Black rose stems

Question:

I am originally a Florida native, having moved to New Mexico six years ago. I noticed a problem that I have never experienced before in growing roses. When I cut the flowers from the plants or prune out the bushes, I get black on the branch from the cut down about four or five inches. This is concerning me since I have never had this experience before growing roses in Florida. What could be causing this?
 - Marcie S.
   Melrose, New Mexico

Answer:

A common problem faced by New Mexico rose growers is the presence of the carpenter bee. This bee tunnels into the soft pith in the center of the stem to lay her eggs. Her tunneling causes the stems to first yellow, then dieback and turn black to at least the depth to which she tunneled into the pith. If this appears to be the case (evidenced by hollow stems), you can cut the stem to a bud or stem below the dieback. A drop of casein glue (white glue) over the cut surface will discourage further tunneling and further dieback.

Another possible cause is the dry air in New Mexico. We have fewer disease problems than gardeners in humid climates because the dry New Mexico air makes it more difficult for many common diseases of ornamentals to attack our plants. However, the benefit of dry air also comes with a cost. The dry air can harm the plants. This is most common on vigorous new growth, which is very tender.

If the plants are given a lot of fertilizer (especially nitrogen) and plenty of water, the growth that develops is very "soft" and subject to damage by environmental factors such as wind and heat.

Although it is possible that this year's higher-than-normal precipitation in much of the state could have contributed to increased diseases, environmental stress is the most likely cause of the blackened stems on your roses. The rain this year may have resulted in very soft growth, subject to heat, wind, or carpenter bee damage. However, it would be good to take a sample to your local Cooperative Extension Service office for the County Agent to confirm the cause or, if necessary, send a sample to an Extension specialist for diagnosis.

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Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!