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Shady Pathogens

Monday, January 15, 2007
filed under: Disease

Sclerotinia and downy mildew are still the main villains when it comes to sunflower diseases, at least for now. Hybrids with better tolerance to both diseases are already commercially available, with more on the way.

There are other shady pathogens to be wary of, however - diseases that can ambush sunflower fields, weather and other circumstances permitting. Here’s a profile of yield robbers that showed up in the 2006 U.S. Sunflower Survey, and what can be done to manage them.

Three conditions are needed for diseases to occur: 1) the presence of a susceptible crop host; 2) a pathogen strain or strains specific to the crop host; and 3) the right environment for infection to proliferate, which explains why any one disease is spotty from year to year and amongst locations.


Background: Rust races are crop specific. The fungus Puccinia helianthi is specific to sunflower (and wild sunflowers also), but sunflower isn’t affected by either Asian soybean rust or wheat rust. So rust in the previous year’s wheat crop doesn’t mean an increased chance of rust in the following year’s sunflower crop, and vice versa.

Sunflower rust spores overwinter on debris from infected sunflower leaves and stems. In the spring, spores germinate to infect volunteer seedlings, wild sunflower or new young plants in nearby fields.

This is a weather dependent disease that can explode when conditions are right for spores to multiply. Disease development is typically favored by warm temperatures, cloudy days and wet foliage for at least 8-12 hours, and extended wet periods of 3 to 4 days can cause serious damage.

Symptoms: Sunflower rust pustules are found on the underside of sunflower leaves, beginning on the lower leaves of the plant and moving higher in the canopy under favorable conditions. Rust can be yield impacting when as little as 3% of the plant’s upper leaves and bracts are covered with rust pustules prior to the plant reaching growth stage R-6, which is when the ray flowers (petals) are wilting. If rust pustules are found on the upper leaves of confection sunflower at budding, then the crop may benefit from a fungicide application.

Damage: Rust can reduce yield and affect oil content, seed size, test weight and kernel-to-hull ratios as well. Thus it affects overall yield and many “quality factors” by which confection sunflowers are graded. Late planted fields of susceptible hybrids are generally more severely damaged by rust than earlier planted fields.

Management: Hybrid selection is the first line of defense. Since there can be many rust races present, even if a hybrid is marketed as “rust resistant,” that designation may not mean the hybrid is resistant to all races found in a given area. Destroy volunteer plants and wild annual sunflower occurring in the vicinity of commercial fields. Plant early, which may help the crop escape rust infection buildup. Follow recommended planting rates, avoid high plant populations and high nitrogen rates, which lead to dense foliage that favor disease.

Research in Colorado has shown an economic benefit to a timely fungicide application for rust control in irrigated confection sunflower (see Dec. 05 article online, – go to ‘Sunflower Magazine,’ then ‘view archives’ and ‘Disease.’)

Treatment timing is critical. Fungicide treatment timing may coincide with insecticide spraying for seed weevil and head moth; check label for recommended treatment timing and tankmixing compatibility. Folicur 3.6F and Headline are fungicides that have received labels for rust in the past; see labels for registered treatment status in 2007. Headline® provides no curative (systemic) activity. It is a strobilurin fungicide and must be applied prior to infection (preventative activity only) to have any effect.

Verticillium wilt

Background: Verticillium wilt is caused by a persistent soilborne and seedborne fungus, Verticillium dahliae, that can remain in the soil for several years. The fungus has a wide host range and causes wilt of several other broadleaf weeds and cultivated broadleaf crops, including potato. The disease infects root tips of sunflower plants and eventually affects all parts of the plant. Sunflower hybrids have had resistance to Verticillium for several decades, governed by a single gene. In the last three years, however, a new strain of Verticillium has been found which is able to infect “resistant” hybrids.

Symptoms: Symptoms usually are not observed until flowering but under severe conditions they may occur as early as the six-leaf stage. Diseased plants usually occur in groups of a few to many plants. Symptoms develop first on the lower leaves and gradually appear on leaves farther up the stem. Leaf tissue between the veins yellows and becomes necrotic, while areas near the veins remain green, producing a mottled appearance. Affected leaves rapidly become completely dry. Severely diseased plants will turn brown, starting at the bottom leaves, and die prematurely. Severely diseased plants may contain masses of tiny black fruiting bodies (microsclerotia) inside the stalk, which are too small to be visible without the aid of a microscope. In severe infections, the black microsclerotia will form a black layer on the pith tissue..

Damage: Verticillium wilt can be a serious disease on lighter soils with a history of sunflower cropping, but generally is not a serious problem on heavier soils. This disease will cause some yield loss if a susceptible crop is planted.

Management: Plant resistant hybrids, and ask your dealer if the hybrid is resistant to both the “old” and the “new” strain of Verticillium. Damage can be minimized by following a three to four-year rotation (away from both sunflowers and potatoes), using nonhost crops such as cereals and corn, and avoiding fields with a history of Verticillium wilt.

Phomopsis Stem Canker

Background and symptoms: The distinguishing feature of Phomopsis stem canker (caused by Phomopsis helianthi) is a large tan to light brown lesion or canker which typically surrounds the leaf petiole on the stem and be up to 6 – 8” in length. Phomopsis t can be confused with Phoma black stem,but Phoma lesions are generally jet black in color,and much smaller (1-2”). Phomopsis also causes more extensive pith degradation than Phoma, so the stalk may be crushed with moderate thumb pressure, which would be unlikely with a Phoma lesion.

Damage: Phomopsis is most severe under conditions of prolonged high temperatures and high humidity. Yield losses result from smaller heads and lighter seed, and from lodging due to weakened stems.

Management: Since the fungus overwinters best in infected sunflower debris on the soil surface, thorough disking in the fall to bury plant residue and crop rotation can reduce disease incidence and severity. Some hybrids are more resistant to Phomopsis than others.

Rhizopus Head Rot

Background and symptoms: Two fungal species, Rhizopus arrhizus and R. stolonifer, have been implicated in development of this disease. Symptoms first become noticeable as dark spots on the back of ripening heads, followed by a watery soft rot that later turns brown. As disease progresses, heads dry prematurely, shrivel, and tissues appear to shred. Inside shredded tissues, coarse, thread-like mycelial strands are observed, followed by the appearance of small black dots (sporangia). Sporangia are filled with spores that are easily released and wind-blown to other plants. Symptoms on the flower side of heads include the appearance of mycelium, a grayish, fuzzy substance that is covered with sporangia.

Damage: Rhizopus differs from Sclerotinia head rot in that Rhizopus does not have the black sclerotia that form within the heads. Further, for Rhizopus to develop, some type of mechanical injury on the head in combination with high temperatures and high relative humidity are required for infection and disease progress. Infection is initiated in heads through wounds created by hail, birds, or insects. The two also differ in that Sclerotinia head rot is generally more apt to be the problem in the Northern Plains, and Rhizopus in the High Plains.

Damage and economic losses are dependent upon time of the season that wounding and infection occurs. Infection rarely occurs before flowering, and greatest yield reductions result when infection occurs before seeds are properly filled. Losses to Rhizopus head rot in some fields have been near 100% in the High Plains.

Management: Although no chemical controls are available, development of Rhizopus is strongly correlated with sunflower head moth infestations. It has been demonstrated that a good sunflower head insect control program will limit infection and yield losses due to Rhizopus head rot.

Rhizopus spores are found everywhere since it is a soilborne fungus (and is the same fungus responsible for “bread mold.” Therefore rotation will have little impact on Rhizopus occurrence. There have been some observations that hybrids with upright heads are more frequently infected than varieties with bending heads. Planting date studies have indicated that early planted fields that bloom before late July are more vulnerable to sunflower moth and thus Rhizopus damage.

More information on sunflower moth control, as well as other insects and diseases in sunflower, can be found in NDSU, High Plains, and Manitoba Production Handbooks and Guides online at – go to ‘Growers’ then ‘Production Handbooks.’ – Tracy Sayler

Sources for this article: NDSU Extension Bulletin 25; The High Plains Sunflower Production Handbook; Manitoba Sunflower Production Guide; Bob Harveson, extension plant pathologist, University of Nebraska; Carl Bradley, plant pathologist, University of Illinois; Tom Gulya, plant pathologist, USDA-ARS Fargo; Doug Jardine, plant pathologist, Kansas State University.

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