Blackspot: What a Pest! Some Ideas About Prevention and Mitigation

“Black spot...is the most serious disease of outdoor- grown 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:  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:

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 Earth- Kind ® varieties and from the list of recommended roses on 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