective is to maintain a first-position square retention rate of 70-85%. If squares are shedding before bloom, plant bugs are likely the culprit. Use a sweep net to monitor for plant bugs prior to bloom, and the threshold for treatment is eight or more plant bugs per 100 sweeps and square retention of 80% or less. Neonicotinoids generally provide better performance during pre-bloom. To manage resistance, it is advisable to avoid using pyrethroids, organophosphates, and carbamates during this stage.

Sampling methods and plant bug thresholds change once the cotton plants enter the bloom stage. The threshold for treatment at this stage is the detection of 2-3 plant bugs per drop cloth sample (equivalent to 0.6 per row foot). During the early bloom, both sweep nets and drop cloths can be utilized to monitor plant bug populations. There are several insecticides available that effectively control plant bugs during the post-bloom period, but selection should be based on individual field conditions. For instance, growth regulators such as Diamond only target immature plant bugs and should be tank-mixed with other insecticides if significant numbers of adults are present. Regular scouting and rotation of insecticides are key factors for long-term success in managing plant bugs.

Insecticide recommendations for plant bugs in cotton (cotton fleahoppers, tarnished plant bugs, and clouded plant bugs).

*Note – based on previous field trials, the number of insecticide applications for tarnished plant bugs is reduced with the recently released ThyrvOn Cotton.

Stink Bugs in Soybean

Redbanded stinkbugs (RBSBs) are arguably the most important insect pest of soybean in Louisiana. RBSBs are medium-sized stink bugs with a distinctive reddish band across their backs. When flipped, you will notice a characteristic spine protruding between the hind legs. Their piercing-sucking mouthparts allow them to feed on various plant parts, including stems, pods, and seeds. RBSBs are primarily found in the southern regions, where soybean production is prevalent, but their range has been expanding in recent years. Unlike other stinkbug species, RBSBs exhibit extended feeding behavior, necessitating continued control measures beyond the R6 growth stage. If left unchecked, RBSBs can cause significant reductions in soybean yields and quality, especially as they pierce deeper into pods, leading to weight reductions and potential elevator dockage.

To effectively manage RBSB, regular scouting using a sweep net should be conducted. Early detection allows for timely intervention and more effective control. The threshold for RBSB is 4 bugs (nymphs and adults) per 25 sweeps. Whereas for southern green, green, and brown stinkbugs, the threshold is 9 bugs (nymphs and adults) per 25 sweeps. Extend control measures for RBSB until maturity to protect soybeans from sustained feeding. Rotate chemistries to avoid resistance development.

Insecticide recommendations for redbanded stinkbugs in soybean

*Note – 0.5 lb of acephate applied alone does not provide satisfactory control.

Don’t Forget About Nematodes When Diagnosing Irregular Patches of Poor Soybean Growth

David Moseley and Tristan Watson, LSU AgCenter Scientists

If you find irregular patches of soybean plants that look generally stunted, yellow, or dead, the cause could be nematodes. Generally, nematodes jeopardize the root system of the plant. Therefore, the plants will often have symptoms that mimic other maladies such as drought and nutrient deficiency. It is not uncommon to find nematodes throughout Louisiana. A recent survey (2019-2021) detected Southern root-knot nematodes in 22% of 164 fields. Another damaging species, the reniform nematode, was found in over 58% of the fields. During the early months of the growing season it can be difficult to determine if the problem is from nematodes by examining the roots because few if any root symptoms are likely to have developed by this point. Therefore, a sample should be sent to the LSU AgCenter's Nematode Advisory Service to determine if nematodes are present in the soil. For more information on the Southern Root-knot Nematode, read “Southern Root-Knot Nematode on Soybean” from the Louisiana Crops Newsletter Volume 12, Issue 7 – August 2022. If nematodes are determined to be a problem, please evaluate the performance of varieties tested by the LSU AgCenter such as in the 2023 Soybean Variety Yields and Production Practices.

Figure 1. Irregular patches of poor growing soybean plants that were infected by Southern root-knot nematodes. This picture was taken in Morehouse parish in 2022.

Figure 2. Southern root-Knot infected soybean plants that have poor growth and development symptoms. This picture was taken in Morehouse parish in 2022.

LSU AgCenter Specialists