Title

Sunflower Disease Diagnostic Series

(PP1727, Reviewed Jan. 2023)
File
Publication File:
Summary

This series aids in disease identification.

Lead Author
Lead Author:
Samuel Markell, Extension Plant Pathologist, North Dakota State University
Other Authors

Robert Harveson, Extension Plant Pathologist, University of Nebraska
Charles Block, Plant Pathologist, USDA, Ames, IA
Thomas Gulya, USDA Sunflower Pathologist, (Retired), Fargo, N.D.
Febina Mathew, Broadleaf/Oilseed Crops Pathologist, North Dakota State University

Availability
Availability:
Web only
Publication Sections

Table of Contents

Bacterial head rot

Pectobacterium carotovorum, subsp. carotovorum and P. atrosepticum

FIGURE 1 – Watery lesions forming on heads as a result of infection through wounds
FIGURE 1 – Watery lesions forming on heads as a result of infection through wounds
FIGURE 2 – Slimy masses of bacterial growth within infected head tissues
FIGURE 2 – Slimy masses of bacterial growth within infected head tissues
FIGURE 3 – Affected tissues dry out and turn black after a period of warm, dry weather
FIGURE 3 – Affected tissues dry out and turn black after a period of warm, dry weather

Pectobacterium carotovorum, subsp. carotovorum and P. atrosepticum

AUTHORS: Bob Harveson, Sam Markell, Tom Gulya and Charlie Block

SYMPTOMS

• Coalescing lesions develop watery, soft-rot symptoms that become dark brown as disease progresses
• Heads give off an odor of rotting potatoes, and slimy masses of bacterial growth are present within infected tissues

FACTORS FAVORING DEVELOPMENT

• Thunderstorms with hail; insect or bird damage to heads
• Warm temperatures with high humidity levels

IMPORTANT FACTS

• Mechanical injury (from insects, birds or hail) is required for infection
• Pathogen is found ubiquitously in soil and is spread by rain splashing and driving winds
• More common in the U.S southern Great Plains states
• Can be confused with other head rot diseases (Sclerotinia, Botrytis or Rhizopus)

Rhizopus head rot

R. stolonifer, R. oryzae (syn. R. arrhizus) and R. microsporus

FIGURE 1 – Note wound from hail stone with subsequent development of watery, soft rot
FIGURE 1 – Note wound from hail stone with subsequent development of watery, soft rot
FIGURE 2 – Rotted area of head drying, shriveling and beginning to shred
FIGURE 2 – Rotted area of head drying, shriveling and beginning to shred
FIGURE 3 – Grayish fungal strands growing through head;
FIGURE 3 – Grayish fungal strands growing through head;
reproductive structures
reproductive structures

R. stolonifer, R. oryzae (syn. R. arrhizus) and R. microsporus

AUTHORS: Bob Harveson, Sam Markell, Charlie Block and Tom Gulya

SYMPTOMS

• First appears on heads as dark spots of varying sizes as a result of wounding, followed by a watery, soft rot that later dries and turns dark brown
• Rhizopus is distinguished from other head rots by the presence of grayish, threadlike mycelial strands within infected heads; small black reproductive structures the size of a pinhead also may be present

FACTORS FAVORING DEVELOPMENT

• Thunderstorms with hail; insect or bird damage on head
• Warm temperatures with high humidity levels

IMPORTANT FACTS

• Mechanical injury (from insects, birds or hail) is required for infection
• Pathogen is found ubiquitously in soil, and infective spores are released into the air easily
• More common in the U.S southern Great Plains states
• Can be confused with bacterial and/or Sclerotinia head rots

Sclerotinia head rot

Sclerotinia sclerotiorum

FIGURE 1 – Apothecia (grows from sclerotia and produces ascospores)
FIGURE 1 – Apothecia (grows from sclerotia and produces ascospores)
FIGURE 2 – Soft brown area on the back of head
FIGURE 2 – Soft brown area on the back of head
FIGURE 3 – A shredded sunflower with sclerotia
FIGURE 3 – A shredded sunflower with sclerotia

FIGURE 4 – White mycelium and black sclerotia on the face of a skeletonized sunflower head
FIGURE 4 – White mycelium and black sclerotia on the face of a skeletonized sunflower head

Sclerotinia sclerotiorum

AUTHORS: Sam Markell, Tom Gulya, Charlie Block and Bob Harveson

SYMPTOMS

• Lesions begin as large, soft (mushy), brown areas on the back of heads that turn tan-cream, typically odorless
• White mold (mycelium) and hard black structures (sclerotia) form inside head
• Heads will shred, and disintegration and/or decapitation may occur

FACTORS FAVORING DEVELOPMENT

• Wet soils prior to bloom (facilitates apothecia production)
• Frequent wetness during or after bloom, including rain, fog, heavy dew
• Temperatures 85 F or below

IMPORTANT FACTS

• The same pathogen causes sclerotinia wilt and sclerotinia mid-stem rot
• The pathogen can survive for many years in the soil as sclerotia
• Management tools are limited
• Most common in the U.S. northern Great Plains
• Can be confused with Rhizopus head rot

Bacterial stalk rot

Pectobacterium carotovorum, subsp. carotovorum and P. atrosepticum

FIGURE 1 – Affected tissues blacken and are often on petiole axils
FIGURE 1 – Affected tissues blacken and are often on petiole axils
FIGURE 2 – Infected stalk splitting longitudinally
FIGURE 2 – Infected stalk splitting longitudinally

FIGURE 3 – Development of a foam on stalk wounds due to bacterial infection
FIGURE 3 – Development of a foam on stalk wounds due to bacterial infection

Pectobacterium carotovorum, subsp. carotovorum and P. atrosepticum

AUTHORS: Bob Harveson, Charlie Block, Sam Markell and Tom Gulya

SYMPTOMS

• Infected stalks soften and dry up, becoming dark brown to black and may split open
• Plants often lodge under the weight of maturing heads
• A foam may appear on infected tissues as a result of bacterial-causing fermentation of sugars in plant

FACTORS FAVORING DEVELOPMENT

• Thunderstorms with hail
• Warm temperatures with high humidity levels

IMPORTANT FACTS

• Mechanical injury (from insects, birds or hail) is required for infection
• Pathogen is found ubiquitously in soil and is spread by rain splashing and driving winds
• More common in the U.S southern Great Plains states
• Can be confused with other stalk rots

Charcoal rot

Macrophomina phaseolina

FIGURE 1 – Gray lesion at the base of sunflower stalks
FIGURE 1 – Gray lesion at the base of sunflower stalks
FIGURE 2 – Microsclerotia inside sunflower stem
FIGURE 2 – Microsclerotia inside sunflower stem
FIGURE 3 – Stem with severe charcoal rot
FIGURE 3 – Stem with severe charcoal rot
FIGURE 4 – Field with charcoal rot
FIGURE 4 – Field with charcoal rot

Macrophomina phaseolina

AUTHORS: Sam Markell, Charlie Block, Bob Harveson and Tom Gulya

SYMPTOMS

• Gray to silver basal lesion starting at the soil line
• Premature senescence and plant death
• Abundant dusty black microsclerotia inside lower stem (visible with a hand lens)
• Vascular tissue compressed into layers

FACTORS FAVORING DEVELOPMENT

• Field history with charcoal rot, including soybeans, corn and other crops
• Wet weather in spring followed by hot, dry weather in reproductive growth stages
• Water stress (sandy soil, heat, drought, etc.)

IMPORTANT FACTS

• The same pathogen causes charcoal rot on soybeans, corn and other crops
• Infection begins early in the season but manifests in late reproductive stages if plants are stressed
• Most common in the U.S. southern and high Plains states
• Can be confused with Verticillium wilt and Sclerotinia wilt

Downy mildew

Plasmopara halstedii

FIGURE 1 – Stunting and chlorosis (yellowing) from systemic infection: Healthy (left), infected (right)
FIGURE 1 – Stunting and chlorosis (yellowing) from systemic infection: Healthy (left), infected (right)
FIGURE 2 – Underside (left) and upperside (right) of leaf with systemic infection
FIGURE 2 – Underside (left) and upperside (right) of leaf with systemic infection
FIGURE 3 – Local lesions from secondary infection
FIGURE 3 – Local lesions from secondary infection

Plasmopara halstedii

AUTHORS: Sam Markell, Bob Harveson, Charlie Block and Tom Gulya

SYMPTOMS

• Stunting, leaf chlorosis, white sporulation on underside of leaf, plant death
• Horizontal heads when mature
• Secondary infection: discrete chlorotic leaf spots on upper leaf surface

FACTORS FAVORING DEVELOPMENT

• Cold soils and rainfall shortly after planting leading to waterlogged soil
• Cool nights with dew or rain (for local lesions via secondary infection)

IMPORTANT FACTS

• Secondary infections do NOT cause yield loss
• Pathogen is soil-borne and can survive many years in soil
• Disease is specific to sunflowers
• Fungicide seed treatments and resistant hybrids can be used for management
• Can be confused with herbicide damage

Fusarium root and stem rots

Fusarium species

FIGURE 1 – Pink discoloration caused by an unidentified Fusarium species
FIGURE 1 – Pink discoloration caused by an unidentified Fusarium species
FIGURE 2 – Pink streaks caused by Fusarium spp., associated with black microsclerotia of M. phaseolina (Charcoal rot)
FIGURE 2 – Pink streaks caused by Fusarium spp., associated with black microsclerotia of M. phaseolina (Charcoal rot)
FIGURE 3 – Sunflowers infected with Fusarium
FIGURE 3 – Sunflowers infected with Fusarium

Fusarium species

AUTHORS: Sam Markell, Bob Harveson, Charlie Block and Tom Gulya

SYMPTOMS

• Premature senescence
• Internal pink, orange, red or purple discoloration of pith

FACTORS FAVORING DEVELOPMENT

• Water stress (sandy soil, heat, drought, etc.)

IMPORTANT FACTS

• Many Fusarium species have been found to cause damage to sunflowers
• Many Fusarium species can cause disease and/or survive on crop hosts
• Economic damage is thought to be limited but can occur
• Frequently found with Charcoal rot
• Can be confused with other stalk/wilt diseases

Phoma black stem

Phoma macdonaldii

FIGURE 1 – Phoma lesions centered on petioles
FIGURE 1 – Phoma lesions centered on petioles
FIGURE 2 – A sunflower stalk with numerous Phoma lesions
FIGURE 2 – A sunflower stalk with numerous Phoma lesions
FIGURE 3 – Phoma (bottom black lesion) and Phomopsis (upper brown lesion) occurring on the same stem
FIGURE 3 – Phoma (bottom black lesion) and Phomopsis (upper brown lesion) occurring on the same stem

Phoma macdonaldii

AUTHORS: Sam Markell, Bob Harveson, Tom Gulya and Charlie Block

SYMPTOMS

• 1- to 2-inch black lesion, usually superficial
• Lesions centered on petioles
• Multiple lesions may occur on the same stem

FACTORS FAVORING DEVELOPMENT

• Frequent rainstorms
• Insects (such as stem weevils) can facilitate infection
• Sunflower residue nearby or short rotation

IMPORTANT FACTS

• Rarely economically important
• Typically the most common stem disease in the northern Great Plains
• Infection begins on leaves and progresses into the stem
• Can be vectored by black sunflower stem weevils (Apion)
• Can be confused with Phomopsis stem canker

Phomopsis stem canker

Diaporthe helianthi, D. gulyae

FIGURE 1 – Leaf bronzing
FIGURE 1 – Leaf bronzing
FIGURE 2 – Stem lesions at different stages of development
FIGURE 2 – Stem lesions at different stages of development

FIGURE 3 – Stem lesion and lodging
FIGURE 3 – Stem lesion and lodging

Phomopsis helianthi, P. gulyae

AUTHORS: Sam Markell, Tom Gulya, Bob Harveson and Charlie Block

SYMPTOMS

• Leaf bronzing
• Large (often greater than 6-inches) brown stem lesion that is centered on petiole
• Stem will become hollow and is easily punctured with thumb
• Premature senescence and/or widespread lodging may occur

FACTORS FAVORING DEVELOPMENT

• Frequent rainstorms
• Infested sunflower residue nearby and short crop rotation

IMPORTANT FACTS

• Infection begins in leaves and spreads into the stem
• High disease pressure can devastate the crop
• Most common in the U.S. northern Great Plains
• Can be confused with Phoma black stem and Sclerotinia mid-stem rot

Sclerotinia mid-stem rot

Sclerotinia sclerotiorum

FIGURE 1 – Leaf lesion caused by Sclerotinia infected flower
FIGURE 1 – Leaf lesion caused by Sclerotinia infected flower
FIGURE 2 – Sclerotinia lesion with white mycelium
FIGURE 2 – Sclerotinia lesion with white mycelium
FIGURE 3 – Shredded stalk resulting in lodging
FIGURE 3 – Shredded stalk resulting in lodging
FIGURE 4 – Abundant small black sclerotia in a shredded stem
FIGURE 4 – Abundant small black sclerotia in a shredded stem

Sclerotinia sclerotiorum

AUTHORS: Sam Markell, Charlie Block, Tom Gulya and Bob Harveson

SYMPTOMS

• Large (greater than 6-inch) tan to manila lesion on the stem, centered on petiole
• White mold (mycelium) and hard black structures (sclerotia) may be visible
• Stalk may shred at lesion, and plant eventually will lodge

FACTORS FAVORING DEVELOPMENT

• Wet soils before bloom (facilitates apothecia production)
• Temperatures 85 F or below
• Prolonged wet canopies (rain, fog, dew, etc.)

IMPORTANT FACTS

• The same pathogen causes Sclerotinia head rot and Sclerotinia wilt
• Infection begins on leaf when ascospores colonize senescent leaf tissue, florets or pollen
• Most common in the U.S. northern Great Plains states
• Can be confused with Phomopsis stem canker

Sclerotinia wilt/Basal stalk rot

Sclerotinia sclerotiorum

FIGURE 1 – Tan to manila basal lesion; note white mycelium
FIGURE 1 – Tan to manila basal lesion; note white mycelium
FIGURE 2 – Lodging and shredding (left plant only) caused by Sclerotinia wilt
FIGURE 2 – Lodging and shredding (left plant only) caused by Sclerotinia wilt
FIGURE 3 – Sclerotia and mycelium on infected sunflower
FIGURE 3 – Sclerotia and mycelium on infected sunflower
FIGURE 4 – Wilted sunflower plant
FIGURE 4 – Wilted sunflower plant

Sclerotinia sclerotiorum

AUTHORS: Sam Markell, Bob Harveson, Charlie Block and Tom Gulya

SYMPTOMS

• Tan to manila basal lesion at soil line
• White mold (mycelia) and black sclerotia on basal lesion
• Whole-plant wilt, basal shredding and lodging may occur

FACTORS FAVORING DEVELOPMENT

• Field history with Sclerotinia diseases
• Tight crop rotation with broadleaf crops

IMPORTANT FACTS

• The same pathogen causes Sclerotinia white mold on other broadleaf crops
• Unlike Sclerotinia head and mid-stalk rot, fungus invades through roots
• Sclerotia can survive for many years in the soil
• Most common in the U.S. northern Plains states
• Can be confused with Verticillium wilt and Charcoal rot

Verticillium wilt

Verticllium dahliae

FIGURE 1 – Sunflower with Verticillium wilt. Note leaf chlorosis progressing upward.
FIGURE 1 – Sunflower with Verticillium wilt. Note leaf chlorosis progressing upward.
FIGURE 2 – Leaf symptoms
FIGURE 2 – Leaf symptoms
FIGURE 3 – Vascular browning
FIGURE 3 – Vascular browning
FIGURE 4 – External Verticillium lesion on lower stem (L) and shrunken and blackened pith (R)
FIGURE 4 – External Verticillium lesion on lower stem (L) and shrunken and blackened pith (R)

Verticllium dahliae

AUTHORS: Sam Markell, Tom Gulya, Charlie Block and Bob Harveson

SYMPTOMS

• Interveinal chlorosis and necrosis starting at lowest leaves and progessing upwards
• Damaged vascular tissue; initially, a brown ring may be present
• Wilting occurring at bloom, usually in patches or rows
• Pith shrunken and black at maturity

FACTORS FAVORING DEVELOPMENT

• Water stress (sandy soil, heat, drought, etc.)
• Field history with Verticillium wilt

IMPORTANT FACTS

• The same pathogen causes Verticillium wilt on other crops (potatoes, etc.)
• Can be economically devastating with high disease pressure
• Leaf symptoms can be confused with Phomopsis stem canker
• Can be confused with Charcoal rot and Sclerotinia wilt

Albugo/White rust

FIGURE 1 – White sporulation on underside of leaf
FIGURE 1 – White sporulation on underside of leaf
FIGURE 2 – Chlorotic lesion on upper surface of leaf
FIGURE 2 – Chlorotic lesion on upper surface of leaf
FIGURE 3 – Dark, bruiselike lesion on the stem
FIGURE 3 – Dark, bruiselike lesion on the stem

AUTHORS: Sam Markell, Tom Gulya, Bob Harveson and Charlie Block

SYMPTOMS

• Raised chlorotic pustules up to 3/8 inch in diameter on upper side of leaf
• Spores on underside of leaf opposite of chlorotic pustules
• Lesions on stem, petiole and head are dark and bruiselike

FACTORS FAVORING DEVELOPMENT

• Cool nights (50 to 60 F) and warm days (70 to 80 F)
• Rain splash

IMPORTANT FACTS

• Disease is very rare in the U.S.
• When found, it often is observed in single horizontal layer of leaves across a canopy
• Can be confused with downy mildew local lesions and powdery mildew

Alternaria leaf blight

Alternariaster helianthi, Alternaria zinniae

FIGURE 1 – Characteristic necrotic and chlorotic leaf blight lesions
FIGURE 1 – Characteristic necrotic and chlorotic leaf blight lesions
FIGURE 2 – Stem lesions
FIGURE 2 – Stem lesions
FIGURE 3 – Lesion coalescence and necrosis near leaf tips
FIGURE 3 – Lesion coalescence and necrosis near leaf tips
FIGURE 4 – Yellow leaf spots with little necrosis on resistant cultivar
FIGURE 4 – Yellow leaf spots with little necrosis on resistant cultivar

Alternariaster helianthi, Alternaria zinniae

AUTHORS: Charlie Block, Sam Markell, Bob Harveson and Tom Gulya

SYMPTOMS

• Young leaf spots are small, dark, angular
• Leaf spots usually are found between major leaf veins, along leaf margins and tips and will coalesce
• Extensive yellowing (chlorosis) occurs, followed by browning and leaf death
• Defoliation occurs from the ground up
• Stem lesions are dark, narrow, elliptical and about ½ to 1½ inches long

FACTORS FAVORING DEVELOPMENT

• Rainfall shortly after planting
• Warm, humid weather

IMPORTANT FACTS

• Disease development is highly dependent on rain and dew
• Plants at flowering and seed filling stages more susceptible than young plants
• Fungus survives on plant residue
• Crop rotation and tillage of residue to encourage decomposition to help manage disease
• Can be confused with Septoria leaf blight, bacterial leaf spot

Apical chlorosis

Pseudomonas syringae pv. tagetis

Figure 1 - Young plant infected systemically; note bright yellow chlorosis and stunting
Figure 1 - Young plant infected systemically; note bright yellow chlorosis and stunting
FIGURE 2 – Plant nearing bud formation (R1) exhibiting systemic chlorosis symptoms
FIGURE 2 – Plant nearing bud formation (R1) exhibiting systemic chlorosis symptoms
FIGURE 3 – Distribution of apical chlorosis corresponding to low areas of water accumulation in field
FIGURE 3 – Distribution of apical chlorosis corresponding to low areas of water accumulation in field

Apical chlorosis

Pseudomonas syringae pv. tagetis

AUTHORS: Bob Harveson, Tom Gulya, Sam Markell and Charlie Block

SYMPTOMS

• Distinctive bright yellow to nearly white chlorosis of newest leaves
• New leaves will be unaffected in warm weather
• May occur on isolated plants, patches or in rows
• Stunting if plants infected at a young stage

FACTORS FAVORING DEVELOPMENT

• Wet, cool conditions
• Water-logged soils

IMPORTANT FACTS

• Can be observed on plants of all growth stages, but most common on young plants (pre-bloom)
• Damage is minimal unless young plants are infected
• Chlorotic symptoms due to a toxin produced by the pathogen
• Related to bacterial leaf spot pathogen
• Can be confused with fertility problems, downy mildew and/or viruses

Bacterial leaf spot

Pseudomonas syringae pv. helianthi

FIGURE 1 – Multiple leaf spots surrounded by yellow halos
FIGURE 1 – Multiple leaf spots surrounded by yellow halos
FIGURE 2 – Small necrotic leaf spots on lower leaves
FIGURE 2 – Small necrotic leaf spots on lower leaves
FIGURE 3 – Coalescing of spots to form linear lesions
FIGURE 3 – Coalescing of spots to form linear lesions

Pseudomonas syringae pv. helianthi

AUTHORS: Bob Harveson, Sam Markell, Tom Gulya and Charlie Block

SYMPTOMS

• Angular, necrotic spots of varying size
• Leaf spots form linear lesions that crack and fall out
• Necrotic spots may be surrounded with yellow haloes

FACTORS FAVORING DEVELOPMENT

• Wounds created by hail, sandblasting and other forms of mechanical damage
• Warm temperatures with high humidity levels

IMPORTANT FACTS

• Often is restricted to lower leaves and, thus, not generally economically damaging
• Can be seed-borne and soil-borne; spread by splashing rains and high winds
• Related to apical chlorosis pathogen
• Can be confused with Alternaria leaf blight and Septoria leaf blight

Powdery mildew

Erysiphe cichoracearum

FIGURE 1 – Discrete spots of white mycelium forming on a seedling
FIGURE 1 – Discrete spots of white mycelium forming on a seedling
FIGURE 2 – White spots forming on mature leaf (brown spots are rust)
FIGURE 2 – White spots forming on mature leaf (brown spots are rust)
FIGURE 3 – Sunflower leaf completely covered in mycelium
FIGURE 3 – Sunflower leaf completely covered in mycelium

Erysiphe cichoracearum

AUTHORS: Sam Markell, Tom Gulya, Bob Harveson and Charlie Block

SYMPTOMS

• White spots of fungal mycelium on upper leaf surface, can be rubbed off easily
• White mycelium will eventually cover the entire leaf
• Black specks (cleistothecia) may develop late in season

FACTORS FAVORING DEVELOPMENT

• High humidity
• Plant maturity and leaf senescence

IMPORTANT FACTS

• Usually doesn’t appear until after full bloom (R5)
• Symptoms are often more severe on lower leaves
• White fluffy growth on the top of leaves and late onset of disease help distinguish from downy mildew
• Can be confused with local lesions of downy mildew

Rust

Puccinia helianthi

FIGURE 1 – Pycnia (L) on upper side of leaf and Aecia (R) opposite pycnia on underside of leaf
FIGURE 1 – Pycnia (L) on upper side of leaf and Aecia (R) opposite pycnia on underside of leaf
FIGURE 2 – Uredinia surrounded by yellow halos; note spores on finger
FIGURE 2 – Uredinia surrounded by yellow halos; note spores on finger
FIGURE 3 – Pustules on stem and petiole (L) and bracts (R)
FIGURE 3 – Pustules on stem and petiole (L) and bracts (R)
FIGURE 3 – Pustules on stem and petiole (L) and bracts (R)

Puccinia helianthi

AUTHORS: Sam Markell, Bob Harveson, Charlie Block and Tom Gulya

SYMPTOMS

• Pycnia = yellow/orange bump on topside of leaf (early season)
• Aecia = cluster of orange cups opposite pycnia (early season)
• Uredia = dusty cinnamon-brown pustule (throughout season), spores can be easily rubbed off, yellow halo common
• Telia = hard black pustule (crop maturity)

FACTORS FAVORING DEVELOPMENT

• Frequent leaf wetness; dew, fog, light rain, etc.
• Temperatures between 55 and 85 F
• Proximity to wild, volunteer or sunflower residue that has or had rust

IMPORTANT FACTS

• Sunflower rust is specific to sunflowers (cultivated and wild)
• Economic losses can be devastating in epidemics
• Fungicide threshold = 1 percent severity on upper leaves at or before bloom (R5)
• Found in all U.S. Great Plains states
• Can be confused with soil splashed on lower leaves or other foliar diseases

Septoria leaf blight

Septoria helianthi

FIGURE 1 – Young developing lesions
FIGURE 1 – Young developing lesions
FIGURE 2 – Mature lesions of Septoria leaf spot
FIGURE 2 – Mature lesions of Septoria leaf spot
FIGURE 3 – Pycnidia visible as black specks inside large, round lesions (with hand lens)
FIGURE 3 – Pycnidia visible as black specks inside large, round lesions (with hand lens)

AUTHORS: Charlie Block, Bob Harveson, Sam Markell and Tom Gulya

SYMPTOMS

• Circular leaf spots up to ¾ inch in diameter, with dark margins and tan to gray centers
• Leaf spots often, but not always, surrounded by a narrow yellow halo
• Fungus survives on plant residue; infection spreads from bottom leaves upward
• Mature leaf spots become dotted with black specks, or pycnidia, on the upper leaf surface

FACTORS FAVORING DEVELOPMENT

• Cool temperatures and rain in the spring and fall
• Symptoms develop most rapidly after flowering, but finding leaf spots on seedlings is common
• Frequent wetness during or after bloom, including rain, fog and heavy dew

IMPORTANT FACTS

• Disease tends to go dormant during hot, dry weather
• Seldom a problem in drier sunflower-production areas
• Can be confused with Alternaria leaf blight and bacterial leaf spot. Larger rounded lesions with pycnidia help distinguish Septoria leaf spot from Alternaria leaf spot.

Virus Diseases

Nebraska mottle/ringspot virus? Sunflower mosaic virus

FIGURE 1 – Greenhouse-inoculated seedlings showing small, yellow spots (NMRV?)
FIGURE 1 – Greenhouse-inoculated seedlings showing small, yellow spots (NMRV?)
FIGURE 3 – Typical sunflower mosaic virus symptoms (SMV)
FIGURE 3 – Typical sunflower mosaic virus symptoms (SMV)

Nebraska mottle/ringspot virus? (NMRV?)Sunflower mosaic virus (SMV)

AUTHORS: Tom Gulya, Bob Harveson, Sam Markell and Charlie Block

SYMPTOMS

NMRV?

• Begins as small, yellow spots on new foliage
• Chlorotic ringspots may develop as plants mature

SMV

• Leaf mosaic symptoms

FACTORS FAVORING DEVELOPMENT

• Unknown

IMPORTANT FACTS

• Viruses are not typically an economic problem due to low incidence
• Identity of virus pathogen and potential vectors are unknown in many viruses
• Sunflower mosaic virus can be seedborne and vectored by aphids