Presentation on theme: "Blackspot: What a Pest! Some Ideas About Prevention and Mitigation."— Presentation transcript:
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
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 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
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 www.chattanoogarose.org, and realize that your garden will probably suffer from Blackspot www.chattanoogarose.org 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
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