By Kiersten Wise, University of Kentucky; Tom Allen, Mississippi State University; Febina Mathew, South Dakota State University; and Daren Mueller, Iowa State University.
Interveinal chlorosis, the term used for yellowing and browning of tissue between leaf veins in the upper soybean canopy, is a common symptom in soybean production systems (Figure 1). More often than not, it is assumed that interveinal chlorosis indicates the presence of soybean sudden death syndrome (SDS).
However, it is important to remember that there are several other diseases and disorders that can cause interveinal chlorosis in soybean besides SDS. Some of these diseases are new to the US and Canada, or are expanding into new regions, which can complicate disease diagnosis. This article will discuss the most common diseases and disorders associated with interveinal chlorosis of soybean leaves.
Figure 1. Sudden death syndrome is often the cause of interveinal chlorosis of soybean leaves, but not always. Image courtesy Albert Tenuta.
Brown stem rot
Brown stem rot (BSR) is a disease caused by the soil-borne fungus, Cadophora gregata, which often produces interveinal chlorotic leaf symptoms typically found in the mid to upper canopy. These symptoms are usually noticed in the mid to late reproductive stages of soybean (R3 and later). Plants with foliar symptoms should be removed from the soil carefully and stems split to observe the internal stem tissue. Soybean stems affected by BSR will have brown pith tissue, especially near nodes (Figure 2). Roots will be healthy and not rotted or woody. It is important to note that there are two strains (genotypes) of the fungus that causes BSR. Only the ‘A’ genotype causes typical interveinal chlorosis, while the ‘B’ genotype does not. Both genotypes cause the pith to turn brown. In general, the geographic area can also help with identifying BSR. This disease is primarily observed in northern soybean growing regions, and is rarely, if ever, diagnosed in the Mid-South and southern states. Planting resistant soybean varieties (especially those with soybean cyst nematode resistance) and rotation to a non-host crop for two years or more are management options for BSR.
Figure 2. Foliar symptoms of brown stem rot can appear similar to other diseases, but splitting stems reveals brown pith tissue. Image courtesy Adam Sisson.
Stem canker is caused by the fungi Diaporthe caulivora and D. aspalathi. Foliar symptoms (Figure 3) on soybean plants can vary depending on the causal fungus. For example, D. aspalathi is known for causing interveinal chlorosis symptoms on infected plants. In the past, this fungus was referred to as the cause of “southern stem canker,” while other Diaporthe fungi were thought to cause “northern stem canker” that resulted in wilted leaves or plant death, but not interveinal chlorosis.
However, recent soybean checkoff-funded research indicated the two fungi reported to cause stem canker are more widely distributed throughout the US, and are not confined by geographic areas, as was previously thought. Therefore, plants exhibiting interveinal chlorosis in northern states may be affected by the pathogen causing stem canker and should be submitted to diagnostic laboratories for accurate diagnosis. The symptoms associated with stem canker are typically observed during the early reproductive stages of soybean, beginning shortly after flowering (R1). Regardless of the species of Diaporthe associated with the observed symptoms, stem canker results in reddish brown, sunken lesions on the outside of the stem (Figure 4). These lesions are often associated with nodes and spread up the stem. Roots of plants affected with stem canker should appear healthy. Research is ongoing to better understand stem canker management. At this time, variety resistance and crop rotation are the most viable management options. For more information see the CPN publication Stem Canker (CPN 1006): //cropprotectionnetwork.org/resources/publications/stem-canker
Figure 3. Foliar symptoms of stem canker on soybean. Image courtesy Tom Allen.
Figure 4. Sunken, reddish brown cankers are symptoms of stem canker on soybean. Image courtesy Adam Sisson.
Sudden death syndrome (SDS) is primarily caused by the soilborne fungus Fusarium virguliforme. This fungus produces a toxin that moves into the leaves, causing the characteristic interveinal chlorosis that is often the first indication of SDS (Figure 5). Symptoms are most commonly observed in the mid to late reproductive stages (R3 and later). Symptoms begin as yellow spots between leaf veins which expand and turn brown, leading to interveinal chlorosis. Symptoms will be most severe in the youngest leaves, and leaves will die and fall prematurely, leaving petioles attached to the stem. SDS can result in plants with rotted roots, and the cortex of the stem will be brown or gray, while the pith remains white. Integrated management of SDS is important. Variety resistance, fluopyram fungicide seed treatment, and soybean cyst nematode management can all help manage SDS. For more information see the CPN publication Sudden Death Syndrome (CPN 1011)://cropprotectionnetwork.org/resources/publications/sudden-death-syndrome
Figure 5. Characteristic foliar symptoms of sudden death syndrome on soybean. Image courtesy Daren Mueller.
Taproot decline was initially observed in 2007 and was confirmed in several southern states in 2017. The disease is caused by various species of the soilborne fungus Xylaria. It causes interveinal chlorosis of plants beginning in the mid-vegetative (V4-V6) to early flowering growth stages (R1-R2). Symptomatic leaves are often first observed in the lower canopy, and increase in severity as the season progresses (Figure 6). Affected taproots will have a dry, rotted appearance, and root systems often break off when plants are pulled from soil. Plants that are removed from the soil often have a dark taproot with brown internal discoloration, and roots are easily snapped off. In certain conditions, fungal growth on residue that remains on the soil surface or from buried soybean residue can be observed, known as “dead man’s fingers.” While the geographical distribution of taproot decline is not fully known, it has been reported across southern states (Alabama, Louisiana, Mississippi) and as far north as southern Missouri and Tennessee. Currently, management options are unknown, but research is underway to determine the impact and management for this disease.
Figure 6. Taproot decline foliar symptoms on soybean. Image courtesy Tom Allen.
Triazole fungicide injury
Application of fungicides in the demethylation-inhibiting class known as triazoles (Fungicide Resistance Action Committee Group 3) can cause a phytotoxic effect that results in interveinal chlorosis on soybean leaves (Figure 7). Symptoms only appear on leaves, typically in the upper canopy, and stems and roots remain healthy. Injury is worse when certain triazole fungicides are applied during hot and dry conditions. Triazole fungicide injury is more commonly observed in southern or Mid-South production regions due to the environmental conditions that favor injury; however, reports of phytotoxicity occurred as far north as Iowa during the 2018 season. Some specific fungicides are much more prone to producing phytotoxicity (e.g., products that contain prothioconazole or tebuconazole). In addition, the injury associated with this disorder is generally observed 21-28 days following the application of a fungicide that contains a triazole in either a pre-mix product or a tank-mix combination. Soybean varieties vary in their response to triazole fungicide injury, which can confuse diagnosis. Yield losses due to triazole fungicide injury have not been observed; however, research continues to determine the role of the injury as well as potential methods to reduce the injury observed.
Figure 7. Triazole fungicide injury foliar symptoms on soybean. Image courtesy Daren Mueller.
There are several other soybean diseases that may temporarily cause interveinal chlorosis depending on environment, variety, or geographic location. These diseases include southern blight, Phytophthora root and stem rot, red crown rot (Figure 8), and white mold (Figure 9). If these diseases are the cause of the interveinal chlorosis, leaves will eventually turn brown and the interveinal chlorosis symptoms can no longer be observed.
In all cases discussed here, a field diagnosis should not simply rely on foliar symptoms. Stem and root symptoms of plants exhibiting interveinal chlorosis can help distinguish among these diseases and disorders. If you are unsure about the cause of the interveinal chlorosis observed, send samples to a diagnostic laboratory to get an accurate diagnosis.
Figure 8. Red crown rot foliar symptoms on soybean. Image courtesy Tom Allen.
Figure 9. White mold foliar symptoms on soybean. Image courtesy Brandon Kleinke.