October 26, 2013

1 - It is possible to test for chemicals injuring plants and for diseases infecting plants to best know how to manage problems.

Yard and Garden October 26, 2013


Is there a test to determine if my lilac bush is being sprayed with an herbicide? The plant is old but still blooms in the spring. However, certain areas have dried up and appear dead this fall. Happened last year, too, and the plant recovered.

Lois A. N.

Via University-Wide Extension


There are tests for chemicals that may have been applied to the soil or a plant, but these tests may be expensive and you have to identify which chemical to test for. Did you use herbicides in the vicinity of your lilac? Which chemical did you use? Was it applied to the foliage or to the soil? Even "weed and feed" type products can cause problems, if that was used. To test for a wide spectrum of chemicals would be extremely expensive. If you used weed control product near the lilac, the label will tell you what chemicals are in the product, identifying what chemical or chemicals to look for in the text. Do you suspect someone else applied the chemical? In that case it will be more difficult to test without some idea of the chemical involved. Your local NMSU County Extension Service agent can help you identify what chemicals may be involved whether you applied the chemical or you think someone else applied chemicals. They should see samples of the foliage before it dries up as symptoms first appear. You may need to contact your Extension Service agent next summer to check for you. You can contact them now to see if there are symptoms present that can guide a determination even now.

HOWEVER, there are other considerations besides herbicides. There are foliar diseases that can cause the symptoms you described. These appear in the summer after monsoons, or even earlier if you are irrigating the plants in the evening. Evening irrigation raises the humidity of an area and can contribute to foliar disease development. Powdery mildew is a very common foliar disease in lilac that develops in late summer following high humidity at night. There are other diseases also possible. There are even vascular diseases that infect the stems or roots that could cause the symptoms you described. Your local NMSU Extension Service agent can help you diagnose disease problems. If the agent cannot immediately identify the problem, they can send samples to NMSU to have experts investigate the problem and determine causes and solutions to the problem.

Since you described the recovery of the lilac in the spring, I suspect disease may be the culprit. Some diseases can be managed fairly easily, others are more difficult. Some can be minor problems; some can ultimately kill your lilac. Getting advice from your NMSU Extension Service agent will help determine what you should do.

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.

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

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