March may seem a little early to worry about fungal diseases, but recent prolonged wet weather and warmer temperatures have made the climate just right for a variety of leaf diseases in the Florida Panhandle. One in particular, Blackspot (Diplocarpon rosae), is particularly problematic for the discerning rose grower. Blackspot is just beginning to be seen this season on non-resistant rose cultivars throughout the central Florida panhandle.
Monsieur Tillier, a rose that shows promise for blackspot resistance, is also part of the Earthkind program. Image Credit Matthew Orwat
Blackspot spores are spread from the canes or leaves of the previous season’s growth by rain or overhead irrigation. To minimize innoculum (black spot spores) remove all leaves from the previous season and any dead leaves at the base of the plant. After the annual pruning in mid to late February, it is a good idea to spray roses with sulfur or copper based fungicidal soap. This helps kill spores that have been laying dormant throughout the winter. Be careful not to spray these products when temperatures rise above 80 ° F. Doing so could burn the plants, although these products will damage tender young growth as well at temperatures below 80 ° F .
Wet, blackspot affected leaves. Image Credit Matthew Orwat
While dormant sprays often allow rose gardeners to get ahead of the disease, the best defense for the home garden is to plant resistant cultivars. There are several old and new resistant cultivars on the market. Some new resistant cultivars of note include Home Run and Knockout. Also, several older cultivars have demonstrated excellent resistance, such as Mrs. B.R. Cant and Spice. The author has been working with several older rose cultivars at the UF / IFAS Washington County Extension demonstration garden that have shown resistance during the last three years. While this demonstration garden is not a scientific study, several roses that have demonstrated some merit include Belinda’s Dream, Monsieur Tillier, Mme. Antoine Mari, Rosette Delizy and Souvenier de la Malmaison. Another resource for information on disease resistant roses is the Texas Earthkind rose program from Texas A&M University. While disease resistance is not always identical in different parts of the county, their recommended list gives the beginning rose gardener a good place to start.
Mrs. B. R. Cant, a tea rose from 1901 has shown very good blackspot resistance at the UF IFAS rose trials in Quincy, FL. Image Credit Matthew Orwat
Several other cultural methods that will prevent spread of this disease among the rose garden include avoiding overhead irrigation, planting in well ventilated areas and proper plant spacing. These techniques attempt to make the garden environment inhospitable for blackspot proliferation.
Regardless if these management techniques are followed, if traditional large, long-stemmed, high maintenance Hybrid Tea roses are desired, most Florida rose gardeners will need to begin a spray schedule to prevent the onslaught of blackspot. Several fungicides are on the market to aid in this process, but remember that they must be used regularly (every 7-14 days) and modes of action must be rotated. This means that different types of fungicide must be rotated so that resistance to a particular fungicide by the blackspot organism does not develop. Below is a table of recommended fungicides for homeowner blackspot control. For more information, please consult the UF IFAS Extension publication P268, Blackspot of Rose.
Fungicide products marketed toward homeowners for control of black spot on roses
|
Active ingredient |
Fungicide group |
Trade name |
|
Copper hydroxide |
M1 |
Hi-Yield ® Copper |
|
Copper Sulfate |
M1 |
Bonide® Copper Dust |
|
Copper Octanoate |
M1 |
Bonide® Liquid Copper, Natural Guard Copper Soap, Ortho® Disease B Gon® Copper Fungicide |
|
Sulfur |
M2 |
Bonide® Sulfur Dust, Ferti-lome® Dusting Sulfur, Green Light, Hi-Yield® Dusting Wettable Sulphur, Ortho® Bug-B-Gon® Rose & Flower Care |
|
Mancozeb |
M3 |
Bonide® Mancozeb |
|
Captan |
M4 |
Bonide® Captan 50WP, Hi-Yield® Captan 50W Fungicide |
|
Chlorothalonil |
M5 |
Bonide® Fungonil, Ferti-lome® Broad Spectrum, Hi-Yield® Vegetable, Flower, Fruit and Ornamental Fungicide, Monterey, Ortho® Disease B Gon™ Garden Fungicide , Monterey Fruit Tree, Vegetable & Ornamental Fungicide |
|
Myclobutanil |
3 |
Spectracide Immunox® Multi-Purpose Fungicide |
|
Propiconazole |
3 |
Ferti-lome® Liquid Systemic Fungicide, Monterey Fungi-Fighter |
|
Tebuconazole |
3 |
Bayer Advanced™ Disease Control for Roses, Flowers & Shrubs |
|
Tebuconazole + Imidacloprid |
3 + |
Bonide® Rose RX Systemic Drench, Feti-lome® 2-N-1 Systemic |
|
Triforine |
3 |
Ortho® RosePride® Disease Control |
|
Calcium Polysulfide |
NC |
Hi-Yield® Lime Sulfur Spray |
|
Neem Oil |
NC |
Bonide® RX 3 in 1, Green Light® Neem Concentrate, Green Light® Rose Defense®, Monterey |
|
Acetamiprid + Triticonazole |
NC + 3 |
Ortho® Bug B Gon® Insect & Disease Control |
|
Acephate + Resmethrin + Triforine |
NC + NC + 3 |
Ortho® RosePride® Insect, Disease & Mite Control |
| Fungicide Group (FRAC Code): Numbers (1-37) and letters (M) are used to distinguish the fungicidal mode of action groups. All fungicides within the same group (with same number or letter) indicate same active ingredient or similar mode of action. This information must be considered in making decisions about how to manage fungicide resistance. M=Multi-site inhibitors, fungicide resistance is low; NC= not classified. Source: http://www.frac.info/ (Fungicide Resistance Action Committee, FRAC).
Always read a current product label before applying any chemicals. |
||
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