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Life After Downy Mildew
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Printable Version

Is There Life in the Rose Garden After Downy Mildew?

Howard E. Jones, Consulting Rosarian

Tidewater Rose Society

May 2000

 

 

History of Downy Mildew

 

Downy Mildew was first reported in England in 1862 and in the early 1900’s was reported throughout Europe.  It was reported in the United States in 1880 and seems to occur wherever roses are grown, given the right weather conditions.  All rose cultivars are susceptible, and it also attacks such crops as tomatoes, grapes, and tobacco and has been pinpointed as the responsible fungus in what was called the potato blight in the disastrous famine in Ireland in 1848.

 

Symptoms

 

Downy Mildew is a fungus disease caused by Peronospora Sparsa and is manifested by purplish-red to dark brown irregular spots or elongated blotches on the upper leaf surface and grayish mycelia and conedia on the lower leaf surface beneath the lesions.  It lives on the inside of the leaf and may have gotten its name because of the downy gray mold on the underside of the leaf, but does not resemble Powdery Mildew, which lives on the top and bottom surface of the leaf.

 

Unlike Blackspot (Diplocarpon rosae) that attacks the lower foliage first, Downy Mildew attacks the new growth at the top (apical) of the bush and works its way down to the lower foliage.  Excessive wet conditions can trigger an attack of either of these rose diseases--Blackspot or Downy Mildew.  Three to four days of rain with moderately warm temperatures (65F to 80F) will trigger Downy Mildew when the spores are present.  The spores need to be in water for at least four hours to germinate.

 

1992 Attack in the Jones Garden

 

In the fall of 1992, I attended a Colonial District meeting and Rose Show in Roanoke, Virginia.  Several of the exhibitors present were from North Carolina and shared that some of them had Downy Mildew in their rose gardens.  I thought to myself how lucky I was that I had ever had this dreaded rose disease.  When I returned home, I saw some foliage damage which looked suspiciously like what they described.  Although I first thought that it might be just Blackspot, I eventually decided that it was, in fact, Downy Mildew.  The young apical growth, rather than the older foliage on the lower part of the bush, were manifesting the typical discolored blotches of Downy Mildew and the top foliage dropped off at the slightest touch.  This was a dead giveaway, as leaf abscission (drop off) can be severe and happens very quickly.

 

1999 Attack in the Jones Garden

 

This brings us to the growing season of 1999.  The months of July, August, September and October inundated us with just over 40 inches of rainfall in Virginia Beach and there were three hurricanes.

 

Hurricane Dennis blew in on August 30 and there was rain every day for seven days.  On September 16-17, Hurricane Floyd provided 4.2 inches of rain and winds of 50-60 mph.  Finally, at


the end of the hurricane season on October 18, Hurricane Irene dumped another 8.5 inches of rain in our rain gauge for the “coup de grass” as it passed by.

 

The rainfall, averaging 10 inches per month, for the four months was almost equivalent to the average rainfall for the entire year for our area.  Talk about ripe conditions for Blackspot and Downy Mildew.  I was not able to spray from August 23 until September 6 because of continuous wet conditions, and by the middle of September I began to see a fair amount of Blackspot in our garden, but not enough to be overly alarmed.

 

On September 27, while spraying with a mix of Banner Maxx and Orthene for Blackspot and Aphid control, I first noticed symptoms of what turned out to be Downy Mildew.  In one of our beds of Hybrid Teas I noticed that leaves had fallen off and were lying on top of the pine bark mulch.  Also, when my spray wand bumped against canes as I sprayed, additional leaves fell off.  Upon examining the fallen leaves, I observed that the entire underside had a purplish cast, while the tops of the leaves remained green.  No lesions or irregular purplish-brown spots were evident at this time.  However, the rapid and severe dropping of the upper leaves still made me think it might be Downy Mildew that I was dealing with.

 

Several days passed as I was trying to diagnose my problem, and gradually infected leaves began to show the classical discolorations of purplish-red to brown spots and irregular blotches of red, brown, and purple on a background of yellow before falling off (see color plates 26-29 in Horst’s Compendium of Rose Diseases).

 

As luck would have it, by the time I decided that it was definitely a case of Downy Mildew, wet weather had set in again and it was a week before I could start my spraying program.  Keep in mind that I already had a mild case of Blackspot before the attack of Downy Mildew due to the rainy weather that made it impossible to spray every seven days on a regular basis.  On some leaves it was difficult to tell where the Blackspot stopped and the Downy Mildew began. 

 

My supply of Pace (a specific fungicide for Downy Mildew that has protective as well as reportedly eradicant capabilities) was seven years old.  I also had a supply of Fore (Dithane M-45) that was two or three years old.  Determining that I could not wait to order in (not available locally) a fresh supply of Pace,  I decided to use the Fore first because it was fresher.

 

On October 4 I sprayed with Fore along with Funginex in the mixture for Blackspot and Mavrik for Aphids.  On October 8 I sprayed with Pace, Funginex and Orthene, not knowing if the Pace was still viable.  Between October 4 and December 5 I sprayed six times using either Fore or Pace in the spray mixture.

 

The Downy Mildew started in one bed which had about 40 Hybrid Teas and gradually spread to the rest of my 170 Hybrid Teas and finally throughout most of the garden of 340 rose bushes.  Only some potted Hybrid Teas on the driveway, away from the rest of the rose garden, were not affected.  As late as December 20 I was still seeing damaged foliage that showed the classical discolorations of Downy Mildew.

 

Unfortunately, the weather conditions continued to be ideal for Downy Mildew as well as Blackspot.  Several days of wet, rainy, cool weather followed by sunny, warmer dry days, and then wet cool days again provided ideal conditions for the spreading of Downy Mildew.

 


Plan of Control

 

Where do I go from here, and what do I intend to do to prevent a recurrence of this virulent fungus disease that many rose growers believe is the most devastating of all the fungus diseases that attack roses?

 

Since Downy Mildew spores over-winter in infected leaves, stems, and flowers, sanitation is extremely important.  Therefore, I started by diligently cleaning up and destroying all potential sources of reinfection.  This is not easy to do, and it is impossible to remove all sources of reinfection since the canes on the bushes after pruning will contain spores just waiting for the right weather conditions to germinate and trigger a new attack.

 

Immediately following my Spring pruning around the first of March, I plan to add Fore or Pace or one of the other fungicides for preventing Downy Mildew to my regular Spring clean-up spraying mixture.  I will also spray the mulch and ground in the rose beds thoroughly.  In addition to Fore and Pace, there are several fungicides that are effective in preventing Downy Mildew, including Ridomil, which is sold under the brand name of Subdue.  Pace contains both Fore and Ridomil.

 

After my bout with Downy Mildew in 1992, I used Pace on a regular basis in the Spring of 1993; but, by the middle of the growing season, not seeing any signs of Downy Mildew, I began to ease up on my preventive spraying for this disease.  I do not intend to make this same mistake again.

 

Following my clean-up spraying in early March, I intend to every 7-10 days use one of the effective fungicides for Downy Mildew along with a fungicide for Blackspot and an insecticide, if necessary.  Since Downy Mildew spores are reportedly killed at 80F, I may back off from June 15 to August 15 and then start up again for the rest of the growing season using one of the specific fungicides for Downy Mildew at each spraying until around December 1.  This way I hope to avoid having a recurrence of this devastating fungus disease.

 

Sources of Information

 

Several good articles have been published on Downy Mildew that I have used as a source of information and as reference materials for my personal account in this article.  They are listed below.

 

  Downy Mildew by John Mattia, August 1991 issue of the American Rose

  More than Just Spots by Paul Desmet, 1993 American Rose Annual

  Beware of Downy Mildew by Beth V. Parks, 1973 American Rose Annual

  Compendium of Rose Diseases, R. Kenneth Horst, published 1983 by the American Phytopathological Society

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