Stagonospora Nodorum Leaf and Glume Blotch
CPN-3003. September 22, 2020. DOI: doi.org/10.31274/cpn-20200922-2
Stagonospora nodorum leaf and glume blotch (SNB) is a disease that affects wheat leaves and glumes. The fungus Parastagonospora nodorum (formerly named Stagonospora nodorum, Septoria nodorum) causes the disease. The fungus can infect and damage wheat at the seedling stage through maturity, which can affect the quantity and quality of the grain.
In susceptible wheat varieties, yield losses up to 50 percent have been reported. SNB occurs throughout wheat production areas in the United States and Canada. Over the last decade, the incidence of SNB in the United States has increased, perhaps because of increased conservation tillage use in wheat production systems.
Symptoms and signs of Stagonospora nodorum leaf and glume blotch on foliar tissue.
Symptoms and Signs
The fungus that causes SNB can infect wheat throughout the growing season. Signs and symptoms of the disease are visible on both leaves and glumes.
On foliage, symptoms generally begin with small yellow flecks or spots. These spots then develop into lens-shaped lesions that are surrounded by yellow halos, which may become brown or grayish brown (Figure 1). The lesions may further coalesce to produce large areas of dead, brown tissue.
Figure 1. Stagonospora nodorum blotch symptoms on wheat foliage
During periods of high humidity, fungal fruiting bodies called pycnidia (small round structures) develop in the lesions. The pycnidia are generally brown to orange and are difficult to distinguish within the brown lesions.
Stem infections also occur and may be most pronounced near nodes. On wheat heads, the fungus causes glume blotch symptoms (Figure 2). Glume infections develop as purple-brown or grayish brown streaks and blotches. These symptoms often start at the tips of glumes, and then eventually spread throughout the whole spike simultaneously. Pycnidia will develop in symptomatic tissue.
The fungus can infect seeds, which reduces test weights. Seedlings that emerge from infected seed may die soon after emergence or develop dark coleoptile lesions.
Figure 2. During later disease stages, P. nodorum produces fruiting bodies (pycnidia) within lesions on wheat leaves.
Development and Disease Cycle
The main ways this fungus overwinters are in fungal growths (called mycelia) in wheat residue and fungal fruiting bodies. Seed can transmit the fungus (Figure 3). Other reservoirs of inoculum include volunteer wheat and wild grasses infected by P. nodorum.
Initial infections arise from spores that were produced from fungal fruiting bodies and then dispersed by wind or splashing rain. Infection requires at least 12 hours of free moisture and temperatures between 59-77°F (15-25°C). After infection, symptoms are not observed for at least 10-14 days. Symptom development is favored by temperatures between 68-81°F (20-27 °C). New pycnidia develop in the lesions 10-20 days after infection. Glume blotch occurs when wet, humid conditions persist.
Figure 3. The life cycle of Stagonospora leaf and glume blotch.Image: Iowa State University Integrated Pest Management
Yield Losses and Impact
When conditions favor disease development, SNB can cause up to 50 percent yield loss and increase grain dockage due to poor test weight and reduced quality. Yield loss is attributed to leaf and glume infection, which reduces the area the plant can use to perform photosynthesis and fill grain.
The timing of infection and disease development influences the risk of yield loss. Infections that occur before heading generally result in greater yield losses.
Diseases, Disorders, and Injury with Similar Symptoms
Septoria Tritici Blotch (Septoria tritici)
Initial SNB symptoms are chlorotic flecks on the lower leaves, which expand to irregular brown lesions; however, these lesions are typically vein-restricted. One can see fruiting bodies (pycnidia) on the lesions. The pycnidia are sphere-shaped, gelatinous, and grayish brown.
How to distinguish Septoria tritici blotch from Stagonospora nodorum blotch:
It is difficult to distinguish Septoria tritici blotch from SNB without microscopic observation. However, the lesions of Septoria tritici blotch can be restricted by leaf veins and appear side-by-side, while SNB lesions are oval to lens-shaped and surrounded by yellow halos. Also, it is easy to see S. tritici pycnidia with the naked eye as small black specks, On the other hand, P. nodorum pycnidia are brown, smaller, and less visible with the naked eye.
Figure 4. Septoria tritici blotch, also known as speckled leaf blotch, can be distinguished by the large black fungal structures present within lesions. Fungal structures produced by Stagonospora nodorum are much smaller not easily observed without the aid of a hand lens.
Tan Spot (Pyrenophora tritici-repentis)
Foliar symptoms of tan spot include small, oval to diamond-shaped spots that occur sporadically on wheat leaves. Later, these spots enlarge and turn tan with a well-defined yellow border and a small dark spot in the center.
How to distinguish tan spot from Stagonospora nodorum blotch:
Tan spot produces lens-shaped lesions with a well-defined yellow halo and a dark brown spot in the center. SNB lesions are surrounded by diffuse yellow boundaries and the centers of the lesions contain pycnidia.
Figure 5. Tan spot on a wheat leaf.
Fusarium Head Blight (Fusarium graminearum)
Symptoms and signs of Fusarium head blight (FHB) include bleached spikelets and pink to salmon-orange spore masses on wheat heads. FHB-affected plants often have small, shriveled kernels.
How to distinguish Fusarium head blight from Stagonospora glume blotch:
FHB causes bleaching of the wheat head whereas SNB produces purple to brown lesions on the glumes.
Figure 6. Bleached wheat heads symptomatic of Fusarium head blight.
Sooty Mold (Cladosporium, Alternaria, Stemphyllium, and Epicoccum)
Sooty mold symptoms include dark olive green to black discoloration of infected heads, which gives a weathered appearance.
How to distinguish sooty mold from Stagonospora nodorum blotch:
SNB causes purple blotch symptoms on glumes. By contrast, sooty mold has a characteristic speckled-black head symptom and can lead to black point discoloration of harvested kernels. Sooty mold spores may also easily transfer to hands and clothes, resulting in a black, charcoal-like residue.
Figure 7. Discoloration and weathered appearance of wheat heads with sooty mold.
Successfully managing SNB requires integrating several management practices, including:
- Plant certified disease-free seed to minimize the risk of seed-transmitted SNB.
- Select wheat varieties with SNB resistance. Resistance to SNB on the leaves and glumes may differ, so check with seed companies to determine how they established their resistance ratings.
- Apply a foliar fungicide when SNB risk is high. When selecting a fungicide to manage SNB, consider the crop growth stage. Many fungicides are labeled to control SNB; however, you should not apply fungicides that contain a quinone outside inhibiting (QoI/Strobilurin, FRAC group 11) active ingredient after head emergence. QoI fungicides may increase mycotoxins produced by the Fusarium head blight fungus, which is also favored by the warm, wet conditions that promote SNB.
- Populations of S. nodorum resistant to QoI fungicides have recently been reported in the United States. To minimize the development of fungicide resistance, farmers should use fungicide products that contain active ingredients from multiple fungicide classes.
More information about products that may have efficacy against SNB are available in Fungicide Efficacy for Control of Wheat Diseases (CPN-3002).
Nathan Kleczewski, University of Illinois; Navjot Kaur, Virginia Tech; Gary Bergstrom, Cornell University; Carl Bradley, University of Kentucky; Martin Chilvers, Michigan State University; Alyssa Collins, Pennsylvania State University; Christina Cowger, North Carolina State University; Erick DeWolf, Kansas State University; Andrew Friskop, North Dakota State University; Alyssa Koehler, University of Delaware; Hillary Mehl, Virginia Tech; Pierce Paul, Ohio State University; Jorge Salgado, Ohio State University; Adam Sisson, Iowa State University; Damon Smith, University of Wisconsin; Kiersten Wise, University of Kentucky; and Heather Young-Kelly, University of Tennessee
Kaitlyn Bissonnette, University of Missouri; Emmanuel Byamukama, South Dakota State University; Alfredo Martinez Espinoza, University of Georgia; Travis Faske, University of Arkansas; Bob Hunger, Oklahoma State University; Juliet Marshall, University of Idaho; Lucky Mehra, Kansas State University; Daren Mueller, Iowa State University; Nidhi Rawat, University of Maryland; and Albert Tenuta, Ontario Ministry of Agriculture, Food and Rural Affairs
All photos were provided by and are the property of the authors and reviewers.
This project was funded in part through Growing Forward 2 (GF2), a federal-provincial territorial initiative. The Agricultural Adaptation Council assists in the delivery of GF2 in Ontario. The authors thank the United States Department of Agriculture - National Institute of Food and Agriculture and the Grain Farmers of Ontario for their support.
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