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Tuesday, June 2, 2015

Ash Dieback or Ash Decline? They Both Kill the Tree.

Q. I have a 23-year-old ash on a drip system with 10, five gallon per hour emitters watered twice per week for 30 minutes. It is located on a southern exposure and planted in a rock mulch.  It has a drip system.  The tree has begun to lose leaves and limbs are dying. Is the tree just old or am I doing something wrong?
Ash tree showing die back. This is not the tree mentioned here.

A. You are watering this tree about 50 gallons a week if all the emitters are working properly. As hot weather is upon us, I don’t believe that this will be enough water to support this tree.

That is probably enough emitters for that tree if they are spaced 2 to 3 feet apart under the canopy of the tree. If the emitters are grouped too closely together much of that water will be wasted. You want to wet about half the area under the canopy of the tree each time you water to a depth of 18 to 24 inches.

Instead of just placing emitters under the canopy of the tree, an alternative method is to place other plants under the canopy that require water as well. I believe there are several advantages to the tree in doing this.
Ash decline
May 1st is usually the critical time when we increase the frequency of applied water from once a week (beginning the first week of February) to twice a week.

Unfortunately a couple of the ash tree varieties have been developing this kind of problem since before I arrived in the Valley in 1984. I remember during the mid-1980s I was called out to look at some very large Modesto ash trees near downtown North Las Vegas. These ash trees were on city property and surrounded by turfgrass or lawns that were in very good condition. The trees had leaves which were curling up and dying as well as branch dieback.
Ash decline will kill the branches of ash trees. If the trees are not pruned and maintained properly these trees will become a safety hazard. This ash tree limb from an ash tree with ash decline broke from the tree and hit this house during a mild wind storm.
The plant pathologist for the state of Nevada and I sent samples to pathology labs looking for some answers. It first came back as a disease called ash yellows but it was later found that this diagnosis was incorrect by the same laboratory. The landscape contractor I was working with, Nanu Tomiyasu, had tried fertilizers, increasing the amount of water to the tree even though it was in a lawn and nothing seemed to help. He was desperate for answers.

Dieback occurring on fantex ash
Fast forward now to 2015. This disease on ash, first found on Modesto ash, has been found on other ash varieties like Arizona ash that have been introduced into the Valley since that time. Plant pathologist from the University of Arizona has acknowledged this disease and written about it http://ag.arizona.edu/pubs/diseases/az1124/ calling it Arizona Ash Decline and the University of California has termed it Ash Dieback.

There are some assumptions about this disease but everyone seems to agree that it is not controllable and eventually the tree will become a hazard and should be removed as the disease progresses and limbs die more and more. Increasing the fertilizer and water the plant receives will not help.

I have no evidence for this but I assume that this disease is transmitted by insects such as the Apache cicada which is common here or whiteflies. Putting ash trees in less stressful environments such as lawns or supplying enough water to the tree roots, seems to help prolong its health. I am telling people that this tree will be short-lived in our environment, 10 to 20 years, until this problem is resolved.

They should not be confused with dieback of limbs when older, established ash trees are no longer in lawns but part of a landscape retrofit to desert landscaping. In my opinion, this will shorten the trees life. Before the tree becomes a hazard, please remove it. If this is ash decline there is nothing you can do to save it.


  1. Dear Bob,

    I have been seeing ash decline in Yavapai County for the last 4 years or more. It impacts F. velutina 'Modesto' most often. You mentioned the U of A information. I thought you might have seen the work of Bricker and Stutz on this disease. I have visited with Jean Stutz about it and it is difficult to conduct a reliable assay for the disease. The papers linked below describe locations and some of the difficulties. Here are the two papers:


    I have seen a couple of trees recover (maybe temporarily) from the disease under heavy irrigation. White flies do not occur in the locations where I've seen the disease, but cicadas do.


    Jeff Schalau, Agent, ANR
    University of Arizona Cooperative Extension, Yavapai County

  2. I was surprised to read that 50 gallons of water was insufficient in a week for this Ash tree. I'm used to most people watering their trees with two - 2 gal emitters for 60 mins., three times a week, though I know this is insufficient. I tend to talk about placing 4 - 4 gal emitters on a new tree, out from the trunk, and running for 60 to 90 minutes during the summer, 3 times per week but also adding additional emitters as the drip line/leaf line expands with the age of the tree to make sure the feeder roots get the water that is necessary. I worried I am not giving out the right info now. Is there a simple formula I can use to give people the correct gallons for their tree and its size?

    1. Estimating plant water use is extremely difficult to do because there are so many variables that have to be taken into consideration. One of these variables is plant size. We know plants use more water as they get larger. The tree mentioned above is 23 years old. I have not seen the tree but at 23 years of age I'm assuming this is a pretty big tree. If this tree were five to 10 years old I think you're 50 gallons of water use might be approximately right. But this depends on the condition of the tree, the size of it, the density of its canopy, exposure to sunlight and wind and climate. I have participated in several research projects estimating plant water use which you can see at Research Gate https://www.researchgate.net/profile/Robert_Morris12 We did this work very early on before there was much research done on plant water use. The estimates today are mostly done by experts sitting around a table and discussing each tree and putting it into a category; low, medium and high. I have had people ask me how much water plants use and I have come up with a formula that is probably very inaccurate but at least gives a rough approximation of what that water use might be. I will put it on a post for people to critique in the very near future.

    2. I read your posting again and have another comment. Bottom line about enough water is whether the amount of water you are giving a tree is sufficient enough to keep it healthy and growing adequately.There are some other factors such as water quality that should be taken into concern as well. One irrigation water originates from the Colorado River, or at least most of it, then salts have to be taken into consideration.At our location in Las Vegas, Colorado River water carries about 1 ton of salt per acre foot of water or about 326,000 gallons. We like to see overwatering ( technically called leaching fraction) of about 20% added to that amount of water to keep salts moving through the soil profile. So when we estimate how much water to apply we have to include how much the plant is using, what percentage is not being applied correctly ( distribution uniformity) and a leaching fraction. So the amount of water applied may have to be adjusted upward 30 to 60% depending on how it is applied and the quality of the water. I do agree with you, if trees generally get about 20 to 30 feet tall, four emitters placed in a rectangular pattern around the tree with the trunk in the center will probably be adequate if the trees developed their root structure under this type of pattern. If this is a retrofit to desert landscaping, I think you could be in big trouble if these trees were in a lawn.

  3. A mature Ash tree in Phoenix in June can transpire (take up) 300 gallons of water a week. Ashes are stream side trees not desert trees.