Part 1 - Disease Pressure - Internal vs External
Part 2 - The Amplification Effect
Part 3 - Avoidance of RRD in a Garden
Part 4 - Chemical Control of RRD

  Part 1 - Disease Pressure - Internal vs External

The term "Disease pressure" isn't listed in the American Phytopathological Society glossary, nor is it in the dictionary. But, it is a phrase that can be useful for assessing both potential and real problems in a rose garden that is at risk from RRD. I use the term "Disease Pressure" to describe the amount of risk of RRD a garden is facing (that impacts a rose or a rose garden).

For example, roses grown in glass (green) houses in the mountains of South America have zero RRD disease pressure. Roses grown within view of and downwind from a field of RRD-infected multiflora are under high disease pressure.

Based on the gardens we saw in our the first two years of looking at RRD, I came up with the following numbers.

RRD Disease Pressure based on % loss first year

% loss    characterization

< 1 %        caused by random mite drop
1-5%        not random, gardener should look up wind for sick plants or in garden for local
        pathogen source
5-20%        the source can be found, either upwind or in the garden
20-50%    RRD source(s) most likely visible from the rose garden
> 50%        Rose garden cannot survive

Zero Disease Pressure is where there is NO chance of RRD infection entering the garden. This would be a garden where no new roses are brought in, and where there is no RRD upwind of the garden. The upwind danger is the part that is often underestimated.

There is a difference between internal and external disease pressure. When there are roses with RRD in a garden, the internal disease pressure all but guarantees that mites will spread from the infected rose(s) to other nearby roses. A mite that develops from eggs hatched in that garden will be dropped by wind onto nearby roses and may decide not to move further. In a densely grown rose garden, if a mite falls from one rose, there is a high probability that it will either land on another rose or be carried to another nearby rose by minor puffs of wind. The mite could even walk to another rose as fast as its four legs could carry it. Internal disease pressure can be controlled by rogueing out disease plants as soon as they are recognized. It is internal disease pressure we have greatest control over and the one that can do the most damage if not controlled. The sooner the sick plants are removed, the faster this disease pressure is reduced.

A number that the Iowa State workers included in their 1997 paper illustrates the problem with allowing one rose with a year-old RRD infection to remain in a rose garden. "Previous research has shown that eriophyid mite populations in newly infected plants are low and that mite populations increase on plants 14 to 22 times the initial population levels in the 2nd year of symptom expression." (Epstein, Hill and Nutter, 1997, p.177) This is the equivalent of having 14 to 22 newly infected roses for every year-old RRD infeced rose in the garden. All of those mites will be infection vectors. I don't think that there can be a stronger argument for removing RRD sick roses immediately.

Internal Disease Pressure is defined by the continued presence of a RRD infected rose in a garden. If there are no mites in the garden (if there were totally effective acaricides to kill the mites), then one sick rose could be tolerated. The hitch in this system is that a rose with RRD grows rapidly (regardless of the system) and that the mites multiply rapidly in the roses with lots of fresh growth. Even if that infected rose were treated with Cygon 2E (a systemic insecticide) there is no information about the translocation speed of Cygon 2E within a rose bush, much less within a rose bush that is growing more rapidly than normal.

External Disease Pressure is a combination of factors as diverse as winds, topography, mite reproductive success, location of wild roses, condition of your neighbors roses, even quality of mainterince of agricultural fields miles away. Needless to say, we don't have total control over this but can try to exert some influence.

External Disease Pressure may be high in new subdivisions where soon-to-be suburban lands have been overgrown with multiflora that will live there until it's bulldozed so more subdivisions can be built. Land near interstate highways and expressways and tollroads may be an added pressure as multiflora was used as crash barriers, especially along the Washington D.C. beltway. Those barrier roses contributed a lot of seeds to where ever the well-roseseed-fed birds rested and left droppings. Roses were also used at horse farms, especially in Delaware and at some farms in Tennessee and Kentucky. In areas where mining happend, the extent of the multiflora problem can be seen at this website that discusses land reclamation in an anthracite area in Pennsylvania.
http://wilkes.edu/~kklemow/GG/New-Nan.report.html

Methods of killing Rosa multiflora are discussed in the following website from Alabama. A number of chemicals are listed. http://www.bugwood.org/weeds/forestexotics.html

Seasonal Multiplier

Because mite availabilty is the cornerstone of disease pressure, seasonal mite abundance is an important factor in determining pressure on a garden. It can be considered to be a multiplier or a third dimension to the disease pressure consideration. "Mite populations were low in spring, increased steadily during the summer and tended to peak in September. Populations on RRD-infected plants were 15x greater than on healthy plants." (Amrine et al. 1989, p. 317.) This warns us of when the threat to roses in southern Indiana is higher. If your growing conditions mirror those of southern Indiana, this is a model to use.

From infections I've seen in Tennessee, I expect that mite counts may spike in spring as well, but until someone does counts, this must remain a working hypothesis. Drought depresses mite populations (Epstein and Hill, 1997; Amrine et al. 1989), so the mid to later summer droughts may be a negative.

  Part 2 - The Amplification Effect

At first we were surprised to find RRD at great distances beyond what we thought was the RRD front. Then for two years we watched roses in Grainger County TN (our home). We have a lot of multiflora along fencelines in our county and when we first saw RRD in Claiborne County (one county north of Grainger) we saw no RRD in Grainger Co. In 2001, we saw RRD on two bushes in Grainger: one on a gallica in Washburn TN, and the other on a rambler near the Cleveland Cemetery just southwest of Washburn. That fall, the "RRD front" moved over (more probably around) Clinch Mountain and into our garden, but only on two roses. The following year, RRD appeared on one rose between Rutledge and Knoxville (a thirty six mile long drive). Two years later, at least 25% of the roses along fence rows along that road were sick with RRD.

Now it's 2007. About a third of the multiflora along the initially affected fencerows are infected.
In some areas, my roses and I are fortunate that the fencelines have been cleaned and nothing is there. None of the plants initially infected have died. Most still have some alive canes and not all of the alive canes have RRD. My area is turning into subdivisions, and that may be good as some fields that were becoming choked with multiflora now are cut for hay or groomed as front yards.
Less multiflora is good.
We have been trying to kill some RRD-sick multiflora along the road to our house, in our woods. For a puny plant that doesn't get enough sunlight, it keeps coming back (three years now) even though each time, we have thought we grubbed the above ground parts out and hit the remaining roots with concentrated herbicide.

This to me defines a moving front of RRD infection. Yet, at these times, we found infected roses at some distance (hundreds of miles) to the south and east of the front. Roses east of Atlanta in Watkinsville appeared to be ahead of the front. One valley in western North Carolina (in Yancey County) appeared in 2002 to be ahead of the front for that area, but that county had large quantities of multiflora along fencelines along ridgetops, and an acre field planted in roses in Winston-Salem NC was infected in 2001.We attribute Yancy Co. NC to "augmentation" based on the density of infection, but the others were more random. We've seen RRD east of Charlotte in several counties and up into Virginia.
This past year 2006, Dave Booty send me reports of RRD in the Reiman Rose Garden in Iowa as well as in a large private garden in Madison Wisconsin. This represents a northward expansion that mirrors an increase in RRD reported on the internet from the greater Chicago area.

When I asked Dr. Jim Amrine about this discontinuous distribution of the disease, he mentioned seeing this elsewhere (including in disjunct valleys in West Virginia) and he attributed it to an "amplification effect." Although he hasn't defined the term, and it's not a term in the literature, it was so descriptive that the meaning was obvious to me and it is quite useful.

Let's look at a hyptothetical county and its disease pressure. At some distance beyond the disease front, County X has an average of one RRD-carrying mite land on each acre of land. Most places in that county, there are few folks who grow roses and those roses are small. Even if the mite lands within a hundred feet of a rose, the odds of it finding its way to that rose by means of transport by puffs of air are very small. If that county has fencelines or creeks where 'R. multiflora' is abundant, the odds of establishing RRD infection rise, because that fence acts as a wind break that may drop any mite blown in its vicinity, because the mite will keep riding puffs of wind until it reaches a rose. If that mite lands in a yard like mine, it will land where ten percent of the ground is covered with roses. It has ten times the chance to survive than a yard that is only one percent rose covered. Puffs of wind will probably give that mite a ride to one of my roses. If that mite were to land in the middle of a field where roses are grown, the long term effect of that single mite would be amplified by the number of roses and the probability of the mite finding a place to feed easily and rapidly is amplified.

The moral of the story: Production fields in which 50% of the area is covered by roses are at risk from even the most random infected mite that has blown many miles and would most likely have starved to death. Mass plantings in a large rose garden are in the same catagory. Once the mite lands and feeds, we switch back to Internal Disease Pressure and it's a brand new situation with RRD spreading from there. So, the RRD front moves forward in clusters rather than as a line on a map.

  Part 3 - Avoidance of RRD in a Garden:

The comments in this section are my best estimates of how to deal with RRD over the long term as compared to the FAQ - How do you treat an infected garden, which deals with immediate actions. It is based on what we have learned during our study of RRD. We hope that we will learn more and have successes to report. I request that you the share your insights, experiences and suggestions with me by e-mail at: abpeck@att.net. I will not place any attributed comment here without your permission.

Avoidance is a principle of plant disease control in which plants are grown at times or locations where
the pathogen is inactive or not present. It works well for annual crops that are harvested quickly, but
less well for plants such as roses, american chesnuts and dutch elms.

When Rose Rosette Disease enters an area some gardens are under greater disease pressure than others
- see FAQ - What roses are at greatest risk? Those gardens fortunate enough to be under low disease
pressure may not develop RRD for years. Alternately RRD may be treated as a minor annoyance tied to rogueing out unproductive plants. Failure to remove an infected plant guarantees that RRD will spread within a garden regardless of the external disease pressure. You may have as long as a year before one rose infects its neighbors, but the time of infection will be determined by unpredictable weather and
culture conditions. As stated elsewhere in this book, you should look in your area for wild roses that can serve as a source of infection. By finding and removing them, you can reduce the disease pressure
facing your garden. Most of the disease pressure will be external in origin; when disease pressure
switches to internal pressure, your garden is at highest risk.

A worse case scenario is a garden down wind from a national park or other large tract of land owned by an uncooperative owner where established infected roses remain in large numbers. Untended fields,
abandoned from their former agricultural use and un-bush-hogged are excellent reservoirs for RRD.
This circumstance can define high disease pressure. Disease pressure is multiplied when one is too
conservative in rogueing out suspected RRD infected plants. If you can't bring yourself to remove a
plant, at least dig it up and keep it in isolation. Isolation can be physical as in at a distance from and
downwind of your roses. Isolation can also be chemical as in treating that rose with miticide on a
regular basis.

Many land owners will let you remove weed roses even if they are not willing to do it themselves or pay for it themselves. Such removal increases the value of their fields. Rose growers whose gardens are under heavy disease pressure are faced with three bad options; give up roses, accept a very high loss rates (we are seeing staggering losses in some gardens in east Tennessee and elsewhere) or greatly change rose cultural practices. My suggestions for avoidance of RRD fall into four categories; resistant roses, cultural practices, planting within beds, and garden layout. (Resistant roses are a hope rather than a reality, sadly.)

Unlike folks who deal with plants subject to viruses vectored by aphids, white fly, leaf hoppers and
other insects that are visible to the so-called naked eye, (more likely the bespeckled. bi-focaled eye ) the mites that transmit RRD are essentially invisible.

The question of roses resisting a virus.... becomes a question of a potentially infected rose that does NOT exhibit RRD symptoms. There is a possibility that a rose may be infected with RRD, but show
no symptoms. Until an ELISA test is developed, the only way an asymptomatic rose can be tested for RRD is to graft budeyes from healthy looking but suspect roses onto RRD - free 'R. multiflora'. The best time of the year for that grafting is Spring and even then, not all grafts successfully transmit RRD, even from plants with blatent symptoms. We know of no garden that has lost 100% of its roses; we do know of losses approaching 90%. Are there roses that are intrinsically resistant to RRD? (Fall, 2003, we now know of two gardens, one in Tennessee and the other in Virginia, that have lost all their roses.)

Resistant Roses: The need for ornamental roses that are resistant to RRD already exists although the
roses don't. In a more perfect world, rose breeders would have been working on this for several
decades, ever since Glen Viehmeyer sounded the alarm in The American Rose in the early 60s. No resistant roses have been introduced. Although "disease-resistant" roses are touted in sales brochures, the disease-resistance is selective and does not mean RRD. Dr. Zary brought it up in 1994 in the American Rose Annual, but as far as we know, nobody - not even his own company- followed his suggestions. Fortunately for rose growers, he's breeding beautiful roses. Unfortunately, they are susceptible to RRD. We worry that nobody has cared about this until it is too late. Genetically resistant ornamental roses are likely to be few in number if they exist at all. Roses that we expected to be resistant, such as ‘American Pillar’ (1/4 'Rosa setigera') have turned out not to be (two examples of infection, one in Georgia, the other in Alabama). (And a third American Pillar in my own yard in 2006.)

I am hesitant to recommend the few roses I suspect might have resistance without testing them first, because so many I had hopes for have turned up sick in gardens. That will take time. Even now, many rose growers have never heard of RRD or worse yet, believe the misinformation put out by proponents of RRD as a weed control and think it only infects 'R. multiflora'.

Even some state websites indicate that there are resistant roses. Unfortunately a lot of "resistant roses"
have proven to show lots of symptoms of RRD when moderate disease pressure reaches them and deposits disease-bearing mites on susceptible growth areas of the roses.

In addition to a genetic resistance, there are two other ways for a rose to have RRD resistance. One is
for the rose to present a physical barrier to the mite. If a rose is “thick skinned” - that is to say, the
cuticle of the rose is so thick that the mite can not bite through, it that can afford some protection. This
is a characteristic the folks at Iowa State attributed to ‘Bonica’. Unfortunately, at some stage of growth,
that is not the case. In Iowa, the mite migration and this growth stage may not overlap, but it does in
South Carolina, and Missouri so even these roses are vulnerable to infection. Based on what I had read from ISU, I had great hopes for ‘Bonica’ and its kin (and had started saving open pollinated seeds from 'Bonica' and its sport, 'Royal Bonica'), but we saw a badly infected bed of 'Bonica's at Orangeburg, S.C. (Pictured) and later in St Louis. Even if such resistance isn't perfect, it’s better than nothing, so I would have these roses in a garden that is under high disease pressure. We do grow the Bonica offspring, Royal Bonica, and are watching it.                    Click on Pictures to Enlarge

Infected 'Bonica' on left and on right, new RRD growth after an attempt to prune it off. The disease pattern does NOT mimic herbicide drift, it was much too sporadic with an infected cane here, skip a plant and an infected cane on the next side of the third or fourth bush. Where we have seen herbicide drift, it affects uniformly with decreasing intesity to the margins of the drift, it' s not a checkerboard pattern of effects that affects one cane but not the four same aged canes closest to it.

The other way a rose can be resistant to RRD is for the rose to prove unsuitable to the infection
carrying mites. This has been reported by Jim Amrine based on his observations of the mite on 'Rosa
bracteata'. The mites simply don't want to live on it. One would hope this feature would also apply to
bracteata offspring including 'Mermaid'. Some of the roses in this family (especially 'Mermaid') become
huge and can never be considered to be mannerly plants for flower beds. There is an additional
hardiness constraint on bracteata and at least some of its offspring. Sadly, we have had several reports of RRD on 'Mermaid' in three different states with different climates and different heat patterns and presumably different times of mite dispersal.

Rose Cultural Practices: Thanks to the excellent work done at West Virginia University, much of
which was summarized in the March 1998 American Rose Magazine we know quite a bit about
Phyllocoptes fructiphilus, the mite that transmits the RRD pathogen. That is not the subject of this
book, so I refer you to the Bibliography for that and other sources of more information on the mite. As rose gardeners we can use that knowledge to reduce the chance of infection. We know the mites don't migrate very much at temperatures below 80 F so a preemptive spraying program with Cygon 2E when temps approach and pass 80 degrees would be advisable if you are under high disease pressure. This is especially true after a period of rain has caused new growth. Except Cygon 2E is no longer approved for use in home gardens.

A well watered and heavily fertilized rose garden provides succulent new growth at any given time and
provides potential homes and food for incoming mites. One defensive measure is not to water or
fertilize in summer. The problem is that people grow roses for their bloom and rose hybridizing for
almost two hundred years has largely been directed at increasing bloom frequency and quality. So
cultural practices that discourage summer rose bloom negate almost two hundred years of rose breeding.
A good example of the effect of cultural practices that reduce RRD infection rates, presumably done
unintentionally, can be seen in the review of the Iowa State Field Tests. The problem is, most people
wouldn't want small, emaciated, weed engulfed roses in their gardens.

Planting within beds: Once a rose is thoroughly infected, there is no cure. I realize many rose growers
are already facing space limitations, but the more space you have between your roses the better.
Eliminating canes crossing from one rose to another is good, but not practical for people (like me) who like large OGR's. Roses inter planted with other plants in a cottage garden style will have less chance of massive infection than a solid mass of roses. Interplanting roses could help although many rose growers (including my husband) don't like the look. They tend to mutter that any plant that grows in a rose garden that isn't a rose, is a weed. Cottage gardens may be in our future.

Garden layout: We have changed our garden from traditional parallel beds to rooms with roses on four
sides. This makes it easier to see all the roses, so odd growth or colors stand out and can be removed quickly. Additionally, the old beds were 6 feet apart; now there are 16 to 24 feet of open space in the new rooms. That provides more space for a mite falling off an infected rose to fall to the ground and starve to death, rather than falling onto a nearby rose and spreading infection. If each row of roses acts as a 'snow fence', where the snow (mites) accumulate will be grass, not roses. Quite honestly, it looks better too!

Pruning: I like the look of dense rose bushes because it increases the density of bloom. Dense bushes
make excellent wind baffles and will drop any air-borne mites that flow through the growth. Those mites may land on that rose. Now I prune our large roses so that more air will flow through them. I'm also experimenting with removing the twiggy lower growth from previous years so that the mites that do drop out will find no new succulent growth where they land- no fresh succulent growth will, I hope, mean no transmission of RRD.

I have only tried to encourage summer dormancy in the garden I call my Hybrid Musk jumble. This is a massive planting of Hybrid Musks that are a jumble of canes. I neither water nor fertilize this area that is over a hundred square feet that are five to seven feet high. So far, no RRD and I am temporarily pleased that RRD hasn't come into this garden although roses near it have succumbed to RRD.   With these roses I have chosen less summer bloom and limited fall bloom. I hope I am decreasing the potential disease pressure for this garden by limiting the fresh and succulent growth when mite pressures are high.

  Part 4 - Chemical Control of RRD:

According to Dr. Philley of Texas A&M; "only one miticide proved effective at controlling Phyllocoptes fructiphilus (the mite) - Dimethoate (Cygon)”- specifically Cygon 2E. The acaricide Avid is also approved for use on Eriophyoid mites and is suggested by Virginia Tech as an alternative to Cygon 2E.

This book emphasizes cultural control of RRD to reduce disease pressure within a garden and outside the garden if possible. We rely on early recognition of symptoms, a small amount of remedial chemical spraying (Cygon 2E) of the infected and surrounding bushes, and finally rogueing out the infected bush and cutting canes from neighboring roses where an infected mite could have fallen. In some cases, for large bushes, we may attempt to save the bush. For large “own root” bushes we have even split off the infected “side” of the bush, splitting the roots down the middle and removing the potentially infected roots along with the sick cane. If it’s worth the risk, and how much to take out is a judgement call, but we have seen this used successfully.

We regard chemical control as a temporary solution, perhaps seasonal, to correspond to times of mite migration, or for the entire garden while trying to find the external source of a new and massive infection. Chemical control of RRD boils down to control of the vector mite Phyllocoptes fructiphilus, but there are serious limitations to chemical control as a long term method for dealing with RRD.

Some species of eriophoid mites live in the open on leaves, canes or fruit and are easily directly sprayed with a pesticide. P. fructiphilus was first described from inside rose hips and in the east it lives most of its life sheltered in leaf bracts or between petioles and stems except when it is migrating or' mating'. Thus, we can not assume a surface pesticide application will touch a mite. Cygon 2E, the chemical recommended by Dr. George Philly is systemic. It enters the plants tissue. Contact is established when the mite feeds. The mite feeds and then dies. In a perfect world, all mites would be adversely affected before they move to the parts of the plant where stylets can puncture undifferentiated tissues (that will become phloem and through which the RRD pathogen will spread within the plant). But it’s not a perfect world.

Dr Philly’s method was developed quickly in responce to a need. It was not the result of extensive testing, but it works. Va. Tech recommended Avid, which is approved for home usage and is registered for use on eriophoid mites. A warning; a pesticide that works on one eriophoid mite species may not work on another species.   All of these pesticides are general; they act on a multitude of genera and species, but not necessarily against all species in a genus. Dr. Amrine of West Virginia University had three miticides that he did test on Phyllocoptes fructiphilus in the early 90's. Below is a quote from Chapter 4.1.2 of the book "Eriophyoid Mites - their Biology, Natural Enemies and Control" (edited by Lindquist, Sabelis and Bruin, 1996) - Phyllocoptes fructiphilus and Biological Control of Multiflora Rose by J. W. Amrine Jr.

"PROTECTING ORNAMENTAL ROSES

Valuable ornamental roses can be severely affected and killed by RRD. Tests were conducted in Madison, Indiana, in 1989 to find miticides to control P. fructiphilis on ornamental roses. Six compounds were selected and applied to four one-plant-replicates, with four controls. The materials evaluated were amitraz, carharyl, dicofol, acephate, diazinon and avermectin. The three best materials were carbaryl, amitraz and diazinon, all showing reduction of mite populations for about 30 days; carbaryl appeared to have some slight systemic activity (Amrine, unpublished). Mite numbers in Indiana were too low to repeat the tests in 1990, and too few RRD-infected plants with mites were available in West Virginia. We recommend treatment of ornamental roses with one of the three materials every two weeks, from mid-May until September. Study plots were established at West Virginia University to evaluate protection of six classes of ornamenta~ roses (hybrid tea, florihunda, ~grandiflora, miniature, climbing arid shrub) and muitiflora rose in a split plot design with three replicates ~nd controls. The plants were treated with one of the six materials biweekly; P. fructipliilus from RRD-infected multiflora were then applied biweekly, one week after treatments. To date, four Rosa multiflora (two controls, one treated with avermectin and one treated with diazinon) became infected; none of the ornamental roses, including controls, produced symptoms of RRD."

Carbaryl is ..

Amitraz is ..

Diazinon is

It is a small irony of life that researchers who did the initial work on eriophoid mites had trouble keeping their mite colonies alive due to invasion of spider mites. Since we don’t spray for spider mites in our garden, we hope they and their predators will feed on eriophoid mites before our roses are a main vegetable course. Small predators about the same size as Phyllocoptes fructiphilus can get into their hiding places where nonsytemic pesticides can’t - if preditors remain after use of pesticides. After times of heavy rainfall death of Phyllocoptes fructiphilus due to fungal attack have been reported (        ). What does this say about the use of broad spectrum fungicides on our roses. Am I preventing blackspot and promoting RRD?

As it turns out some fungicides such as sulfur also kill some eriophoid mites. “The fungicides zineb and maneb were found to be effective in controlling p. oleivora on Florida citrus (Johnson et al., 1957) and zineb was recommended for rust mite control from 1958 to 1965. Control failures occurred and its use began to decline after the 1960-61 season in Florida (Childers, 1990). Also, zineb failed to control citrus rust mites at certain locations in Israel during 1963 (Swirski et al., 1967).” (Childers et al., 1996).

Any spray program for an insect the reaches adulthood in seven days that does not kill 100% of the mites can quickly become a breeding program to produce chemical resistant mites. It kills the most susceptible mites and allows the most resistant to breed. At this time this problem for roses is flying way below radar. If some eriophyoid mites developed resistence to certain chemicals in three years, how long do we have if we rely heavily on chemical control for Phyllocoptes fructiphilus?

When resistance starts, rose growers will start to see much higher losses to RRD as their attempts to eliminate the disease vectors no longer work. It won't be a matter of expired chemicals, but of having bred mites with resistance to the chemicals.   As with any chemical controls, alternate different chemicals.

The kind of information we need and don't have: One of the few translocation times given is for Merit, and it suggests that Merit be used as a soil drench from one week to three months before it is needed based on the size of the plant, because Merit has been tested and found that movement through the plant is not instantaneous. http://www.aragriculture.org/news/ReLeaf/2003/v4n1.asp . I could find no translocation times for Cygon, so I must suggest that we err on the side of caution, and spray newest growth, especially in the fall. 2007 I did find that there is a rapid loss of efficacy if any attempt is made to store mixed Cygon from one use to the next.