Bacterial leaf streak of wheat

Bacterial leaf streak of wheat DEFAULT

Bacterial Streak and Black Chaff of Wheat

Bacterial streak, caused by Xanthomonas campestris pv. undulosa, appears first as small water-soaked lesions that rapidly develop into long, thin, tan-brown, water-soaked lesions, confined between the leaf veins. Lesions may appear translucent when backlit, especially when leaves are wet. Unless leaves remain wet or there is high humidity, the water-soaked appearance will fade and symptomatic tissue will become necrotic. On susceptible hosts, lesions may coalesce. The bacterium exudes milky droplets of bacterial ooze within lesions when free-moisture is present on leaves. The ooze will give the leaves a shiny or glazed appearance when dry and small, amber-colored granules of dried exudates may be evident in lesions. Symptoms are often most evident on the middle third of the leaf blade.

Black chaff symptoms are present on the peduncle, rachis, and glumes. Infected peduncle and rachis tissues are dark brown, dark purple, or black. Glume lesions are similarly dark, though streaking may be evident. In awned varieties, alternating bands of green and pigmented awn tissue may be evident. In severe cases, grain fill is reduced, and grain may not be marketable, in part because infected seed can spread the pathogen. Bacterial leaf streak leaf lesions. Image: E. Byamukama

Black chaff symptoms on wheat. Image: M. Burrows

Symptoms of bacterial streak and black chaff are most apparent after heading. Humid, warm weather favors infection, as does overhead irrigation. Anything that injures wheat leaves, such as hail or high winds may increase the risk of bacterial streak. Black chaff risk is higher if bacterial streak lesions are evident on the flag leaves.

Less susceptible varieties are available. Plant clean seed to prevent introducing inoculum into a field. Avoid using overhead irrigation which will contribute to pathogen spread.

Gallery Images: E. Byamukama, M. Burrows, and J. Marshall

    Unless leaves remain wet or there is high humidity, the water-soaked appearance will fade and sympto

    Unless leaves remain wet or there is high humidity, the water-soaked appearance will fade and sympto

    Field symptoms of black chaff.

    Field symptoms of black chaff.

    Black chaff symptoms on wheat head.

    Black chaff symptoms on wheat head.

    On susceptible hosts, bacterial streak lesions may coalesce.

    On susceptible hosts, bacterial streak lesions may coalesce.

    Leaf symptoms of bacterial streak on wheat.

    Leaf symptoms of bacterial streak on wheat.

    Black chaff symptoms are present on the peduncle, rachis, and glumes.

    Black chaff symptoms are present on the peduncle, rachis, and glumes.


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    Causal Organism, Occurrence and Spread

    Bacterial leaf streak (BLS) of wheat is caused by the bacterium Xanthomonas translucens pv. undulosa. A similar bacterium, Xanthomonas translucens pv. translucens, causes disease on barley.

    Bacterial leaf streak is observed frequently in wheat across North Dakota, Minnesota and South Dakota. Yield losses due to BLS and black chaff are variable, ranging from negligible to greater than 50%, depending on the stage of infection and severity.

    The causal bacterium is primarily seed-borne and also can survive in crop debris and grassy weeds. The bacterium is spread by splashing or wind-driven rain, and enters the plant through wounds or natural leaf pores called stomata.

    Symptoms of this disease are most noticeable in areas that have had frequent storms associated with high winds, especially during and after the flag leaf growth stage. Overhead irrigation may increase BLS risk due to splashing and prolonging leaf wetness periods.

    Symptoms and Comparison to Other Common Wheat Diseases in North Dakota

    Leaf symptoms: Early leaf infections are characterized by irregular translucent water-soaked streaks and readily observed on the flag leaf (Figure 1A and 1B). After prolonged periods of leaf wetness, bacterial exudate (ooze) can form within lesions.

    Early symptoms of bacterial leaf streak
    Figure 1. Early symptoms of bacterial leaf streak onwheat (1A) and barley (1B). Note water-soaked streakswith bacterial ooze. (Andrew Friskop, NDSU)

    Streaks eventually will turn yellow (chlorotic) and then brown (necrotic), reducing the photosynthetic potential of the flag leaf (Figure 2). Streaks also may appear shiny as clumps of bacteria dry on the leaf surface (Figure 3).

    Chlorotic and necrotic lesions on the flag leaf
    Figure 2. High levels of bacterial leaf streak on asusceptible variety causing chlorotic and necroticlesions on the flag leaf. (Andrew Friskop, NDSU)
    Bacterial ooze can give bacterial leaf streak lesions a shiny appearance.
    Figure 3. Bacterial ooze that hardens can givebacterial leaf streak lesions a shiny appearance.(Andrew Friskop, NDSU)

    Leaf symptoms of BLS can be confused with the fungal leaf spots tan spot and Septoria blotch. A couple of tips to differentiate between BLS and fungal leaf spots include the onset of disease, appearance of lesions, color of lesions and presence of fungal structures.

    Tan spot lesions have a distinct ellipsoid shape with a tan or dark brown center and a yellow halo (Figure 4A). Tan spot also tends to appear earlier in the season and move up the crop canopy gradually.

    Septoria causes lens-shaped or irregular lesions that often have a grayish, non-water-soaked appearance and sometimes harbor “pepper-grain” sized spore-bearing structures of the fungus (Figure 4B).

    Comparison of wheat foliar disease symptoms
    Figure 4. Comparison of wheat foliar disease symptoms:(A) tan spot, (B) Septoria/Stagonospora nodorum blotch and(C) bacterial leaf streak. (Andrew Friskop and Andrew Green, NDSU)

    Symptoms of BLS tend to become most apparent after thunderstorms and high-wind rain events (Figure 4C). However, these leaf diseases are difficult to differentiate at the later growth season when lesions coalesce and often they occur together as a leaf disease complex.

    Glume symptoms: For BLS, the bacterium may infect the glumes during grain fill and cause dark purple to black streaks, a symptom called black chaff (Figure 5). A purple to yellow lesion on the peduncle (stem tissue below head) often is associated with black chaff. Severe black chaff may result in discolored kernels.

    Black chaff symptoms can look similar to glume blotch caused by fungi in the Septoria species complex. However, Septoria glume blotch symptoms are characterized by brownish-gray lesions on the glumes and awns (Figure 4B).

    Bacterial exudate (ooze) on glumes
    Figure 5.Bacterialexudate (ooze)on glumes of anewly infectedwheat spike (left).After infection,dark purple toblack streaks willdevelop on theglumes andawns (right).(Andrew Friskop, NDSU)

    Management of BLS and Black Chaff

    Clean seed: The BLS pathogen is seed-borne (low seed transmission rate), so the use of clean seed can prevent infections and spread. However, because of the prevalence of this disease in North Dakota (residue and grasses), prevention of BLS through clean seed may not be practical.

    Research is being conducted at NDSU to develop means for bacterial detection in seeds and other samples. No chemicals are recommended for use as a seed treatment.

    Variety resistance: None of the available wheat varieties in North Dakota are immune to BLS. However, a large range exists in susceptibility among commonly grown wheat varieties.

    Variety reactions to BLS for hard red spring wheat, hard red winter wheat and durum are presented in the annual variety selection guides prepared by NDSU Extension. Variety resistance is the best management tool to help reduce losses caused by BLS.

    Foliar products: Foliar fungicides and antibiotic compounds have been tested in field environments for the reduction of BLS on spring wheat. Although some products significantly reduced disease severities in a laboratory and greenhouse setting, field trial results have been inconsistent, and more field data is needed before recommendations can be made for any foliar product.

    This publication was written by Marcia McMullen, former Extension plant pathologist, and Tika Adhikari, former plant pathologist, NDSU Department of Plant Pathology.

    NDSU Extension does not endorse commercial products or companies even though reference may be made to tradenames, trademarks or service names. NDSU encourages you to use and share this content, but please do so under the conditions of our Creative Commons license. You may copy, distribute, transmit and adapt this work as long as you give full attribution, don’t use the work for commercial purposes and share your resulting work similarly. For more information, visit

    For more information on this and other topics, see

    County commissions, North Dakota State University and U.S. Department of Agriculture cooperating. NDSU does not discriminate in its programs and activities on the basis of age, color, gender expression/identity, genetic information, marital status, national origin, participation in lawful off-campus activity, physical or mental disability, pregnancy, public assistance status, race, religion, sex, sexual orientation, spousal relationship to current employee, or veteran status, as applicable. Direct inquiries to Vice Provost for Title IX/ADA Coordinator, Old Main , NDSU Main Campus, , [email protected] This publication will be made available in alternative formats for people with disabilities upon request,

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    Bacterial Leaf Streak Developing at Low Level in Winter Wheat

    Figure 1. Bacterial leaf streak symptoms on winter wheat leaves.

    Written collaboratively by Emmanuel Byamukama, Shaukat Ali and Sunish Kumar.

    A few winter wheat fields were found with bacterial leaf streak (BLS) starting to develop at low levels (Figure 1). BLS is usually observed starting from flag leaf growth stage and can be severe on the upper leaves, when there is prolonged leaf wetness resulting from frequent rains. Symptoms start as small brown to dark brown, water-soaked lesions limited within the veins. These can coalesce to form large necrotic areas on a wheat leaf (Figure 1).

    Causal Agent and Symptoms

    Bacterial leaf streak is caused by a bacterium called Xanthomonas translucens pv undulosa. The bacteria survive primarily on and in the seed but also on crop residue and in the soil. The bacteria can survive on the plant surface without causing disease until weather conditions are conducive for infection to take place. The bacteria are spread from one plant to another mainly by rain splash. The bacteria enter the plant through wounds (caused by high winds or insect feeding) and natural openings.


    Because the bacteria are primarily disseminated through seed, planting certified, pathogen-free seed is the most effective management strategy for bacterial leaf streak. Unfortunately, there are no seed treatment products available for managing this disease. Also, crop rotation may help to reduce the inoculum level or plant BLS tolerant varieties. SDSU Wheat Breeding Programs have varieties that are moderately resistant to BLS. For BLS rating of wheat cultivars, see the latest Winter Wheat Variety Trial Results.

    Bacterial Leaf Streak in Corn #979 (Air Date 1-8-17)

    Bacterial leaf streak

    Bacterial leaf streak (BLS), also known as black chaff, is a common bacterial disease of wheat. The disease is caused by the bacterial species Xanthomonas translucens pv. undulosa. The pathogen is found globally, but is a primary problem in the US in the lower mid-south and can reduce yields by up to 40 percent.[6] BLS is primarily seed-borne (the disease is transmitted by seed) and survives in and on the seed, but may also survive in crop residue in the soil in the off-season. During the growing season, the bacteria may transfer from plant to plant by contact, but it is primarily spread by rain, wind and insect contact. The bacteria thrives in moist environments, and produces a cream to yellow bacterial ooze, which, when dry, appears light colored and scale-like, resulting in a streak on the leaves. The invasion of the head of wheat causes bands of necrotic tissue on the awns, which is called Black Chaff.[14] The disease is not easily managed, as there are no pesticides on the market for treatment of the infection. There are some resistant cultivars available, but no seed treatment exists. Some integrated pest management (IPM) techniques may be used to assist with preventing infection although, none will completely prevent the disease.[2]

    Host and symptoms[edit]

    Bacterial leaf streak is a pathogen known to infect and damage wheat varieties. The pathogen has also been known to infect other small grain all cereal crops such as rice, barley and triticale. The strains of the pathogen are named differently according to the species they infect. It is one of the most destructive diseases in rice.[15] Resistant wheat cultivars offer the best protection against yield loss, but little is known about the inheritance of resistance.[12] The disease is most common on wheat and can be found on winter and summer wheat varieties.

    Leaf streak and lesion symptoms of Bacterial Leaf Streak

    Symptoms of this pathogen can be seen on the stem, leaves, and glumes. Stem symptoms are not always present, but can be seen as a dark brown to purple discoloration on the stem below the head and above the flag leaf. [14] In the early stages of the disease, translucent water soaked streaks can be seen on the leaves often accompanied by a shiny glaze or clumps of dried bacteria on the leaf surface. [8] These markings turn to brown lesions after just a few days, and may be surrounded by a lime green halo. Lesions can stretch the entire leaf blade. BLS exhibits similar symptoms to those of Septoria nodorum, a common fungal infection. A common sign that will distinguish this pathogen from Septoria nodorum is the lack of spores on the leaves, which appears as tiny black spots on the leaf surface with a Septoria infection. A cream to yellow colored bacterial ooze produced by BLS infected plant parts is also a distinguishing sign of the pathogen.[7] Infected glumes, known as black chaff, are darkened and necrotic. Severe symptoms will result in kernels that are discolored due to black and purple streaks. [9] Plants infected with bacterial leaf streak will exhibit an orange cast from leaf symptoms and suffer yield and quality loss.[14]

    Black Chaff symptoms shown on wheat plant

    Disease cycle[edit]

    Bacterial leaf streak of wheat is a seed-borne disease and is primarily transported by seed, but can also be transmitted by plant-to-plant contact within its lifecycle. Epidemics are typically observed late in the growing season and in wet environments. Moisture facilitates the release of the pathogen from the inoculated seed, which leads to leaf colonization and invasion of tissue. Bacteria on the moist leaf surface enter through leaf openings, such as the stomata or wounds. Under moderately warm weather conditions ( degrees Celsius) bacteria will begin to release and multiply in the plant parenchyma tissue.[4] The bacteria then inject a number of effector proteins into the plant with a Type 3 Secretion System (T3SS)[5]. The T3SS effectors in Xanthomonas are Transcription Activator-like effectors (TAL effectors). These TAL effectors activate the production of plant genes that are beneficial to bacterial infection.[1] The bacteria spreads and progresses vertically up the plant.[3] The bacterial masses cause elongated streaks along the veins of the plant. Water collection on leaves also leads to the spread of the organism, increasing the number of lesions on leaves. Rain, wind, insects, and plant-to-plant contact may spread the disease in the season and cause reinfection.[4] After the plant is harvested or dies, the bacteria may overwinter in the soil; although, its survival rate is much greater when crop debris is present. However, the bacteria cannot survive on decomposing matter alone and free bacteria cannot survive for more than 14 days in hot dry weather.[7] Bacteria is primarily spread by infected or contaminated seed.[10] In some cases, the bacterial will overwinter on other perennial plants and weeds.[4] Residual bacteria in the soil, debris or other plants may also cause new infection in clean seed.


    The disease can tolerate warm or freezing temperature,[2] but favorable conditions for the disease include wet and humid weather. Irrigated fields provide a favorable environment for the disease.[14] The disease has become quite prevalent in semi-tropical regions, but can found all over the world where wheat is grown.[13] Strong winds that blow soils help contribute to the spread of disease. When the spread is initiated by wind blown soil particles, symptoms will be found most readily towards the edges of the field.[6]

    Management of BLS[edit]

    Bacterial leaf streak of wheat is not easily prevented, but can be controlled with clean seed and resistance. Some foliar products, such as pesticides and antibiotic compounds, have been tested for effectiveness, but have proven to have insignificant outcomes on the bacterial pathogen. Using clean seed, with little infection, has yielded effective results for researchers and producers. The pathogen, being seed-borne, can be controlled with the elimination of contaminated seed, however, clean seed is not always a sure solution. Because the pathogen may still live in the soil, the use of clean seed is only effective if both the soil and seed are free of the pathogen. Currently, there are no successful seed treatments available for producers to apply to wheat seed for the pathogen. Variety resistance is another option for control of the disease. Using cultivars such as Blade, Cromwell, Faller, Howard or Knudson, which are resistant to BLS may reduce the impact of the disease and potentially break the disease cycle. Avoiding susceptible cultivars such as Hat Trick, Kelby, and Samson may also reduce the presence of the disease and reduce the amount of bacterial residue in the soil.[6] Using integrated pest management techniques such as tillage to turn over the soil and bury the infection as well as rotating crops may assist with disease management, but are not a definitive control methods.[9] Depending on conditions, the bacteria may survive for up to 81 months.[4] Because the bacteria is moisture driven, irrigation may also increase the risks of BLS infection.[8]


    Bacterial leaf streak is a major bacterial disease of wheat. Yield losses are normally less than 10%, but can be up to 40%. The loss in yield is due to lower weight of the seed rather than lack of production. Risk of damages due to bacterial leaf streak is variable, but should not be overlooked because there is always a chance of an epidemic. Research is still being done on the pathogen, specifically on finding resistant strains. Farmers should avoid planting susceptible cultivars, and avoid infected seed in order to ensure there will be no infection of the pathogen.[4] This disease can survive in freezing to warm climates, threatening wheat populations throughout the world.


    • Boch, Jens; Bonas, Ulla (). "Xanthomonas AvrBs3 Family-Type III Effectors: Discovery and Function". Annual Review of Phytopathology. 48: – doi/annurev-phyto PMID&#;
    • Boehm D. "Understanding Bacterial Diseases in Wheat." AgriPro: Syngenta. <[permanent dead link]>
    • Dean, Paul. "Functional Domains and Motifs of Bacterial Type III Effector Proteins and Their Roles in Infection." FEMS Microbiology Reviews (): Federation of European Microbiological Societies. Web. 4 Dec. <>.
    • Duveiller E., Bragard C., and Maraite H. "Bacterial Leaf Streak and Black Chaff." FAO Corporate Document Repository. <>
    • Gardiner, Donald M.; Upadhyaya, Narayana M.; Stiller, Jiri; Ellis, Jeff G.; Dodds, Peter N.; Kazan, Kemal; Manners, John M. (). "Genomic Analysis of Xanthomonas Translucens Pathogenic on Wheat and Barley Reveals Cross-Kingdom Gene Transfer Events and Diverse Protein Delivery Systems." Ed. Turgay Unver". PLOS ONE. 9 (1): E doi/journal.pone PMC&#; PMID&#;
    • Hershman D.E. and Bachi P.R. "Wheat Bacterial Streak/Black Chaff." () University of Kentucky Extension Service. <>
    • Lindbeck, Kurt. INDUSTRY BIOSECURITY PLAN FOR THE GRAINS INDUSTRY. N.p.: Plant Health Australia, n.d. Plant Health Australia. June Web. 4 Dec.
    • McMullen M. and Adhikari T. "Bacterial Leaf Streak and Black Chaff of Wheat." () NDSU Extension Service Plant Disease Management. <>
    • McMullen, M.P., and Lamey, H.A. Symptoms and controls of crop diseases. NDSU Extension Bulletin PP
    • Mehta, Yeshwant Ramchandra. "Spike Diseases Caused by Bacteria." Wheat Diseases and Their Management. London: Springer International, Print.
    • Smith, M. "Small Grains Disease Update." (). University of Minnesota. <>
    • Tillman B.L. and Harrison S.A. "Heritability of Resistance to Bacterial Streak in Winter Wheat." () Crop Science: Vol. 36 No. 2, p.&#; <>
    • Tillman B.L., Harrison S.A., Russin J.S. and C. A. Clark. "Relationship between Bacterial Leaf Streak and Black Chaff Symptoms in Winter Wheat."() Crop Science: Vol. 36 No. 1, p.&#; <>
    • Wegulo S.N. "Black Chaff." University of Nebraska, Plant Disease Central (). < at the Wayback Machine>
    • Xie X, Chen Z, Cao J, Guan H, Lin D, et al. () Toward the Positional Cloning of qBlsr5a, a QTL Underlying Resistance to Bacterial Leaf Streak, Using Overlapping Sub-CSSLs in Rice" PLoS ONE 9(4): e <>

    Wheat of leaf bacterial streak

    Bacterial leaf streak (BLS), also known as black chaff, is a disease of wheat, barley, triticale, oats and many other cool- and warm-season grasses (Figure 1). BLS is more prevalent with warm, humid weather.

    Impact on farmers

    Severe outbreaks are commonly associated with weather incidents that damage leaf tissue. The disease can be found across the globe and, although not new to the region, has become more prominent in Minnesota, North Dakota and South Dakota since

    The disease’s impact on grain yield and quality isn’t well-documented. However, yield losses up to 40 percent have been reported.

    Unlike many fungal pathogens, BLS is a sporadic disease, occurring in some fields but not others. Even within a field, symptoms can be very patchy; some areas can appear severely affected, while immediately adjacent areas are completely devoid of symptoms.

    Stages of bacterial leaf streak infection

    About bacterial leaf streak (BLS)


    BLS is caused by the bacterium Xanthomonas translucens. Two common types of this pathogen that can infect wheat and barley are currently recognized. These differ in pathogenicity, and are therefore referred to as pathovars:

    • Xanthomonas translucens pv. undulosa is pathogenic on wheat, barley, triticale and other grasses.

    • Xanthomonas translucens pv. translucens is specifically pathogenic on barley.

    Research efforts are in progress to more clearly delineate the pathovars associated with recent disease outbreaks in Minnesota. BLS infection initially becomes present as water-soaked lesions on leaves, with advanced infections developing black chaff symptoms on glumes (Figure 2).

    Distinguishing from bacterial leaf blight (BLB)

    BLS infection caused by Xanthomonas translucens shouldn’t be confused with bacterial leaf blight (BLB) caused by Pseudomonas syringae. BLB isn’t commonly found in Minnesota, and causes spotty and blotchy leaf lesions rather than the streaked lesions of BLS.

    Black chaff symptoms

    BLS symptoms generally appear during the grain-fill period.


    Earlier in the day, look for dark brown, water-soaked streaks on the leaves (Figure 1, part B). You may find milky droplets of bacterial exudate on the lesions (Figure 1, part A). This ooze will dry up and form a scale-like glaze (Figure 1, part B).

    If you hold your fingers against the leaf’s underside or hold the leaf up to the light, the lesions may appear transparent, especially when the leaf surface is wet. Symptoms of black chaff include brown-black discolorations of glumes (Figure 2).

    Glumes may also be discolored at the base and, at times, you can see banding. You may also find bacterial exudate on the glumes.

    Distinguishing from pseudo black chaff

    This exudate is one way to distinguish BLS from a brown, black or purple discoloration of the glumes (melanosis) that’s associated with a genetic syndrome called pseudo black chaff. Pseudo black chaff can occur in varieties that carry the Sr2 stem rust resistance gene. The discoloration is expressed under warm and humid conditions.

    Xanthomonas translucens survives on and in seed, crop residue and perennial weeds (Figure 3). It has a limited ability to survive in soils without crop residue.

    How it spreads

    Infected seed is thought to be a primary method of distance dispersal. However, seed transmission studies haven’t been reproducible and there are no reliable seed detection assays.

    BLS invades the plant either through leaf pores, known as stomata, or wounds caused by wind, hail, frost or farm machinery. The bacteria can spread by rain splash, irrigation, plant-to-plant contact and feeding insects, such as aphids.

    Favorable conditions

    Xanthomonas translucens has a relatively wide temperature range, but grows optimally at temperatures above 78 degrees Fahrenheit. It doesn’t appear to need free moisture for infection and disease development.

    Although initial infection is probably favored by humidity, once the bacterium is inside the plant, humidity is no longer needed. Temperature becomes the important factor in the rate of bacterial growth.

    Disease cycle of BLS

    BLS is more likely to occur in years when conditions are warm and humid, especially if the varieties planted are more susceptible and there are bacterial cells either in the environment or seed. Because of the incidental nature of BLS, it’s been hard to amass a consistent body of data regarding yield and quality losses.

    Researchers found that, on average, yield losses were below 5 percent when the flag leaf’s infected area was less than 10 percent. When 50 percent of the flag leaf was infected, they observed losses of up to 20 percent. Preliminary research at the University of Minnesota indicates yield losses exceeding 30 percent are possible.

    Management options

    Fungicides and seed sanitation

    Fungicides don’t control BLS. There are few bactericides labeled for use in wheat and barley (copper-containing compounds such as Kocide, Cuprofix Ultra and Champ). However, early trials show that while these can provide some control of BLS, results are often inconsistent.

    Seed sanitation has also shown to be ineffective in Minnesota.

    Genetic resistance

    Consequently, as with many plant diseases, genetic resistance offers the most economic and effective control at this time. There are significant differences in how wheat cultivars react to BLS. Barley screening results are also available.

    Variety trial results: Relative disease resistance of hard red spring wheat cultivars

    Variety trial results: Relative disease resistance of barley varieties

    Duveiller., E., Bragard, C., & Maraite, H. (). Bacterial leaf streak and black chaff caused by Xanthomonas translucens. In E. Duveiller, L. Fucikovsky & K. Rudolph (Eds.) The bacterial diseases of wheat: Concepts and methods of disease management (pp. ). Mexico City, Mexico: CIMMYT.

    Duveiller, E., & Maraite, H. (). Study on yield loss due to Xanthomonas campestris pv. undulosa in wheat under high rainfall temperate conditions. Zeitschrift für Pflanzenkrankheiten und Pflanzenschutz, ,

    Madeleine Smith, Extension plant pathologist; Ruth Dill-Macky, small grains pathologist, College of Food, Agricultural and Natural Resource Sciences (CFANS); Rebecca Curland, researcher, CFANS; Carol Ishimaru, plant pathologist, CFANS and Jochum Wiersma, Extension agronomist

    Bacterial Leaf Streak

    Wheat Disease Update: Bacterial Streak and Black Chaff

    Black chaff on wheat heads
    Bacterial leaf streak

    In , drier than normal weather early in the growing season slowed or prevented the development of fungal diseases of wheat in April and May. Bacterial streak, also known as black chaff when it appears on wheat heads, was the predominant disease and was first observed in early June.

    Other diseases observed were leaf rust, which appeared in early June, and wheat streak mosaic, which was observed in May in isolated fields in the southern Panhandle.

    Because of the widespread occurrence of bacterial streak and black chaff, this wheat disease update is devoted to this disease. The following information is modified from NebGuide G, Black Chaff of Wheat. Photos were taken in June

    Bacterial streak and black chaff is a bacterial disease of wheat common in irrigated fields or in areas with abundant rainfall during the growing season. It is also known as bacterial stripe or bacterial leaf streak. The disease also occurs on barley, oats, rye, triticale, and many grasses. It is the most important and most widely distributed bacterial disease of small grains and can cause yield losses of up to 40%.

    Cause and Symptoms

    Black chaff is caused by the bacterium Xanthomonas campestris pathovar (pv.) undulosa. The disease derives its name from the darkened glumes of infected plants (Figure 1). This symptom is similar to that caused by genetic melanism (darkening of tissue) and glume blotch, a fungal disease incited by Parastagonospora nodorum. Black chaff can be distinguished from other diseases by the appearance of cream to yellow bacterial ooze in the form of slime or viscous droplets produced on infected plant parts during wet or humid weather. This ooze appears light colored and scale-like when dry. Bands of necrotic and healthy tissue on awns ("barber’s pole") (Figure 1) are indicative of black chaff.

    A dark brown to purple discoloration may appear on the stem below the head and above the flag leaf. On leaves, symptoms start as small water-soaked spots or streaks that turn brown after a few days. Lesions are irregularly shaped and elongate and may extend the length of the leaf blade (Figure 2). In wheat, a diffuse lime-green halo may surround lesions. Leaf symptoms give plants an overall orange cast.

    Disease Cycle

    The black chaff bacterium survives on and in seed, mainly in the external seed coats. Infection of the plumule (young shoot) occurs during germination and is through wounds or through stomata on the coleoptile (a sheath that protects the plumule). The bacterium invades the coleoptile and comes in contact with and infects the plumule before the first leaf emerges. The bacterium is suspected to survive on crop residue and in the soil, but this is not well documented. During the growing season, the bacterium is capable of living on the plant surface as an epiphyte without causing symptoms. Over short distances, it is spread by splashing water, plant-to-plant contact, and insects. It is spread over long distances by contaminated seed, which is the primary source of inoculum.

    Bacterial exudates on infected plants and epiphytic populations on grassy weeds and volunteer cereals serve as secondary inoculum. Bacteria enter plants through stomata and wounds. Glumes and kernels are infected after head emergence. Infection and disease development are favored by wet, warm weather. The risk for disease epidemics is higher in irrigated than dryland fields.

    Conditions that Favor Epidemics

    • Warm temperatures
    • Wet conditions (free moisture and high humidity)
    • Frost damage and ice nucleation
    • Injury from hail, sandblasting, or other causes
    • Root rot and leaf spot diseases caused by fungi
    • Insects, which may cause injury and also disseminate the bacterium


    The most effective management strategy for black chaff is use of certified, pathogen-free seed. Seed producers should consider testing their seed lots for black chaff before planting. Controlling volunteer cereals and grassy weeds can help to reduce primary inoculum. Irrigation management is critical in creating an environment that is less favorable for disease development and spread. Irrigation should be managed in such a way as to allow the plant canopy to dry completely between irrigations. Although highly resistant wheat cultivars are currently not available, those known to be highly susceptible to black chaff should not be planted.


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