Leaf drop from Chitalpa trees

I have a beautiful Chitalpa, which keeps losing its leaves in great masses. According to the local agricultural extension office, it is not a matter of a disease but rather of cultural practices. I been told the soil might be too alkaline or that I water too much because I planted some day lilies under the tree. What happens is that the leaves start out healthy, then develop yellowish spots, the veins start turning brown, spots like brown spots on roses develop and the leaves become fairly dry and drop off. I would appreciate your advice very much. - Christian T. via internet

This is a problem that has been challenging us as more and more people plant Chitalpa trees. These symptoms develop each summer here in New Mexico creating a mid-summer leaf drop. The trees often refoliate following the onset of monsoon rains and cooling temperatures. However, the leaves usually develop symptoms and drop again once the monsoon rains end. However, as you will see in the information below, supplemental irrigation does not seem to prevent the problem.

Samples of Chitalpa leaves with symptoms that you described have been sent each year to Dr. Natalie Goldberg, NMSU Extension Plant Pathologist, who has looked for a causal organism. The following paragraph is her response to these samples:

"No plant pathogenic microorganisms were isolated from the sample submitted which I am also certain is not Catalpa, but the hybrid Chitalpa, a cross between Catalpa and Desert Willow. I have had a number of samples of this plant over the past few years and I am getting more and more as the tree is being used more in landscapes. I have two at my house and, to be honest, I would not plant them again nor do I recommend them without some reservations. The problems I see are yellow to brown spots on the leaves, leaf blight, wilt and premature defoliation. Trees may exhibit some or all of these symptoms. The symptoms occur on trees that are well watered as well as those that do not receive much water. The symptoms seem to occur in summer when the days really start heating up. Several branches on my trees wilted and defoliated in July of last year. I pruned out the dead limbs and the tree leafed out fine in the spring, but I am just waiting for it to happen again (since it's happened each of the last three years). We have never isolated any pathogen from the samples submitted. I really feel this is a response to environmental stress, but neither increasing or decreasing water seems to help. However, John White, Doña Ana County Horticulture Agent, and I both feel that this hybrid requires watering along the lines of its relative, the Catalpa parent, rather than the Desert Willow parent. Your question about acidifier is a good one. I have not tried this, but it just might help - used at a labeled rate and applied as directed, I'm pretty certain it wouldn't hurt."

Dr. Goldberg stated that she has observed some trees that look good and in our discussion the possibility of pH being a factor was considered. We hypothesized that acidification might help. So, as you mentioned, the alkalinity of the soil may be a factor.

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!