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Mid May treatment of shot hole fungus in apricot and plum, Wheat Ridge, Colorado #868571

Asked May 15, 2024, 2:31 PM EDT

Apparent "shot hole" affecting my Blenheim Apricot tree. Current date, May 15. It's a well-pruned tree about six feet high, fifteen wide. Hundreds of blossoms formed by mid-March, all heavily pollinated by bees. Many of these were clearly fertilized and began to swell with fruit, but most spontaneously aborted during cold winds and snows over the following month, leaving by May a dozen or so reddening healthy looking apricots, now about 2" diameter. I noticed the shot holes and withering leaves perhaps four weeks ago, suspected some kind of fungus, but didn't diagnose it until a few days ago, using online resources that showed pictures. CSU Extension suggests several fungicides but cautions against applications after "shuck fall" stage which in our case was about three weeks ago. What are the recommendations for copper or chlorothalonil applications at this stage wrt fruit edibility or damage to leaves - primary goal being the long term health and recovery of tree and prevention of spore spread to a healthy peach and a newly-infected plum tree.

Jefferson County Colorado

Expert Response

Hello,

Thank you for your question and photos.  Once past the shuck fall, CSU doesn't recommend either chlorothalonil or copper applications until fall as a preventive.  The labels for both fungicides also specify not to apply them after shuck fall.

An Integrated Pest Management approach for tree health and to prevent spread includes thorough fall leaf clean-up (do not compost).  Details are in the Fact Sheet listed below.

Note that, while the photos show symptoms that could be due to Coryneum blight fungus, the symptoms are also consistent with bacterial blight, and treatment methods would be different.  To prepare for any fall microbicide applications, I recommend that you submit a sample of an affected branch to either the Jeffco Extension Diagnostic Desk, or to the CSU Spur lab for identification of a pathogen.  [Fees apply]

References: Best regards,
Mari Hackbarth
Jefferson County Extension of Colorado State University Replied May 26, 2024, 5:16 PM EDT

Mari Hackbarth

CSU Extension Services

Fungicide Questions

 

Hello Ms. Hackbarth,

 

You replied to questions I asked on May 15 (ten days ago) with CSU’s recommendations not to treat the shot hole symptoms of my apricot until fall.  This was kind of a slow turn-around to get this answer, so unencumbered by any real knowledge on the subject, I’m not going to wait. 

 

First of all, your “follow the label” advice is apparently limited to some specific product.  The label on a fungicide spray from ACE Hardware, “Organic” Copper Octanoate, (delivery with a hose end sprayer) recommends weekly use up till harvest for blighted vegetables and fruits.  I started spraying that when I found out about it a couple of weeks ago.  Of course I use products as the label advises.  If the experts at CSU extension services know of any reason why I shouldn’t, perhaps they or you could tell me.  The apricot is flush with pretty new copper colored leaves, but below the canopy many low branches are leaking sap from cankers and the shot holes in many leaves are merging to consume most of the productive leaf area.   This may not be evident in the low-res photos I sent.  But to me it - and the high self-abort rate of the fruitlets is alarming.  Again only a dozen fruit remain and I’m more than willing to sacrifice the crop if fungicidal treatments can stop the spore spreading of this fungus within the apricot or to my peach, pear and plum trees.

 

Again, there is (at ACE and also online) at least one suitable copper compound for backyard fruit and vegetable gardens whose use is encouraged through harvest.  I have to assume that the treatment which should have begun last fall and perhaps in winter must still be efficacious to slow the current blight.

 

Thanks also for the tip about the similarities of my photos to a possible bacterial blight for which, you say, “treatment methods would be different.”  How are they different?  What are some of those treatments and can they be used in conjunction with the copper octanoate to deliver a blanket protection?  (Why or why not?)

 

Finally, are there any experts at the CSU Extension who have had hands on experience with fungicidal treatments - or perhaps could refer me to an orchardist who actually employs them routinely.

 

Thanks again for your replies. I appreciated the links sent. WRT bacterial spot, there were no references to bacterial spot treatments other than leaf and branch removal.  I Googled to find the common treatment of Neem Oil.

 

Sincerely,

Bill Parsons

 

Registration of Copper Octanoate in Canada  (Details of health and environmental risks and efficacy of use against fungus) 

 

https://www.canada.ca/content/dam/hc-sc/migration/hc-sc/cps-spc/alt_formats/pdf/pubs/pest/_decisions/rd2015-19/rd2015-19-eng.pdf   

 

 

 

From: ask=<personal data hidden> [mailto:ask=<personal data hidden>] On Behalf Of Ask Extension
Sent: Sunday, May 26, 2024 3:16 PM
To: bpparsons3 <<personal data hidden>>
Subject: Re: Mid May treatment of shot hole fungus in apricot and plum, Wheat Ridge, Colorado (#0138492)

 

The Question Asker Replied May 27, 2024, 8:08 PM EDT
Hello Bill,

Thank your patience in waiting for a reply, as our office is currently short-staffed, and slow in responding to Ask Extension questions at times.

I am sorry that my response about the pesticide application timing wasn't clear.  I included  examples of labels for two products that contain the active ingredients (chlorothalonil, and copper) listed in the fact sheet for control of Coryneum blight.  Are you using the Bonide brand Liquid Copper Fungicide (contains the Copper Octanoate as active ingredient?  (https://files.plytix.com/api/v1.1/file/public_files/pim/private/assets/43/37/8d/5e/5e8d3743202d9eba64d3af60/texts/23/9b/42/60/60429b2369f6e37cd386c0ca/l811.pdf)

If so, the summary at the top of the label states "Can be used up to the day of harvest", but this is not the case for every crop. Labels are notoriously difficult to read, and directions differ from crop to crop.  So, the user has to read all that tiny print down through the entire label to find instructions for their crop.  And, unfortunately the user is required to do this for each different product they apply, since details may differ.

The crop-specific instructions for Coryneum 
Blight on Stone Fruits in the table on Page 18 of the Bonide label state "Apply as a dormant spray in late fall to before bud-break.  Repeat application every 7-10 days if needed", i.e. during that fall-bud-break period.

Anyway, all this to confirm your statement that the label I referenced gave product specific instruction for the active ingredient "Copper diammonia diacetate complex*[Bis (acetate-O) Diamm".  The active ingredient in the Bonide product (Copper diammonia diacetate complex*[Bis (acetate-O) Diamm) have a different set of product-specific instructions.

As for treatment of Bacterial spot, the product you are using should be labeled for that as well, but with  pathogen-specific instructions.  So, if you are using the Bonide product, for which I gave the URL to the label above, instructions for bacterial control are in the table beginning at the bottom of page 18.  The reference from Oklahoma State does include recommendation for chemical treatment, in addition to cultural controls (pruning, fall cleanup).  For most homeowner situations, cultural controls are sufficient to manage the blight - especially increasing airflow to reduce humidity.

Finally, the sap leaking from branches can result from either insect infestation (borers), or Cytospora fungus.  The cause must be determined before deciding on a management approach.  The CSU Fact Sheet Backyard Orchard: Stone Fruits gives an overview of common pests.

It would be best if you could submit a branch sample to the Jeffco Diagnostic Help Desk for diagnosis ($15 fee).  Branches should be 8-20 inches long, and show both healthy and damaged tissue. Additional photos of the affected branches and the entire tree would also be helpful.  Send them to <personal data hidden>, and reference this Ask Extension Question number.  You could also bring samples of the suspected Coryneum blight for confirmation of disease.  The office is open Tues-Fri 8-5, and is located at the Jeffco Fairgrounds:  15200 W. 6th Ave. Frontage Road in Golden.

Jeffco has a new Horticultural Specialist, and I don't yet know her background in fungicidal applications.  She would be a good place to start.  Rachel Durkan,<personal data hidden>, <personal data hidden>.  If you contact her, reference this Ask Extension question so that she can review the discusion.

Again, thanks so much for your patience!

Sincerely,

Mari Hackbarth
Retired Plant Diagnostician, now a Jeffco Diagnostic Volunteer
Jefferson County Extension of Colorado State University Replied May 31, 2024, 10:33 PM EDT

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