Advice from the Help Desk of the 
UC Master Gardener Program of Contra Costa County

Client:  I wondering if you could help identify the issue with some of the leaves on my Lisbon lemon tree? I typically water once a week and not again until it dries out.

Yellowing Lisbon Lemon Leaves
picture from client

1. There is a virus that can cause vein yellowing, but it is not common in our area, and so is probably not the cause.
2. Herbicide toxicity can cause this type of symptom. If you have used an herbicide containing either diuron or bromacil, that could be the cause. If you have not used herbicides, then the cause is probably nitrogen deficiency.
3. Although nitrogen deficiency symptoms usually present as more general yellowing rather than prominent vein yellowing, nitrogen deficiency can cause vein yellowing when the soil is cold (usually during the winter months), or if stems or the trunk are girdled (mechanically constricted). If there are any ties around your tree, you should remove them.

Lemon trees need regular fertilization, particularly with nitrogen, to remain healthy and productive. If you have not fertilized your tree this spring, we recommend that you do so now.  You can either use ammonium sulfate or a standard citrus fertilizer.  Information on proper nitrogen fertilization of citrus, based on the age of the tree, can be found at this University of California website http://ipm.ucdavis.edu/PMG/GARDEN/FRUIT/CULTURAL/citfertilization.html.Or you can just follow the directions on the fertilizer label.

Citrus has many roots near the soil surface.  Lemon tree roots should not be disturbed by digging or cultivating, since damaged roots will negatively impact water and nutrient absorption. At least several inches of mulch under the entire tree is usually also recommended to protect the roots, keep the roots cool, retain Irrigation moisture, and minimize competing weeds. Keep the mulch at least 6” from the trunk.
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Citrus should be watered every 7 - 10 days during the dry season. Additional information on how to water citrus can be found (and/or downloaded free) at the University of California's website http://ipm.ucdavis.edu/PMG/GARDEN/FRUIT/CULTURAL/citruswatering.html .

I hope that this information is helpful.  If you water and fertilize the lemon tree properly but the leaves do not green up with 4 weeks, please feel free to contact us again.

Help Desk of the UC Master Gardener Program of Contra Costa County (JL)

Note: The  UC Master Gardeners Program of Contra Costa's Help Desk is available year-round to answer your gardening questions.  Except for a few holidays, we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 75 Santa Barbara Road, 2d Floor, Pleasant Hill, CA  94523. We can also be reached via telephone:  (925) 646-6586, email: ccmg@ucanr.edu, or on the web at http://ccmg.ucanr.edu/Ask_Us/ MGCC Blogs can be found at http://ccmg.ucanr.edu/HortCoCo/ You can also subscribe to the Blog  (//ucanr.edu/blogs/CCMGBlog/).