Blackspot: What a Pest!
Some Ideas About Prevention and Mitigation
“Black spot...is the most
serious disease of outdoorgrown roses worldwide as a
result of the potential for
rapid leaf yellowing and
defoliation”
Dr. David C. Zlesak, Univ. of Wisconsin
Acknowledgements
 This presentation is based largely on the blackspot
research done by Dr. David C. Zlesak of the University of
Wisconsin and Dr. Vance Whitaker of the University of
Florida
 The primary source of information is Dr. Zlesak’s paper
“Evaluation of Roses from Earth-Kind® Trials: Black Spot
(Diplocarpon rosae Wolf) Resistance and Ploidy”
 The Earth-Kind® Trials referred to by Dr. Zlesak were
carried out by a partnership of
 Parks and Recreation Department of Farmers Branch,TX
 Texas AgriLife Extension Service – Texas A&M University
 Houston Rose Society
This Evening’s Agenda
 First, the bad news: there’s more than one blackspot!
 Better news: some roses are genetically resistant to
blackspot
 Best news: careful selection of rose cultivars AND a
rigorous spray program can defeat blackspot
What is Blackspot?
 The principal fungal diseases that attack roses are:
 Downy Mildew caused by Peronospora sparsa
 Powdery Mildew caused by Podosphaera pannosa
 And Blackspot caused by Diplocarpon rosae
 The most serious disease for outdoor landscaping with
roses is Blackspot because unchecked it can defoliate a
bush
 Blackspot infects only the genus Rosa
 Its spores overwinter on stems and fallen leaves and are
transported to new growth in water droplets
 If free water remains present lesions may appear in as
little as four days
What is Blackspot? (Cont.)
What is Blackspot (Cont.)
The Bad News
 Multiple studies have been conducted to characterize the
pathogenic race structure of D. rosae
 Yes, there’s not just one Blackspot but a number of
races of Blackspot each with a different ability to infect a
common set of rose genotypes
 Six races have been discovered in Germany, four in
Great Britain, and three in North America
 The races are catalogued by number and the North
American races are numbers 3, 8, and 9
 Dr. Zlesak used these three races to test the
susceptibility to Blackspot of 73 varieties (cultivars) of
Earth-Kind® roses – only nine were resistant to all three
races
Earth-Kind® Roses - What are they?
 The National Earth Kind Rose Research Study is the
foremost environmental rose research program in the
United States and, indeed, the world
 The research is being undertaken in four countries, at
seven universities and in 27 states
 The goal of the research is ”...to identify a collection of
roses that will grow across the country, with cultural
practices that are environmentally responsible.”
 The Earth Kind program is not, however, focused on the
elimination of Blackspot but it will serve to identify roses
that can withstand severe environments
Earth-Kind® Rose Trials

Of greatest interest to rosarians in the south are the trials underway at
Farmers Branch, TX northwest of Dallas

2.5 acres of Gussie Field Watterworth Park were/are dedicated to the
planting of 400 roses – 4 each of 100 varieties

These roses are grown without added fertilizers or soil amendments and
they are never deadheaded, pruned or sprayed

Thus far the Farmers Branch trial garden has yielded 21 varieties able to
withstand the severe conditions of the trial

Pictures of these 21 varieties can be seen at:
http://aggie-horticulture.tamu.edu/earthkindroses/cultivars

Keep in mind that the hardiness of the Earth-Kind roses does not mean
that they are inherently/genetically resistant to Blackspot – they’re just
more able to cope with the disease

A list of the Earth-Kind roses being trialed at the Northern Rose Trials at
the New York Botanical Gardens can be seen at:
http://www.nybg.org/gardens/rose-garden/earthkind.php
The Fairy – Dwarf Shrub
Caldwell Pink – Small Shrub
Belinda’s Dream – Medium Shrub
Sea Foam – Mannerly Climber
New Dawn – Vigorous Climber
The Better News
 Some rose cultivars are genetically resistant to all three
North American (NA) races of Blackspot
 Dr. Zlesak tested 73 rose cultivars most of which were
“survivors” of one of the Earth-Kind trials
 Referring to the handout table, which summarizes Dr.
Zlesak’s test results, only nine of the 73 tested cultivars are
resistant to all three NA Blackspot races
 Not surprisingly, three varieties of Knock Out are among the
nine: Knock Out®, Pink Knock Out®, and Rainbow Knock
Out®
 Sadly, for us Southeastern rosarians Knock Out® was the
only variety from the Houston Earth-Kind trials to be
resistant to all three NA Blackspot races – almost all of the
other Houston Trial roses were susceptible to all three.
A Note About Ploidy
 One of the columns in Dr. Zlesak’s table is labeled Ploidy
 Ploidy is the number of chromosomes in the rose’s cells
 2x signifies a rose is Diploidy and has 14 chromosomes
 3x means a rose is Triploidy with 21 chromosomes
 4x means the rose is Tetraploidy with 28 chromosomes
 The chromosomes contain the genetic material (genes)
which encode, with strands of DNA, the inherited
characteristics of each rose cultivar
 Humans have 23 pairs of chromosomes or a total of 46
Brite Eyes - Climber
Grouse - Shrub
Home Run - Shrub
Knock Out - Shrub
Paprika - Shrub
Peachy Cream - Shrub
Pink Knock Out - Shrub
Rainbow Knock Out - Shrub
Yellow Submarine - Shrub
What to do???
 Approach No.1 – do nothing, i.e., don’t have a rose garden
 Approach No.2 – just plant Knock Outs –this will make a
certain statement about your desire (or not) to grow roses
 Approach No.3 – assuming you like shrub roses, plant the
nine varieties determined to be resistant to the three NA
races of Blackspot – and hope no foreign races show up in
your garden
 Approach No.4 – be realistic, pick roses from the EarthKind® varieties and from the list of recommended roses on
www.chattanoogarose.org , and realize that your garden
will probably suffer from Blackspot
 Approach No.5 – follow Approach No.4 but also (and these
are Earth-Kind® NoNos) care for your roses and spray
regularly and often for Blackspot
Spray!! Oh My!!
 Spraying fungicides may seem environmentally callous – and
it is if not done with the proper safety considerations
 Most popular fungicides are toxic to aquatic life and should
not be applied near ponds, creeks, etc.
 Spraying should be avoided when breezes can cause the
spray to drift outside your garden
 Spray by yourself using proper eye and skin protection and
when other persons or pets are not present
 Always carefully read the label of any fungicide the you use –
in particular, any Precautionary Statements and Directions
for Use
Spray Recommendations
Based on My Experience
 Start a Blackspot preventive spray regimen immediately
after pruning your roses in late March
 Continue spraying regularly every week changing
fungicides week to week to avoid resistance buildup
 For example, use Honor Guard and/or Compass one
week and Cleary’s 3336F and/or Compass the next
 Mix the fungicides with a spreader-sticker/penetrant
adjuvant such as Cadence
 Make sure to cover all the foliage, especially within the
center of the bush – spray the ground around the base
of the bush, as well
The Bottom Line
 If you want to just say you grow roses, but don’t really
want to do any real gardening, plant Knock Outs®
 If you really love roses and want a good-looking
Blackspot-free garden, with roses other than shrubs,
then plant roses with a good “reputation” in the Tri-State
area and spray them regularly and often