“I have really bad black spot Have been using Bayer Advanced Active ingredient is Tebuconazole. I have been spraying them every night for a week now and they don’t look any better. Any suggestions? ~Jim”
Thank you contacting me via email regarding the black spot (Diplocarpon rosae)
problems you have been having on your roses. What type of roses do you
have? If they are hybrid tea roses, they are going to take a little bit
longer to respond to treatments for black spot because they have more
‘sinks’ to put nutrients and resources into. A nutrient source-sink for a
plant is the relocation of nutrients and resources from the part of the
plant where they are being formed (sugars in leaves) or taken up
(water/nutrients in roots) and relocated to areas where they are being used
quickly (for roses, the blossoms). The large size of the hybrid tea rose
blossom and the fragrance both draw much from the plant and make it more
susceptible to disease.
However, do not fear. It is not impossible to cure your plant! You just
have to outsmart the black spot.
In summer, the high humidity and frequent rainfalls (of most areas) promote
fungal pathogen development. Nighttime temperatures between 59° and 80° F
and heavy dews or frequent showers are ideal conditions that allow the
fungus to thrive and continuously reinfect plants.
The fungicide chlorothalonil (Daconil) is effective in controlling blackspot
by killing the fungal spores that spread the disease. However, optimal
disease control with chlorothalonil requires frequent applications to
protect newly developing leaves and to replace fungicide washed off by rain.
Control of blackspot on roses, therefore, may require more than 15 fungicide
applications, at 7-10 day intervals (or more if it is particularly rainy),
during the growing season.
Recent concerns about the safety and environmental impact of frequent
fungicide use have caused homeowners to consider alternatives for control.
Baking soda in solution with horticultural oil (a light petroleum oil
labeled for control of insects) or NEEM oil, for example, has been shown to
reduce diseases on roses in your area. In addition, many people believe
that fertilizing roses with epsom salts (MgSO4) produces more vigorous tea
rose plants that are better able to handle either the chemical treatments or
more organic, natural treatments. The rate is one cup epsom salts per
plant per month.
One note: if you do decide to go with the horticultural oil option, be
careful and use according to the label directions. Research has shown that
using horticultural oil may damage rose foliage due to the higher
temperatures (90+) that prevail during especially warm summers in the north
or regular summers in the south. It has been suggested that applications of
the horticultural oil solutions (oil alone and in suspension with baking
soda) alternated with chlorothalonil fungicide applications avoided this
problem. Oil solutions were applied weekly but were substituted with the
fungicide when rainfall between sprays was less than 0.25 inch. Rain removed
some of the oil and reduced foliar damage.
While complete control of blackspot disease may never be attained without
the use of fungicides, proper management of rose plants will reduce the
amount of fungicide needed. This includes annual replacement of ground
cover, proper pruning and fertilization, and removal of fallen leaves.
Pruning and removal of debris is important because the fungus readily
survives in fallen leaves, buds, or infected canes. Proper fertility will
keep a plant in optimal health, which makes it less susceptible to disease.
I hope this information helps you out. If you have any other questions,
please feel free to ask.
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