Found in nearly every state across the U.S., rose downy mildew has been a serious problem over the past two decades. Here are some tips on cultural practices and fungicides used to control the pathogen.

During the past 20 years, rose downy mildew has become a serious problem, causing significant losses to growers almost every year. The disease has been found in nearly every state across the United States (and Canadian provinces). It does not occur each year since conditions that promote it are not always present.

Various downy mildew diseases can occur on many plants (including roses) in the nursery or landscape, but not all of these are caused by the same pathogen. The downy mildews on most vegetables and flowers are distinct — they do not cross from one plant to another as a general rule, unless the disease hosts are from the same plant family. The cause of rose downy mildew is Peronospora sparsa, which also attacks certain cane berries (blackberry and raspberry) and all types of roses, both wild and cultivated.

Although it may seem that your bare-root roses come with downy mildew, they may be infected from other sources. Roses in landscapes or natural areas around your production site, as well as crop material left over from previous seasons, are likely sources of a downy mildew outbreak. The spores can live in these roses in a dormant state only to become active when the cool, wet weather of late winter and early spring occurs. Be sure to check last year’s roses carefully for signs of downy mildew just as often as you check those in this year’s production. If you are not checking frequently, your first indication of trouble may be extensive leaf drop!

Symptoms

Rose downy mildew is remarkably variable. In the past 20 years the most common symptom has been angular, tan spots with a very small amount of white crystalline sporulation on leaf undersides. The name of the pathogen —Peronospora sparsa — reflects the sparse production of white spores. Sometimes collapsing buds or cankers form on newly emerging rose canes. Different species and cultivars of roses respond differently to downy mildew, making a diagnosis very difficult. Severe leaf drop is common on some cultivars.

Tiny speckles may form that are tan or even bright pink. Such symptoms are difficult to distinguish from early infections with Botrytis, powdery mildew or even rust. Under moist conditions, leaves turn brownish or yellow and drop. A white, downy fungus growth may occur on leaf undersides, but this is usually too hard to see without a microscope. Under drier conditions, leaf spots appear as brown burned areas, mimicking spray injury. Reddish streaks or sunken cankers can also occur on rose canes. Since similar cankers can be caused by other pathogens like Botrytis and Coniothyrium, it is a good idea to have canker symptoms diagnosed by a university or private lab. Sometimes downy mildew sporulation can be observed directly on discolored spots on canes if they are first incubated at a high humidity for a few days.

Cultural Control

Symptoms can appear almost overnight and a severe outbreak can take off if you do not act preventively. Fans and wind easily move spores from plant to plant but are your best allies in preventing downy mildew outbreaks, since they improve air circulation around plants, dry leaves rapidly and reduce relative humidity. Downy mildew weather is cool to warm, humid and rainy. Wet leaves and high humidity will trigger sporulation overnight. When the sun comes up, leaves start to dry and spores are released. Most spores spread by fans or wind and infect new leaves before noon. Six hours of constant leaf wetness are enough for spores of many downy mildew fungi to germinate and infect leaves. Even though fans might move spores, you should use them along with venting to reduce humidity and leaf wetness. Try to water early in the day or whenever leaves will dry quickly, to ensure dry foliage at night. Check all new plants for symptoms upon receipt. Early detection is the only way to control downy mildew. Scout all plants at least once a week, preferably every two to three days. Remember: hungry plants are more susceptible to downy mildew. Maintain a balanced fertility program to protect your crops.

The cultural steps that should be taken when growing garden roses are:

1. Destroy rose debris from previous crops — spores can overwinter in leaves and canes, then the downy mildew can attack new plants.

2. To protect them from infection, isolate new shipments away from last year’s plants.

3. Space plants to allow rapid drying of leaves. If the leaves are touching, they are too close. As the canopy closes in, the humidity increases.

4. Water at a time of day that will promote the fastest possible drying.

5. Apply fungicides preventively.

Fungicides for Control

Choosing the most effective fungicides to prevent or eradicate rose downy mildew can be tough. Although plant pathologists usually like to lump a lot of trials on a single type of pathogen together, we sometimes find that specific results occur on specific crops. This has been the case with rose in our experience. Start early with a rotation of chemicals for prevention.

Chase Horticultural Research, Inc. performed a trial on greenhouse potted roses in 2004. The roses were infected prior to spraying twice on a 10-day interval. Results showed that in an eradication scenario, the strobilurins were not effective on rose downy mildew. Heritage and Insignia were not effective used at very low rates. Cygnus (no longer available) and Compass O were more effective (also used at very low rates) but were still not significantly better than the water-sprayed controls. This is not an unusual result for rose downy mildew based on reviewing all of the trials that have been published or otherwise made available. Using even the highest labeled rates of the strobilurins is rarely as effective on roses as it is on nearly all other plants affected by downy mildew fungi.

In the same trial, excellent control was seen with Segway (all rates tested), Stature DM,

Aliette and Subdue MAXX (used as a foliar spray in this case). Be sure to tank-mix Subdue MAXX with another product for downy mildew control if you are going to spray the product (as per label directions). Remember that the 0 REI is for drenching Subdue MAXX and use as a foliar spray for downy mildew requires a 48 hour REI.

Do not use more than 2 oz/100 gal of Aliette, as higher rates have been shown to reduce downy mildew control on some plants.

Over the past 15 years about 15 trials have been performed from Florida to Alabama to California in the greenhouse and in the field.

The most consistently effective products for downy mildew on roses have been Subdue MAXX (1 to 2 oz/100 gal) used as a foliar spray, Micora (8 oz/100 gal), Stature SC (12.25 oz/100 gal), Aliette (2.5 oz/100 gal) and Segway (3.5oz/100 gal). Remember to follow labels carefully. Subdue MAXX and Adorn must be tank-mixed with another product from a different MOA group that is effective on downy mildew (they can be mixed with each other, for example). You cannot rotate products with the same MOA group number. That means products in MOA group 40(Stature, Micora and Orvego), although really effective, cannot be rotated with one another. Similarly, if you decide to use a strobilurin (or Fen-Stop), these materials all belong to MOA 11 and rotation dictates using something from another MOA group.

Another fact that appeared in reviewing the available trials was that a shorter interval is usually more effective. Thus, using a fungicide once a week is better than using one once a month. Using one every 10 days is better than every 14 days. However, for a spray program to be effective you must remember to apply it. If a 10-day interval is hard to implement, you should switch to every seven days (curative) or 14 days (preventative). Weekly spraying to prevent downy mildew on sensitive crops is common at times of year when environmental conditions favor disease. Spraying more often may be counterproductive.

A final word — make sure you are treating the right disease. It is surprising how many times roses are being sprayed for black spot when they really have downy mildew. Get a diagnosis!

A.R. Chase is plant pathologist at Chase Agricultural Consulting LLC and can be reached at archase@chaseresearch.net. Margery L. Daughtrey is senior extension associate at Cornell University’s Long Island Horticultural Research & Extension Center and can be reached at mld9@cornell.edu.

“This article originally appeared in the May 2013 issue of GPN magazine. Copyright Scranton Gillette Communications.” Re-printed with special permission from GPN Greenhouse Product News Magazine, Scranton Gillette Communications, May 2013.

Disease is a major source of plant damage in roses, with the most common rose diseases being caused by fungus. While some rarely-contracted rose diseases are caused by viruses or bacteria, more often than not, it is fungi that wreak havoc in our rose gardens. Nationwide, blackspot fungus (Diplocarpon rosae Wolf) is the most common rose disease. Powdery mildew (Alphitomorpha pannosa) runs a close second in its commonality, but unlike blackspot outbreaks tend to be seasonal. Occasionally, outbreaks of downy mildew (Peronospora sparsa Berkeley) and botrytis blight (Botritis cinerea) may be found, but these diseases appear much less frequently than either blackspot or powdery mildew.

Damage due to fungal attack can range from one extreme to another on the seriousness scale – from repeated loss of leaves (weakening the plant) to less damaging symptoms of minimal lesions (spots) on foliage and canes. Roses with high levels of infection produce less new growth and fewer blooms because their leaves (which are needed for photosynthesis) are affected by disease and fall off. Because of the important role that foliage plays in the overall health of the rosebush, repeated defoliation ultimately impacts the health and longevity of the plant.

Roses differ in their susceptibility to fungal attack, with modern hybrid teas being the most susceptible. If left untreated, infected susceptible roses can lose a large percentage of their leaves. Fifty years ago, Dr. Griffith Buck identified the correlation of leaves to plant hardiness, finding that roses with the ability to hold onto their leaves even when infected by fungus were hardier, more disease resistant landscape plants. This correlation has been reinforced through the Earth-Kind Rose Research Program being conducted through a partnership between the Texas AgriLife Extension Service and the Houston Rose Society.

Managing Fungal Diseases

Fungal diseases are best managed through a multidisciplinary approach that includesplant selection, planting times, level of fertility, sanitation and applications of fungicides. Some roses exhibit more tolerance than others to attack by common diseases. The susceptibility of a rose variety to disease will dictate the management practices that must be employed to maintain the health of the garden site.

There are many roses on the market that have exhibited tolerance to disease; however, there is an even larger population of roses that are highly susceptible to fungal diseases. The gardener’s job is to balance plant selections so that management of fungal diseases in their roses can be attained within the maintenance parameters the gardener has available. Planting young new roses at times when environmental conditions are ripe for disease development, without protecting those plants against attack, tends to invite the disease process. Some hypothesize that excessive fertilization in early spring and late fall (when weather conditions support disease formation) can initiate the process. There is some logic to this theory as blackspot and powdery mildew tend to attack new plant growth first. Heavy fertilization encourages the development of new growth that, if left unprotected, is fair game for fungal pathogens seeking a host. However, cultural practices alone will not eliminate diseases from the garden.

“Fungicides” are a specific type of pesticide that control disease by inhibiting or killing the disease-causing fungus. They work by attacking and damaging cell membranes, interfering with energy production or interfering with the life cycle of the fungus.

Some of the newer disease treatments on the market do not directly affect the fungus itself, but instead boost the plant’s natural defense system causing the plant to produce thicker cell walls and anti-fungal proteins so that the plant is better able to defend itself when disease pressures are high. Examples of this would be products like Messenger and similar “biorational” treatments.

While most fungicides are only capable of protecting uninfected growth from disease, only a handful are effective against pathogens once infection occurs. For this reason, to be effective most fungicides should be applied when environmental conditions are right for disease development, before disease occurs or at the first appearance of symptoms.

Types of Fungicides

Fungicides fall into two categories — “curatives” (products that attack fungi that have already infected the plant) or “preventatives” (products that inoculate plant tissue so that the plant is better able to fight off disease). Fungicides with curative properties would include products sold under the trade names, Mancozeb, Manzate and Fore and products containing maneb. Preventative fungicides would include products such as Rose Pride (formerly known as Funginex), Rose Defense, Daconi and products containing Neem oil

Some homeowners start applying fungicides to their roses only when the plants lose all their leaves and then stop spraying once new growth emerges. Other gardeners use fungicides for the wrong purpose — for example using a preventative on a bush exhibiting symptoms of heavy blackspot infection. These treatment approaches are wholly ineffective in controlling fungal diseases.

Once defoliation occurs, preventative fungicides become ineffective. In this instance, applications of a curative fungicide, such as Mancozeb, applied every five days for three weeks (or as directed on the manufacturer’s label) is critical to establish disease control. I have found that it takes three weeks of regular applications of a curative fungicide in addition to good garden housekeeping to reestablish disease control. The need to spray should be dictated by the four factors: the health of the rose, the environmental conditions, including the level of disease pressure that may be present, the disease susceptibility of the rose and the desired level of plant perfection.

Disease Control With Fungicides

Fungicides come in powder, granular and liquid form. Most are mixed with water and applied by spraying the mixture onto the plant. A few fungicides are applied to the soil where they break down into compounds taken up by the plant making the plant toxic to the disease. For spray applications, coverage of all parts of the plant (upper and lower leaf surfaces and from the top to the bottom of the plant) is critical because very few fungicides have the ability to be absorbed into plant tissue and move through the plant. One exception would be Aliette, which is applied as a spray treatment and then translocates through the plant’s system from the leaves to the roots and back to the foliage on a “seek and destroy” mission.

On susceptible roses, effective control of fungal diseases may necessitate multiple applications of fungicides — sometimes as frequently as once weekly from the first flush of new growth in the spring until the first hard frost in the fall. These repeated applications are necessary to protect emerging new growth and to replace fungicide product lost by decomposition, degradation by sunlight and/or removal by wind and water. Unfortunately, repetitive applications of the same fungicide can lead to the fungus developing resistance to the treatment — making treatment applications ineffective.

Keeping susceptible roses healthy requires some application of fungicides, especially when conditions support disease development. Blackspot spores germinate and infiltrate cell tissue when temperatures are between 65 – 85° F and when moisture is present on the foliage for seven hours or more. Powdery mildew tends to appear when humidity is high but conditions are dry and cool (warm days/cool nights).

Fungicide labels contain the words “contact” and “systemic”, which is a general description of the method by which the product kills fungal pathogens. Contact fungicides are applied to and remain on the plant surface and do not penetrate into plant tissues. Contact fungicides must come in contact with the disease-causing pathogen to be effective. Examples of contact fungicides are Mancozeb, Fore, Manzate and Daconil.

Often these types of fungicides leave a spray residue on leaves and stems, and only the parts of the plant with spray residue are protected from infection. Contact fungicides are very sensitive to the environment and are usuallyf effective for seven to 14 days, however product life depends on the amount of rain/irrigation the bush receives and the amount of UV rays the chemical is exposed to. One to two inches of rain will reduce the residue of contact fungicides by half, but rainfall of two inches or more will eliminate the product from the plant. The product label will provide direction on application frequency.

Systemic fungicides, sometimes called “penetrants”, are absorbed into the plant and have the ability to move from the application site (similar to how blood moves through our bodies). The distance that systemics are able to move within the plant is dependent on the nature and type of fungicide used. Some systemic fungicides stay in the leaf tissue. Others have the ability to travel from the leaves to the root system, but not back up through the plant structure. Only a few systemic fungicides, like Aliette, can travel up and down freely within the plant. Some of the common systemic fungicides used in rose gardening are Aliette, Fertilome Liquid Systemic Fungicide, Monterey Fungi-Fighter, Rose Pride (Funginex) and Bonide Systemic Fungicide.

Mode of Action – How They Work

How a fungicide works is called its “mode of action.” Fungicides are manufactured in such a way that their modes of action attack either “single” or “multiple” sites within a fungus. Single-site fungicides target one critical component (usually an enzyme or protein) needed by the fungus for survival. The slightest mutation of the fungus impedes the effectiveness of the fungicide treatment by camouflaging the product’s target. Fungicides with single-site modes of action are Rose Pride (Funginex) and Green Light Systemic Fungicide.

Genetic mutation of fungi is more common than we may think. There are 54 known variations of blackspot fungus in North America, and these variations or “races” of the disease are frequently geographically specific. Roses resistant to one race of blackspot fungus may prove susceptible to other races of the disease.

Fungicides designed to attack multiple sites are usually effective against different types of disease components within a fungus. These types of fungicides treat a variety of disease components so that in the event that there is a mutation of one particular enzyme/protein in the pathogen’s make-up, there are usually other non-mutated disease components for the fungicide to attack. Contact fungicides typically affect multiple sites in fungi. Examples of multi-site fungicides are Mancozeb, Manzate, Aliette, FertilomeLiquid Systemic Fungicide, Ortho Garden Disease Control and Daconil.

To Spray or Not to Spray

In North Texas it is not uncommon for hybrid tea roses to require 15 to 20 applications of fungicides per year to maintain plant health. Along the Gulf Coast, environmental conditions can dictate even more applications and for a longer period of time. Sometimes even hardy roses may require a few spray applications when environmental conditions support disease development and disease pressures around the garden are high. A good disease management approach includes cultural and environmental controls, in conjunction with the right type of fungicide, including:

• Plants must get six to eight hours of full direct sunlight;

• Having adequate spacing between plants (a minimum of 2 feet between mature plants);

• Avoiding wetting the leaves during irrigation and not watering plants at night;

• Removing leaf litter in and around the bushes;

• Moderate fertilization; and

• Rotation of fungicide products that have different “modes of action” to delay the development of the resistance to chemical treatments.

A gardener need not hold a Ph.D. in chemistry, toxicology or plant pathology to effectively treat fungal diseases. With all of the rose care products on the market today it may seem a daunting process to select the right product for the disease process in your own garden. This is where a local consulting rosarian can help.

Gaye Hammond (gayeh@lpm-triallaw.com, ‘Fungicides Made Simple’, Summer 2013. Basal Breaks, Jeff and Cindy Garrett (rirjeff@aol.com), eds., Tri-State Rose Society of Chattanooga.