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Archive for category: Septoria Brown Spot

Mid-Season Soybean Issues in Wisconsin

Damon L. Smith, Extension Field Crops Pathologist, University of Wisconsin

The 2015 season has again been an interesting adventure. We managed to plant soybeans early or on-time in much of the state and then the rains started and have kept things fairly wet. Combined with cooler temperatures, this moisture has brought about the risk for several disease on soybean in Wisconsin. Of course the most notable disease is WHITE MOLD, but I already have written about this disease and discussed management strategies. If you would like to revisit this topic, CLICK HERE. You can also watch a short video on the subject HERE. Other diseases to watch out for during the mid-season include Septoria brown spot, brown stem rot (BSR), sudden death syndrome (SDS), stem canker and pod and stem blight, and soybean vein necrosis disease (SVND). I will consider each one of these diseases below.

Leaf spots caused by the fungus Septoria glycines. Photo credit: Brian Hudelson, UW Plant Disease Clinic

Septoria brown spot

Septoria brown spot is a common disease occurring on soybean each year. The spores of this fungus are typically rain splashed from old soybean debris, to the growing plants. Septoria brown spot is usually not considered a yield limiting disease, but in certain cases, it has been attributed to significant yield loss. This is usually the case where a susceptible variety is grown in a location conducive to the disease and rain is frequent and heavy. In a situation like this, fungicides might be required during the reproductive phase of growth to preserve yield. However, most of the time, Septoria brown spot is observed early in the season and again late in the season during periods of heavy rainfall and does not affect yield.

Symptoms include dark brown spots on both upper and lower leaf surfaces. Adjacent lesions frequently merge to form irregularly shaped blotches. Leaves become rusty brown. Symptoms of Septoria leaf spot can also develop on stems and pods of plants approaching maturity. Stem and pod lesions have indefinite margins, are dark in appearance and range in size from flecks to lesions several inches in length, but they are not distinct enough to be diagnostic. Seed are infected but symptoms are not conspicuous.

The onset of Septoria leaf spot symptoms is influenced by the relative maturity of the soybean variety, and symptoms appear earlier in the season on an early-maturing variety. Complete resistance has not been identified in soybean varieties or lines, but varieties do differ in partial or rate-reducing resistance which can be used effectively. Crop rotation is an effective preventative strategy. Septoria leaf spot is more severe in continuously cropped soybean fields. The host range includes most species of Glycine, other legume species, and common weeds such as velvetleaf. For fields with very high levels of Septoria leaf spot, plow under soybean straw to promote rapid decay. Application of fungicides to soybean foliage from bloom to pod fill has effectively reduced the severity of Septoria leaf spot in research trials over the last several seasons. However, yield has not been effected by these applications. Therefore, the odds of a positive return on investment when using a foliar fungicide on soybean to control Septoria brown spot are low. You can check out the results of foliar fungicide trials on soybean in Wisconsin by CLICKING HERE. You can also view the soybean fungicide efficacy table HERE.

Browning of the internal stem (left) is diagnostic for BSR. The middle stem may be developing symptoms. Compare to healthy, white pith in the stem on the right.

Brown Stem Rot (BSR)

Symptoms of BSR are usually not evident until late in the growing season and may be confused with signs of crop maturity or the effect of dry soils. The most characteristic symptom of BSR is the brown discoloration of the pith especially at and between nodes near the soil line. This symptom is best scouted for at full pod stage. Foliar symptoms, although not always present, typically occur after air temperatures have been at to below normal during growth stages R3-R4, and often first appear at stage R5, peaking at stage R7. Foliar symptoms include interveinal chlorosis and necrosis (i.e., yellowing and browning of tissue between leaf veins), followed by leaf wilting and curling. Yield loss as a result of BSR is generally greatest when foliar symptoms develop. The severity of BSR symptoms increases when soil moisture is near field capacity (i.e., when conditions are optimal for crop development).

Foliar symptoms of BSR can be confused with those of sudden death syndrome (see description below). However, in the case of sudden death syndrome (SDS), the pith of affected soybean plants will remain white or cream-colored. In addition, roots and lower stems of plants suffering from SDS (but not those suffering from BSR) often have light blue patches indicative of spore masses of the fungus that causes SDS.

BSR is caused by the soilborne fungus Phialophora gregata. There are two distinct types (or genotypes) of the fungus, denoted Type A and Type B. Type A is the more aggressive strain and causes more internal damage and plant defoliation than Type B. P. gregata Type A also is associated with higher yield loss. P. gregata survives in soybean residue, with survival time directly related to the length of time that it takes for soybean residue to decay. Thus, P. gregata survives longer when soybean residue is left on the soil surface (e.g., in no till settings) where the rate of residue decay is slow. P. gregata infects soybean roots early in the growing season. It then moves up into the stems, invading the vascular system (i.e., the water-conducting tissue) and interfering with the movement of water and nutrients.

Several factors can influence BSR severity. Research from the University of Wisconsin has shown that the incidence and severity of BSR is greatest in soils with low levels of phosphorus and potassium, and a soil pH below 6.3. In addition, P. gregata and soybean cyst nematode (Heterodera glycines) frequently occur in fields together, and there is evidence that BSR is more severe in the presence of this nematode.

Archive for category: Septoria Brown Spot Mid-Season Soybean Issues in Wisconsin Damon L. Smith, Extension Field Crops Pathologist, University of Wisconsin The 2015 season has again been

Septoria brown spot

Septoria brown spot (also called brown spot) is common leaf disease of soybean across the Midwestern U.S. It’s incidence can be high but it rarely develops to cause significant yield loss. Yield losses of five to eight percent may occur under severe conditions when much defoliation occurs. Where soybean rust may occur, it can create diagnostic problems because these two diseases can cause similar small, dark spots on leaves.

Symptoms

The first symptoms usually appear on the lower leaves and then progresses to the mid-to-upper canopy throughout the summer. Initial symptoms are small dark brown spots (less than 1/8 inch in size). It can develop on the first true leaves early in the season. The brown spots often enlarge and grow together into irregular brown areas, which often are associated with yellow patches concentrated more on one side of the leaf than another. The brown areas can contain tiny raised specks called pycnidia (visible with a hand lens) where spores are produced. Infected leaves may fall off prematurely.

Conditions and timing that favor disease

Like most foliar diseases, Septoria brown spot is most common when conditions and leaves are wet and warm, ideally for extended periods of time. Because the pathogen survives and sporuates on soybean residue, minimum tillage and continuous soybeans may enhance this disease.

Causal pathogen

Septoria glycines is a fungus that survives on crop residue and may be seed transmitted.

Disease management

Septoria brown spot typically does not require management because it rarely causes significant losses. Soybean varieties are not available with resistance to this disease, but varieties can vary in their susceptibility. Rotation with non-legume crops and tillage may be beneficial, and foliar fungicides can provide some control under those rare conditions when an application may be warranted.

How septoria brown spot impact Minnesota crops. Learn symptoms, conditions that favor the disease, and disease management.