With the ability to wipe out large portions of sunflower crops and significantly decrease yield, growers know that downy mildew is one of the most devastating diseases to strike sunflower fields. A soilborne pathogen that travels via water with “swimming” spores, downy mildew can be costly for growers. However, by monitoring and proactively employing the right cultural and chemical management practices, growers can manage the disease and preserve yield.
Downy mildew: know the enemy
Downy mildew can occur in all sunflower-producing regions of the U.S. and is most common early in the season, right after planting. The disease spores enter the roots of sunflower and systemically invade the rest of the plant, causing severe stunting and oftentimes killing plants.
“Downy mildew is a very important disease in sunflowers because of the potential damage it can cause,” said Sam Markell, plant pathologist with North Dakota State University Extension. “An infection from this unusual pathogen can result in 100 percent yield loss to the infected plants.”
If infected plants are able to survive, they develop thick, yellow leaves and a white cotton-like substance appears on their undersides. According to Markell, if sunflower growers see these symptoms in their fields, it’s probably beyond the point of control.
“There is a lag time between the moment of infection and when downy mildew symptoms start to show on sunflower crops,” he said. “By the time growers see symptoms, it’s already too late. Downy mildew cannot be managed after it occurs.”
Favoring cool, wet conditions, downy mildew is most prevalent in low-lying areas of sunflower fields with poor drainage, where water can pool and downy mildew spores can freely travel and infect large groups of plants. If infections in a field are spotty and only affect a few plants, resulting yield loss is minimal since healthy sunflowers have the ability to compensate for infected neighboring plants. However, Markell said downy mildew commonly infects large patches of sunflower fields at a time.
“Big patches of downy mildew result in very significant yield loss, which is what we tend to see much of the time,” he said, adding that secondary infections of the disease can also occur, but its effects are starkly different than a primary infection. “We are most concerned about the primary systemic infection. Secondary infections of downy mildew will result in spots on foliage, but that’s about it. Secondary infections of downy mildew will not damage plants or affect yields.”
Downy mildew resurfaces every year, but not always in the same locations, Markell added. Depending on environmental conditions, fields of all sunflower growers, particularly in northern states, are at risk of contracting the disease.
Despite the potentially devastating effects of downy mildew, there are management options for controlling the disease before it destroys yield.
Monitor fields and pinpoint prone areas
Each year, growers should assess their risk for downy mildew by monitoring their fields, identifying low-lying areas and observing environmental conditions like cool temperatures and prolonged rainfall. If growers feel at-risk, they can plant sunflower hybrids that are tolerant against downy mildew, including nine Syngenta sunflower seed hybrids. However, genetic tolerance levels vary and may not provide protection in severe downy mildew infestations.
“Growers should determine their risk for downy mildew because there are areas that will be infected year after year,” Markell said. “For the growers whose fields are more susceptible, varieties with genetic resistance are very important. The infection can happen quickly, and it only needs to happen once.”
The pathogen stays with its sunflower host until it reaches maturity when it forms an overwintering structure, which allows downy mildew to stay in soils for up to a decade. Unfortunately, cultural practices like crop rotation won’t mitigate the disease out of infected soil.
“Although crop rotation has a minimal effect on downy mildew management, growers can work to control volunteer or wild sunflowers near their fields,” said Tom Schmit, South Dakota-based agronomic service representative at Syngenta. “These unwanted plants can provide a host for downy mildew.”
In addition to planting tolerant varieties and eliminating volunteer sunflowers, growers can proactively protect against downy mildew by applying seed treatments before planting.
Treat your seed right
By applying a seed treatment fungicide, growers can prepare for downy mildew infections before the season even starts. In fact, a fungicide seed treatment is the only effective crop protection product for downy mildew control, providing additional protection for less tolerant hybrids and areas most at-risk for the disease. “Foliar fungicide applications may prevent other diseases, but they won’t protect crops from downy mildew,” Schmit explained.
“Sunflowers that have been treated with certain fungicides are much better poised to fend off downy mildew than untreated seed,” Markell said. “I recommend that sunflower growers apply seed treatments for protection from downy mildew and other pests,” advising that growers should be proactive rather than learn the hard way.
Because the disease is known to evolve and adapt quickly, Syngenta will offer Bion® fungicide as an optional component of CruiserMaxx Sunflower seed treatment insecticide/fungicide beginning in 2014. Schmit added that Bion has been extremely successful in field trials, and Syngenta is looking forward to sharing the new option with growers seeking additional downy mildew protection.
While downy mildew can be a nightmare for sunflower growers, crops can be protected through proper field assessments, planting resistant varieties and applying seed treatments like CruiserMaxx Sunflower.
“Once growers see downy mildew, they never forget it. The symptoms are distinct and devastating. There aren’t a lot of diseases like it. It’s unmistakable,” Markell said. “I tell growers to know their fields and take steps to solve downy mildew issues before they begin."