An Overview of Sunflower Diseases

Sunflowers have always been the target of disease. Even though some of them are most dominant in certain locations in the U.S., others manifest wherever the crop is grown. For example, head rot is prevalent throughout California, but downy mildew can be found on crops harvested across the entire country. Over the past few decades, more than 30 diseases have been found on crops (though most of them either won't cause severe damage or can be controlled easily). Below, we'll explore a few of the most important diseases and the type of damage they usually cause.

Rust

Rust is a constant thorn in the side of commercial growers because it evolves, making it difficult (though, not impossible) to cultivate resistant hybrids. It's caused by a fungus called Puccinia helianthi and shows up on sunflower planted throughout the U.S. In mild cases, you'll notice auburn-colored spots showing up on the leaves and stems. When an infestation has occurred, rust can show up on the backs of the heads.

The damage caused by rust can be substantial. It can lead to reduced seed count, smaller seed size, and less oil content. The most effective way to prevent an infestation is to plant resistant breeds, though again, it's hard to keep up with the fungi's evolution. Also, it's worth noting that some types of fertilizer can make the problem even worse.

Downy Mildew

Another disease that affects sunflower crops nationwide is downy mildew. It's usually found in places where the soil is either water-logged or has a significant amount of clay. For years, professional growers were able to use resistant breeds to keep it at bay. However, new strains of the parent fungus (Plasmopara halstedii) have caused growers to revisit their methods. Most hybrids that are used commercially can resist a couple of different strains of downy mildew. However, 9 strains have been identified in the U.S.

This fungus often takes hold at the seedling stage. When that happens, it stunts the growth of the crop and causes the heads to face skyward (as opposed to nodding late in the growth cycle). If the infection spreads across the entire crop, it can have a significant impact on seed yield.

Spots

Spots can appear on the stems and leaves of sunflowers due to 2 different fungi: Alternaria helianthi and Alternaria zinniae. They're dark brown and can gain control quickly during the flowering stages. The infection is mostly seen in crops that are grown in the Northern states and is associated with high humidity and warm temperatures. If you plant sunflowers in soil that is already infected, there's a good chance you'll see spots during the early growth stages.

There are a couple of fungicides that can be used on crops to prevent an infestation. But, most growers prefer to use crop rotation methods. When they notice sunflowers that are infected with spots, they'll typically bury them, so they'll decompose quickly.

Disease Management

The strategies that are used to manage the onset and spread of diseases vary by grower. Some use fungicides and seed treatments. Others rotate crops. Most professional growers make use of hybrids that are resistant to specific infections. That said, there's an ongoing challenge to develop resistance to new strains of the most prevalent fungi. In all cases, it's important to limit the rotation of sunflowers in the same soil to at least 4 years. Not only does that help minimize the spread of diseases throughout the crops, but it also helps increase yield and soil productivity.