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.