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Caution

While sunflowers are drought resistant and thrive in full sun it is important to remember that  the space needed between them to grow is not only for the root system but to allow whatever sun is available to prevent humidity caused by overlapping leaves.

If your plants are too close together the enormous leaves will create an unnatural rain forest effect which promotes unwanted  pests and mildew.

Alternaria leaf blight

SYMPTOMS

Dark brown lesions on leaves surrounded by a yellow halo; lesions coalesce and become irregularly shaped and cause leaves to become blighted; plant becomes defoliated and dies

COMMENTS

Disease emergence favors hot weather and frequent rainfall; fungus may survive in crop debris or on suitable weed hosts; disease can be transmitted through infested seed

MANAGEMENT

Prune out infected leaves; use adequate plant spacing to reduce humidity around plants and promote good air circulation; disease can be controlled by application of appropriate foliar fungicide

Downy mildew

SYMPTOMS

Death of seedlings leading to reduced stand in field; if seedlings survive they may be chlorotic with thickened leaves; white cottony growth is present on leaf undersides; systemic infection causes stunted plant growth and reduced seed production

COMMENTS

Disease emergence favors high humidity; fungus can survive in soil for up to 10 years

MANAGEMENT

Plant sunflower varieties that are resistant to downy mildew; treat seeds with an appropriate fungicide prior to planting; foliar fungicides are ineffective at controlling systemic infections and are not recommended

Phoma blight FUNGUS

Phoma macdonaldii

Symptoms of Phoma blight of sunflower

Symptoms of Phoma blight of sunflower

SYMPTOMS

Symptoms of the disease develop after flowering; large black lesions appear on stem and coalesce to form large blackened areas; dark colored irregularly shaped lesions appear on leaves and flowers; early infections can cause flowers to die; infected plants die prematurely and produce little seed; disease often affects plants in a circular pattern in the field

COMMENTS

Fungus survives in seeds or sunflower debris in the field; disease emergence favors periods of wet weather during flowering

MANAGEMENT

Rotate crop to a non-host (e.g. small grains) a period of 4 years; plant hybrids which are more tolerant of the disease; control stem weevil populations in sunflower fields

Powdery mildew FUNGUS

Erysiphe cichoracearum

SYMPTOMS

Powdery white patches which appear initially on lower leaves but which may spread to all above-ground parts of plants; white patches turn gray in color and black fungal fruiting bodies are visible; severely infected leaves may turn yellow and dry up

COMMENTS

Conditions which favor the development of the disease mach those that are favorable for the host plant; disease emergence is favored by periods of high humidity where leaves remain dry

MANAGEMENT

Allow adequate spacing between plants to promote good air circulation around foliage; plant sunflowers in an area that receives full sun for most of the day; remove and destroy all sunflower crop debris after harvest; applications of appropriate foliar fungicides can help control the disease but care should be taken as some labels do not allow seeds from treated plants to be used as food or feed

Septoria leaf spot FUNGUS

Septoria helianthi

SYMPTOMS

Water-soaked circular or angular spots on leaves with a greasy, greenish appearance on lower leaves; lesions are usually gray with a darker margin; some lesions may have a narrow yellow border; tiny black fungal fruiting bodies may be present in the lesions

COMMENTS

Little is known about the survival and spread of the pathogen which causes the disease; spores are believed to be spread by splashing water; disease will develop rapidly during periods of moderate to warm weather with high rainfall

MANAGEMENT

Plant high quality seed which is free of diseases; rotate crop away from sunflower for a period of 3 years, especially if overhead irrigation is used; fungicides are rarely required for the treatment of Septoria leaf spot

Verticillium wilt FUNGUS

Verticillium dahliae

Verticillium wilt symptoms on sunflower

Sunflowers infected with Verticillium wilt

Sunflowers infected with Verticillium wilt

SYMPTOMS

Lower leaves developing mottled appearance; leaf tissue between veins turns yellow and then brown; infected leaves wilt, dry out and eventually die; stems of plants may become blackened close to the soil line; a cross section of the stem reveals blackened vascular system

COMMENTS

Fungus is soil-borne and enters plants through the roots, invading the vascular system; pathogen can be spread to uninfested fields through contaminated irrigation water or movement of infested soils

MANAGEMENT

Plant high quality, disease-free seed; avoid planting sunflowers in fields known to have been infested with Verticillium previously; plant resistant sunflower hybrids in areas where disease is known to be problematic

References

Darby, H. & Halteman, P. (2011). Sunflower. University of Vermont Extension. Available at: http://northerngraingrowers.org/wp-co…. [Accessed 16 April 15]. Free to access.

Lee, C. (2014). Sunflower for seed. University of Kentucky Cooperative Extension Service and University of Kentucky College of Agriculture, Food and Environment. Available at: http://www.uky.edu/Ag/NewCrops/intros…. [Accessed 16 April 15]. Free to access.

Myers, R. L. (2002). Sunflower. Thomas Jefferson Agricultural Institute. Available at: http://www.agmrc.org/media/cms/sunflo…. [Accessed 16 April 15]. Free to access.

Putnam, D. H., Oplinger, E. S., Hicks, D. R., Durgan, B. R., Noetzel, D. M., Meronuck, R. A., Doll, J. D. & Schulte, E. E. (1990). Sunflower. In: Alternative Field Crop Manual. University of Wisconsin Extension, University of Minnesota Center for Alternative Plant and Animal Products and the Minnesota Extension Service. Available at: http://www.hort.purdue.edu/newcrop/af…. [Accessed 16 April 15]. Free to access.