Pacific Northwest 1998 An Online Guide to Plant Disease Control

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OSU Extension Office
 
Rose -- Black Spot
 
Cause: Diplocarpon rosae, (asexual Marssonina rosae), a fungus that overwinters, or survives through the cold season, on living or dead plant tissue that was infected in the previous growing season. Newly emerging leaves are most susceptible to this fungus. Spores produced on the old plant material are splashed up on to young plant tissue by winter and spring rains, or by summer watering. After conidia land on a leaf, it takes at least 9 hours of leaf wetness for the spore to infect. In the PNW, this moisture requirement is frequently and easily met.

Once the fungus is established in plant tissues, fungal fruiting bodies (acervuli) form in the spots in 11 to 30 days. A new crop of spores is produced and spreads to healthy portions of the plant. The cycle of infection is ready to begin again within 10 to 18 days. Conditions for growth and spread of the black spot fungus are most likely to occur in spring and fall. However, rainy periods in any season will encourage black spot, as long as the temperatures are between 50 and 80° F.

Leaves that are infected with black spot produce the plant hormone ethylene. In leaves, high ethylene content leads to leaf drop. As a result, rose plants infected with black spot are defoliated early and look bare. Leaves may only need one or two lesions for it to drop. The dropped leaves around the base of the plant then serve as a reservoir of fungal spores, reinfecting the plant during subsequent rains. Repeated defoliation will lead to low vigor, inferior blooms and high susceptibility to winter injury.

The natural genetic variability of the fungus means roses found resistant in one location may be susceptible in another location due to the presence of different fungal strains. Also, resistant roses may become susceptible after a few years due to changes in the local fungal population.

Symptoms: Circular black spots, frequently with fringed margins, on the leaf. Yellowing and defoliation are common in susceptible cultivars. In wet weather, spots may become very severe and coalesce to make large irregular spots covering a third of the leaf surface. Few infections are on canes. In cases of severe infections, similar appearing lesions also form on the stems.

Note the black spots with diffuse margins.

Numerous black spots can be seen on the leaves.

Defoliation is the main problem with black spot. This bush has very few leaves left to store energy for the coming winter.

Leaf and stem lesions with lots of defoliation.

 
Cultural control:

  1. Plant cultivars known to be resistant in your area. Note that the cultivar 'Carefree Spirit' was the first landscape shrub to survive for 2 years in AARS tests without any fungicide and voted a winner.
  2. Avoid dense plantings and shaded areas.
  3. Avoid overhead watering that keeps plants wet for extended period of time.
  4. Rake up and burn all leaves in a rose planting at season's end.
  5. Prune canes back to two buds if canes are infected. Remove and destroy diseased canes before budbreak.
Chemical control: Focus applications at the beginning of the season starting at budbreak. Apply frequently in wet spring weather and occasionally in summer (1- to 2-week intervals). May be very difficult to manage once the disease gets started. Almost every fungicide on the market is registered for this disease on this crop but only one or two materials are needed at any one time. Alternate or tank-mix materials with different modes of action to prevent developing resistant fungal populations. Some materials such as the group 11 fungicides (Compass and Cygnus) will not allow more than two (2) sequential applications.

Research in Alabama indicates that monthly dormant season applications of group 3 type fungicides may be helpful in reducing or delaying the onset of this disease.

  1. Bayer Advanced Disease Control at 0.75 fl oz/gal water. A group 3 fungicide.
  2. Captan 50 WP at 2 lb/100 gal water. Hi-Yield Captan Fungicide 50 WP may be used in Oregon home gardens at 5 teaspoons/gal water. A group M4 fungicide. 4-day reentry.
  3. Compass O 50 WDG at 2 to 4 oz/100 gal water. Do not use organosilicate additives or more than two (2) consecutive applications. A group 11 fungicide. 12-hr reentry.
  4. Cygnus 50 WG at 1.6 to 3.2 oz/100 gal water plus a non-organosilicone spreader-sticker. Best used before symptoms develop. A group 11 fungicide. 12-hr reentry.
  5. Daconil Weather Stik at 1 pint/100 gal water. Daconil can be used in home gardens. A group M5 fungicide. 12-hr reentry.
  6. Eagle 20 EW at 6 to 12 fl oz/100 gal water. A group 3 fungicide. 24-hr reentry.
  7. Fixed coppers. Group M1 fungicides.
    1. Champ Formula 2 at 0.66 pint/100 gal water. 24-hr reentry.
    2. C-O-C-S WDG at 2.5 lb/100 gal water. 24-hr reentry.
    3. Copper-Count-N at 1 qt/100 gal water. Foliage and/or blooms may become discolored. 12-hr reentry.
    4. Kocide 2000 T/N/O at 0.75 to 3 lb/A (or 1 to 3 Tbsp/1,000 sq ft) dormant or at 0.75 to 2 lb/A when new growth is present. Foliage and/or blooms may become discolored. 24-hr reentry.
    5. Kop-R-Spray at 3 Tbsp/gal water.
    6. Nu-Cop 50 DF at 1 lb/100 gal water. Foliage and/or blooms may become discolored. 24-hr reentry.
  8. Bonide Fung-onil Multi-purpose Fungicide at 2.25 teaspoons/gal water. A group M5 fungicide.
  9. Insignia at 8 to 16 oz/100 gal water. Do not use with organosilicate-based adjuvants. Use preventatively only. A group 11 fungicide. 12-hr reentry.
  10. JMS Stylet Oil at 1 to 2 oz/gal water. Effectiveness is unknown in the Pacific Northwest when used alone. Tank-mix with another fungicide. Do not use with or near a sulfur application. Do not use during freezing temperatures, above 90°F or when foliage is wet. 4-hr reentry.
  11. Mancozeb-based products. Group M3 fungicides. 24-hr reentry.
    1. Fore 80 WP at 1.5 lb/100 gal water plus a spreader-sticker.
    2. Pentathlon DF at 1 to 2 lb/A or per 100 gal water.
    3. Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
  12. Maneb 75 DF at 1 to 1.5 lb/100 gal water. A group M3 fungicide. 24-hr reentry.
  13. Ortho RosePride Funginex at 1 Tbsp/gal water. A group 3 fungicide.
  14. Phyton 27 at 1.5 to 5 oz/10 gal water. A group M1 fungicide. 24-hr reentry.
  15. Polyram 80 DF at 1 to 1.5 lb/A. A group M3 fungicide. 24-hr reentry.
  16. Propiconazole-based products plus another fungicide. Field-grown roses only. Group 3 fungicides. 24-hr reentry.
    1. Banner MAXX at 5 to 8 fl oz/100 gal water.
    2. ProPensity 1.3 ME at 5 to 8 fl oz/100 gal water.
    3. Systemic Fungicde (ferti lome) at 1 fl oz/gal water.
  17. Spectracide Immunox at 1 oz/gal water. A group 3 fungicide.
  18. Spectro 90 WDG (chlorothalonil plus thiophanate-methyl) at 1.5 to 3 lb/A. A mix of a group M4 and 1 fungicide. 12-hr reentry.
  19. Sporan EC (essential spice oils) at 32 to 48 fl oz/100 gal water. Zero hr reentry.
  20. Thiophanate-methyl-based products. Tank-mix with another product. Group 1 fungicides. 12-hr reentry.
    1. AllBan Flo at 10.75 to 20 fl oz/100 gal water.
    2. Cleary’s 3336 WP at 12 to 16 oz/100 gal water.
    3. Ferti-Lome Halt Systemic at 2.5 teaspoons/gal water is registered for home use.
    4. OHP 6672 4.5 F at 10.75 to 20 fl oz/100 gal water.
    5. Systec 1998 at 20 fl oz/100 gal water.
    6. Tee-Off 4.5 F at 20 oz/100 gal water plus a surfactant.
  21. Wettable sulfur at 3 Tbsp/gal water. May stunt shoots. A group M2 fungicide.
  22. Ziram 76 DF at 1.5 to 2 lb/100 gal water. A group M3 fungicide. 48-hr reentry.
  23. Zyban WSB at 24 oz/100 gal water. Not to be confused with the smoking cessation drug. A mix of a group M3 and 1 fungicide. 12-hr reentry.
Notes: Although Armicarb 100, MilStop, Remedy and Kaligreen are registered, bicarbonates are not recommended for use in the Pacific Northwest due to high disease pressure and poor control using these products.
References:
Bowen, K.L. and Roark, R.S. 2001. Management of black spot of rose with winter fungicide treatment. Plant Disease. 85:393-398.
Horst, R. K. and Cloyd, R. A. 2007. Compendium of Rose Diseases and Pests. Second Edition. APS Press. St. Paul, MN.
Content edited by: Jay W. Pscheidt on January 1, 2009
 
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