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Frog Eye Leaf Spot infection usually occurs after flowering with initial symptoms of small yellow spots on the leaves when conditions are warm and humid. The lesions expand to about 1/4 of inch in diameter and the center becomes dark gray to brown and with the margins being purple. During a severe infection the lesions can coalesce and become larger. Occasionally, the disease can attack the stem and pods as well. The disease survives on soybean stubble and spores are dispersed short distances by air and rain. Management strategies include any tactic to reduce the levels of the inoculum, including tillage and increasing duration that soybeans are out of the rotation. Fungicide applications made at R3, beginning of pod fill, have shown to be the most effective.
Figure 1. Frog eye spot on soybean.
Sudden Death Syndrome (SDS)symptoms begin to appear at the beginning of the reproductive stages, but infection occurs much earlier in the vegetative stage. Infection occurs in the roots and a toxin produced by the pathogen causes the interveinal yellowing and death of tissue on the leaves. While the symptoms sometimes can be confused with southern stem canker, this disease does not produce cankers. The disease can be more severe when planting conditions are cold and wet and when the soil is compacted. Infestations of soybean cyst nematode may also increase the severity of the disease. Foliar applications of fungicide are not effective.
Figure 2. Sudden death syndrome on soybean.
Southern stem canker symptoms begin to appear during the reproductive stages, but infection occurs in the mid to late vegetative stages. Reddish brown cankers are formed, usually on the lower to mid stem area. Green tissue will be present above and below the canker at this early stage. As the disease progresses and the canker enlarges it may gridle the plant and this will contribute to plant death above the canker. Like SDS, the pathogen produces toxins that may also contribute to plant death. Moderate temperatures and wet conditions during the V3 stage is the critical period for infection. Fungicide applications must be applied during the vegetative stages, which is often not practical. Host resistance and reducing inoculum levels by increasing the length of time soybeans are out of the rotation and tillage are the best management tactics.
Figure 3. Soybean Southern stem canker.
Target leaf spot is a disease that attacks both cotton and soybean. In soybean, the symptoms appear on the lower leaves as brown lesions, occasionally with a yellow halo. In general, this disease does not impact yield. However, on cotton this disease can have more of an impact. Warm and wet weather promotes the disease, which forms brown to purple concentric ringed lesions. Lower leaves are the first to show symptoms and when severe can lead to early defoliation. Timing of applications of fungicides should target the 3rd week of bloom are recommended to reduce disease incidence and defoliation and promote higher yields. As there is no known resistance to this target leaf spot in cotton.
Figure 4. Target leaf spot on cotton. Dr. Heather Kelly, University of Tennessee.
Bacterial blight is a disease of cotton that can attack any part of the plant during any period of growth. On the leaf, angular lesions appear initially as water-soaked, then as a red to brown, on the stem or petioles it will cause black cankers (black leg), and if infection occurs in the bolls (boll rot) the lesions are round and initially appear water-soaked. Since it is a bacteria, infection must be through natural opening in the plant, (stomata) or wounds, therefore; infection can occur after heavy rain and or hail. Unfortunately, there are no control options once the plant becomes infected. Reducing the canopy with growth regulators (see next article) to reduce excessive growth. If the field becomes infested, harvest as soon as possible and destroy cotton residue as soon as possible. Planting a bacterial blight resistant variety the following year or rotate out of cotton. 180604090729
Figure 5. Bacterial blight in cotton.