Down with Disease

This shows some marginal necrosis and the more water soaked (dark brown leaves) appearance on surrounding leaves on the pictures. Photos courtesy of Aaron Palmateer, Univ. of Florida, IFAS Tropical Research and Education Center, and OHP.

It’s been reported in the upper Midwest in places such as Minnesota, Illinois, Michigan, Indiana and Pennsylvania. It’s also been sighted in upstate New York, Long Island, Cape Cod, South Florida and in areas of Canada last year. To date in 2012, it has been confirmed in landscapes in central and south Florida and Dallas.

It’s downy mildew. One of the biggest issues facing the horticultural industry recently has been the outbreak of this fungal disease. Whether it’s through water or by air, the spread of downy mildew has been rampant across the United States. The disease has several different forms and each one is causing growing problems for growers, landscapers and the plants the disease attacks.

Downy mildew is not new to the industry. In fact, it was first seen in the late 1800s. More recently, downy mildew was reported in production greenhouses in 2004, with the widespread outbreaks growers are experiencing today developing within the past two years.

But what exactly is this disease? Downy mildew is a fungus that produces two different kinds of spores, which contribute to how easily the disease spreads and the degree of difficulty it takes to prevent it.

“One type of spore is called zoospores and that can travel through water and wind,” says Mark Seguin, vegetative marketing manager at Sakata Ornamentals. “That really accounts for how rapidly this can spread from one location to another. The other type of spore it produces is called oospores. That forms in plant tissue and so that is how the disease can remain in a bed year to year.”

Due to the ease with which downy mildew can spread and recent mild winters across the country, growers are once again going to have to be on the lookout for downy mildew in their plants. Here’s how growers can understand what helps the disease thrive and the necessary steps they should be taking to prevent downy mildew.


Know its favorable conditions
Areas such as the upper Midwest, the Northeast and Florida have seen the largest reports of downy mildew in impatiens due to the environmental conditions those places have during prime growing season.

These “untreated Inoculated” were taken at the beginning of trial. The plants appear to be stunted, leaves turned under and flowers not forming abundantly.

“The pathogen that causes impatiens downy mildew, plasmopara obducens, is an oomycete or water mold,” says Colleen Warfield, corporate plant pathologist at Ball Horticultural Co. “As the name implies, infection and disease development is highly influenced by moisture, both high humidity and free moisture on the leaves. Cool temperatures between 59 and 73 degrees Fahrenheit and at least four hours of leaf surface moisture or high humidity are required for formation of sporangia, which is the white down observed on the underside of infected leaves.”

While daytime temperatures in many of the reported locations may reach levels much higher than this, it appears that cool nights promote disease development.

“High humidity, cool temperatures and reduced airflow are the perfect conditions for this spore to develop,” Seguin says. “The spores can remain in the soil for about five years unless they are exposed to very cold temperatures like minus 5 degrees Fahrenheit.”

In years past, these optimal types of conditions did not occur with enough consistency for downy mildew to be a big issue. Recently, the conditions have been such that downy mildew is in breakout mode.

“In order to have a disease you really have to have the right environmental conditions for it to be a real problem and I think that’s what we saw this past year,” says Nancy Rechcigl, technical representative at Syngenta. “The humidity and moisture levels have to be perfect for the disease to really explode the way it did.”

Recognize the symptoms
While downy mildew causes a number of symptoms on the impatiens it affects, those symptoms can be hard to identify early on.

Growers need to look for the white, downy-like growth of spores on the underside of leaves.

“It can be really difficult to tell that a plant is infected with this particular downy mildew at first,” Rechcigl says. “One of the things that this disease causes is sporulation and sporulation occurs on the underside of the foliage. Unless you’re actively checking regularly and looking at the underside of the leaves, you can have plants that virtually look fine from the top surface, but they could be sporulating on the underside.”

What happens is over time as the disease starts to progress, you’ll get symptoms that can look like other things.

“This disease can be mistaken for some nutritional imbalances or even a spider mite infestation because you get that yellow kind of speckling,” Rechcigl says. “Generally, one of the early signs of the disease is that the leaves will be a dull green and start to turn more pale in color. As the disease advances you might start to see that the foliage starts to curl downward and new growth can be distorted.”

Over time you’ll see the plant begin to wilt. It will start to defoliate and the lower leaves will start to fall from the plant.

“That’s when, unfortunately, people start to notice the disease at that point,” Rechcigl says. “Up until that point you can have a plant that’s sporulating and actually looks quite healthy from the top surface.”

The symptoms of downy mildew usually occur in young plants or new growth.

“The leaf curl is generally a tall-tale sign,” Seguin says. “Also, on the underside of the leaf, there will be a whitish growth. That’s the downy spores forming on the underside of the leaf. That’s when you know you really have it. That’s quickly followed by leaf and flower drop. It can be very aggressive and very quick to where all you have left are bare stems and a dead plant soon after that.”

The symptom that gives the disease its name is the white downy that forms on the underside of the leaf. This symptom is what growers need to be on the lookout for.

“A white, downy-like growth composed of spores may be visible on the underside of infected leaves under cool temperatures and moist or humid conditions,” Warfield says. “When scouting, it is very important to turn leaves over to observe the undersides for this white growth. The time from infection to the appearance of symptoms varies from about five to 14 days depending on the age of plant tissue, temperature and humidity. The latent period between infection and visible plant symptoms is of concern because infected plants could potentially be shipped or moved without even knowing there is a disease problem. This highlights the importance of preventive control at both the production and young plant producer level to ensure cuttings and seed-produced plugs are kept free of this disease.”

If the disease continues to progress, eventually the leaves and flowers will drop, resulting in bare stems with only a few tiny, yellow leaves remaining.

“This symptom is more likely to be observed in a landscape setting where symptoms and early signs of infection are more likely to go unnoticed, in comparison to a greenhouse setting where plants are frequently scouted and suspect plants would be quickly rogued,” Warfield says.

Impatiens downy mildew only affects impatiens walleriana and its hybrids. However, impatiens downy mildew isn’t the only form of the disease.

“There are many different species of downy mildew,” Warfield says. “Most species of downy mildew are host specific, which means they infect only one or a few closely related plants. The species of downy mildew that infects impatiens walleriana only infects impatiens walleriana and a few wild species of impatiens.”

While plants such as coleus, snapdragon, alyssum and rose are susceptible to downy mildew, each is infected by a different downy mildew species that is specific to its host plant.

“The downy mildew species that infects impatiens will not spread to coleus and the species of downy mildew that infects coleus will not spread to impatiens,” she says. “All varieties and intra-specific hybrids of impatiens walleriana are susceptible to impatiens downy mildew including both vegetative- and seed-produced impatiens walleriana. However, there is no evidence that this pathogen is seed transmissible. There are no marketable differences in susceptibility among varieties. New Guinea Impatiens including Fanfare, Divine, Celebration, Celebrette and SunPatiens are highly resistant to this disease and have been successfully grown in greenhouses and landscape beds even under high disease pressure for downy mildew.”

Implement prevention techniques
Prevention of downy mildew is essential to controlling its outbreak. Growers need to take prevention seriously or the disease will beat attempts to stop it.

It is crucial to incorporate fungicides in a rotational spray program for prevention of downy mildew.

“It’s much easier to prevent the disease in general than to cure it,” Rechcigl says. “We know that it doesn’t travel with the seed impatiens, so separating production and separating vegetative cuttings from your seed impatiens is also important.”

The first step in prevention is making sure that your growing environment eliminates ideal conditions for downy mildew.

“Because moisture favors the development of downy mildew, cultural controls should focus on reducing greenhouse humidity and limiting leaf moisture,” Warfield says. “Good air movement through plant spacing and horizontal air flow, adequate drainage and irrigating when leaves will dry most quickly will help to minimize leaf wetness and humidity. Avoiding high humidity and leaf wetness at night is especially important in limiting secondary infections.”

The next critical step is frequent scouting of the crop and paying attention to early leaf symptoms for detection of downy mildew.

“Growers should make sure their staff is familiar with and trained to recognize early symptoms of downy mildew,” Warfield says. “Turn the leaves over and look at the underside of the leaves for the characteristic white, downy-like growth, keeping in mind it may be very sparse and barely visible. It is best to recognize the early symptoms before the pathogen begins to sporulate.”

If an infected plant is discovered, it is crucial that growers properly dispose of the plant as a sanitation measure.

“If an infected, sporulating plant is discovered in the growing facility, place the plant in a bag and seal it before moving it out of the greenhouse to minimize the spread of spores,” Warfield says.

If downy mildew does take hold, the implications are devastating. It destroys the plants and could remain in that garden for years.

“If you’re in an area where downy mildew has been present in the past or you’re at risk, it’s important that growers consider alternatives to impatiens walleriana, particularly for commercial application,” Seguin says.

Implement fungicides and spray programs
Aside from alternatives to impatiens, scouting for symptoms and making sure plants don’t remain wet and have good air flow, growers must be utilizing fungicides and spray programs.

“Growers need to use a rotation program using different fungicides from different FRAC codes or different mode of action groups,” Rechcigl says. “FRAC group 4, 11, 21, 33 and 40. There are a number of different FRAC codes that need to be used in a rotational program. This will make sure that the material they are shipping out is as clean as possible and that it will establish well once it gets into the landscape.”

Because there are multiple ways for this pathogen to enter into a greenhouse facility, there is always some risk for downy mildew developing in the crop.

“Preventive fungicide applications are extremely important in managing this disease,” Warfield says. “The three most important things to remember are to apply fungicides preventively, rotate among products with different modes of action, and be timely in re-applications.”

Growers need to recognize it is crucial to use fungicides incorporated in a rotational spray program as a preventive measure.

“Resistance and selection are two terms that are thrown around,” says Andrew Seckinger, marketing manager at OHP Inc. “In any given population there are organisms or downy mildews that are less susceptible to a certain causal agent, such as strobilurin materials. If you continue to just use them, you basically give these selective organisms an advantage over their competitive downy mildew peers. … If you rotate between chemical classes … you’re taking away that competitive advantage from those selective organisms … .”

FRAC codes or modes of action numbers represent how the chemical compound reacts on a fungal organism. It’s important to understand the FRAC mode of action code of the fungicides that are within a disease program. Many of the most popular and recognized fungicide trade names may be in the same FRAC class and should not be consecutively rotated.

“An example of this is the Qol fungicide class that includes popular brands such as Compass, Disarm, FenStop, Heritage and Insignia,” says Roger Storey, vice president of turf and ornamental, SePro Corp. “These are the same mode of action and should not be used consecutively, and, according to FRAC, are high risk for development of resistance. Rotating different modes of action reduces the potential of selecting for resistant strains. Systemic fungicides are generally at higher risk for developing resistance and, therefore, it is a good practice to combine them with a protectant like mancozeb.”

Just as with FRAC codes, it is important that growers read fungicide labels to make sure the right fungicide is being applied.

“Choosing the product and reading the label and using it in the manner with which it’s labeled are always the best directions for a grower,” says Kathie Kalmowitz, technical specialist at BASF. “The specific grower or the person who’s going to buy the product needs to actually look to make sure their use site is on the label.”

Along reading labels and rotating your fungicides, it is important to make sure the right parts of the plant are getting fungicide applied.

“Downy mildew, while it is a water mold, it is basically a foliar issue,” Seckinger says. “Drenching stains aren’t really going to be effective on downy mildew without that foliage coverage. What I recommend to people is a good thorough foliage spray. Depending on the compound, you would use a good spread or sticker material to hold it on there, particularly if you’re in a situation where you have overhead watering or landscape irrigation.”

To cover the proper areas of the plant, it is important to use spray methods that will ensure the whole plant is covered. “You want to have a very fine spray particle size like we associate with some of the air blast sprayers or ones that have this mist type appearance when you release them in the greenhouse,” Kalmowitz says. “With an air blast, you have really fine particles and the air blast itself picks up the canopy and those particles surround the plant and therefore you really do get a spray that covers all areas of the plant. Those are excellent, however, in many cases a spray to run-off when you’re trying to make sure you’re protecting the stem tissue as well as the entire leaf from top to bottom is better. That means you’re using a greater volume of water. There is a benefit to that because you know you’re getting protection all the way through the canopy.”

These methods depend on where you are in your production schedule and if that plant has a big canopy already or if it’s in an earlier stage.

“There’s a place for both types of sprays and it depends on plant size and also how you think your production is,” she says. “If you feel like you haven’t seen any downy mildew and you feel like you’ve really kept it out, then you probably could do that air blast application and not use quite as much water.”

No matter what fungicides you choose to use or what methods you use to apply them with, you have to make sure you are doing everything you can to prevent downy mildew from infecting your crop.

“Right now with the very vigorous downy mildew that seems to be causing a lot of problems, not only in production but in the landscape this year, growers are going to have to have both a preventive approach and they are also going to have to have those contact fungicides,” Kalmowitz says. “They aren’t going to be able to rely on one or two applications and feel that they’re sending a crop out the door that’s 100 percent clean, and of course that’s what their goal is. They want to make sure that they’re producing the best plant and that they’re sending it out the door clean.”

While preventive fungicides are crucial to protecting your plants, sometimes it is best to simply dispose of an infected plant.

“While some fungicides can give limited control when used as an early curative, it is not realistic to eradicate downy mildew from an infected plant,” Warfield says. “Once a plant is infected, it should be removed and discarded in the trash. Infected plants cannot be cured. If conditions are right and infection goes undetected, it only takes a matter of days for an epidemic to ensue.”
 

Gregory Jones is a freelance writer for Greenhouse Management.