by Ed Bradley

(The Virus and Fungus Among Us)


Two alarming conditions have arisen recently that warrant our special attention.  (This is not a “little boy crying wolf” story – it is REAL.)  From time to time, new hazards come into our gardening world.  These are not new discoveries – but, they are new to our area.  At our April meeting, I advised that both of these diseases had been seen at specific local nurseries.  I am referring to Rose Rosette Disease and Downy Mildew.

Rose Rosette Disease (RRD) is a virus which is transmitted from plant to plant by an extremely tiny mite and is fatal to multiflora roses.  Multiflora roses are heavily grown as hedges (wild) in the mid- and northwest US and parts of Canada.  Because it is very hardy, multiflora is a preferred rootstock used by commercial growers for grafting or budding hybrid roses.  Thus, the connection to our rose gardens – the virus may be “resident” in the rootstock of purchased roses.  It was observed recently in some roses being sold by a local nursery.  I have just received a newsletter from the Central Arkansas Rose Society in which the Editor, Don Adlong, indicates that RRD has been prevalent in that area for some time. 

In her program presentation at our May meeting, Meg Ware showed pictures and described how RRD had “taken over” roses in her daughter’s yard in the Dallas area.  The very next morning, one of the couples at the meeting inspected their roses and “lo & behold” they found one bush with RRD.  (It has been removed.)  Bottom Line:  It is around – be on the alert!

Symptoms:  New leaves will be smallish, thin and pointed, bright or dark red.  New stems will also be bright red and growing in every direction, twisted or snarled looking.  A cluster of these canes and the distorted foliage give the appearance of a “witches broom”.  Infected canes are extremely thorny, even fuzzy looking.  The RRD virus spreads from infected canes to the root system, thence to all canes on the plant.  Mites may also be blown by the wind from one plant to another – for miles.  Infected plants will normally die completely within one to two years.

Cure:  The only practical control for RRD is the shovel.  Dig it and dump it!  Cut it in small pieces for a garbage bag – dig all of the roots, if possible, and put it all in a closed garbage bag.  Sanitize your pruners and shovel with bleach.  You may also want to drench the surrounding soil with bleach to take care of any small root pieces left behind. Spraying regularly with strong miticides, combined with pruning away infected canes, may offer some temporary control; however, the inevitable is probably inevitable – the bush will eventually die.  Go ahead and get rid of it.

Downy Mildew:  Somewhat like RRD, Downy Mildew has been in Texas since about 1995, but it has not been in Bexar County (to our knowledge).  However, this past Spring it was observed on plants in two nurseries.  Downy Mildew is a devastating fungal disease which occurs under humid but fairly cool conditions.  Specifically when relative humidity is 85% or higher and the temperature is between 50 – 75 degrees F.  This disease can be mistaken for blackspot; however, the lesions on the infected leaves will be purplish in color and angular, following the veining on the leaf.  Leaves will quickly turn yellow, then brown, with the bush being rapidly defoliated.  Again, this is a disease that is coming into our area from the wholesale growers, being dormant on the canes until the right conditions occur (humid, cool) to cause it to become active.                       

Downy Mildew:

Controls  are expensive.  Specialty fungicides are available through the internet, and locally from Rainbow Gardens Nurseries.  Some effective fungicides are:  Aliette, Subdue 2E, Subdue Maxx, and Stature DM.  Strangely, if your plants survive, it will eventually disappear during our hot, dry Summer.  That approach, however, is not recommended because the dormant spores will remain in your garden and will again attack your roses when the conditions are right.  If the bush is in a container, a more practical solution might be to quarantine it away from other rose bushes.  Depending on how widespread the disease may be, the shovel may again be the most practical solution.  Spending $150 on a fungicide to save a $20 rose bush is not economical. 

Please be alert and on the watch for these diseases.  One of our members also found Downy Mildew in his garden after it was mentioned at our April meeting.  If you think you have either of these diseases in your garden, call a Consulting Rosarian, or consult with your favorite Nursery specialist.

(For photos of these diseases, go to the web site for Rose Rosette Disease and Downy Mildew.  Color photos are a big help in diagnosing the disease.)


Disclaimer: While the advice and information contained in this web page is believed to be true and correct, neither the authors nor committee members can accept any legal responsibility for any errors or omissions that may have been made. The San Antonio Rose Society makes no warranty, expressed or implied with respect to the material contained herein. 

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