Captan, Myclobutanil and Topsin M questions


I have been reading more on fungicide sprays. I came across this -

“Check the pH of the spray solution, especially when using alkaline well water. While most fungicides are stable over a range of pH values, some fungicides (e.g., Captan, Dithane, Rovral) can degrade under alkaline conditions. For example, the half-life of Captan is 32 hours at pH 5, eight hours at pH 7, and 10 minutes at pH 8. The half-life of Dithane is 32 hours at pH 5, 17 hours at pH 7, and 34 hours at pH 9. (Insecticides in general are more sensitive to pH than fungicides.) For a list of pesticides and their sensitivity to pH, see the 2012 Michigan Fruit Management Guide, pages 59-60. The pH can be adjusted with an acidifying or buffering agent. Avoid letting the spray sit overnight in the spray tank. Fungicides should, whenever possible, be mixed and sprayed as soon after mixing as possible.”

I think someone had mentioned (maybe Olpea or Alan) in the forums of the need for the water to be at the right PH level,.

I am using well water so I assume my PH will be around 7. I will get some PH test strips to confirm, What are common acidifying or buffering agents used with Captan/Water? My first thought is to use vinegar to lower PH but I do not know the pros/cons (hurt trees, sprayers)? Maybe the chemicals used to lower PH for pools? I assume PH of 5 is optimal with Captan from this article.

I see plenty of articles mentioning is Captan and Topsin M combined to together but I never see Topsin combined with
Myclobutanil. Is there a reason for why Topsin M should not be combined with Myclobutanil?


Here is a site that has a chart showing the effectiveness of various fungicides and insecticides -

Here is another article recommending vinegar -

Reading further it looks like Phosphoric acid is used to lower PH. Is there a cost effective way to use Phosphoric acid to lower PH that won’t burn my trees?


Another idea is a rain water tank. Rain water tends to be a bit acidic and so should not need any pH modification.


I expect the well water in your area is at least slightly acidic. If you have copper pipes in your house that leave behind a turquoise stain in the sink or tub, you cab bet your water is acidic.

I could not find examples in commercial spray schedules where the three chemicals are mixed together, but Myclobutanil is active against rust which requires an early spray and Topsin is active against summer rots which normally occur later. I see Captan mixed with other fungicides a lot to help reduce resistance to particular diseases…


Well I ordered the PH tester and some test strips today to find the PH of my water. Maybe I will not need a PH reducer for my tap water.

It looks like this company has PH reducers designed for agriculture use.

Most of there products seem to be multipurpose though and I am not sure if I need anything more than a PH reducer (do I need a surfactant?). At least there products do not seem expensive. Anyway the never ending journey of learning about fruit tree growing …



I use citric acid to lower pH. It’s cheap and recommended by some university extensions. I bought a pH meter and tested how much citric acid it took to lower the pH. For my water, it takes 1 teaspoon per 24 gal. to lower it to 7 pH, or 1 teaspoon per 16 gal. to lower it to 6 pH.

Re: compatibility

According to an MSU compatibility chart, Topsin is fully compatible w/ myclobutanil. Myclobutanil is listed as Nova on the chart (toward the top). Here is a link to the chart (chart is on page 125). Warning, this guide is a big file and crashed my Firefox browser. I had to use Chrome to open it (and it took about a minute to download).

One question I have is that I always see it is recommended to acidify water for glyphosate (the chart you link says 5-6 pH) but I never see any info on the half-life of glyphosate in alkaline water. I always acidify water (along with adding AMS) when spraying glyphosate, but I wonder if I’m wasting my time on the acidifying part? Anyone know of any info indicating the half-life of glyphosate at various pH levels? Glyphosate breaks down slowly in the environment, so I would think it would be stable in the tank at various pH levels, but I always see recommendations the tank water should be acidified.


At what pH does it start?


I use a mix of captan and t-m on strawberries in the spring for botrytis mold - have been acidifying my water after advice here


Glyphosate is a strong metal chelator, so it might be that reducing the pH shifts reduces the amount of glyphosate that can complex with metals (e.g. calcium, magnesium, iron) in hard water. Most phosphate-metal minerals are more soluble at low pH’s and I’d expect similar behavior from the phosphate group on glyphosate.



Our water is very alkaline. I’ve tested it at 9.5 ish, which is in line with our water report, which shows the range from 9.4 to 9.9.

Results of acidfying may vary, even if the pH of the water is the same, since the buffering capacity of different water may be different.

According to Spud’s link above, MSU says 2 ounces of citric acid per 100 gal of water will take water from 8.3 to 5.4. That’s about double what I’m using to take our water from 9.5 to 6.0.

Thanks Levers. It sounds like you’re saying the only reason for lowering the pH is to reduce chelation in the tank water. I already add ammonium sulfate to the tank before adding glyphosate for that purpose, so it sounds like I may be wasting my time adding citric acid as well?


Thanks BlueBerry for the info. The pipe sin the house are pvc so no copper. I had some rot last year before the squirrels cleaned my trees so Topsin will be in my arsenal next year.


Thanks Olpea for the info! :slight_smile:


Wow… reallly nice Consumer Confidence Report to have all that info in as far as metals etc. Kudos to your water utility. Many utilities don’t include that much data in their Confidence Report because they believe they will confuse the customer or get bad press etc about X, Y, or Z being in the water.

Here is a study that suggests that you might not even need to be adding AMS since your total hardness is less than 250 ppm:


You can check out how effective various combinations of materials are for your state in a publication co-authored by the folks at Virginia Tech. The PDF is free but cumbersome.

I believe the bound hard copy version is about $30. 180 pages and covers VA, WV and Md


It has a lot of information that is not contained in similar manuals from Penn State or the one designed for Southeast apple growers



Thank you very much. It never occurred to me AMS might not be necessary. I double checked the water source at the farm. I thought it was the same water source as the house (which is what I pasted in my last post) but wanted to make sure. Turns out the farm is not the same water source as the house.

Here’s the report on the farm (not very comprehensive but has some info.)

They show Ca and Mg as low, but don’t show total hardness. However, they show TDS as potentially high (range of range of 150 to 480). I know it’s impossible to extract hardness simply from TDS, but an internet search of TDS shows anything above 150 TDS on the harder end of the scale.

Is there some way you can estimate hardness from the numbers in the report? Just trying to figure out if I need to add AMS to the farm water, if it’s possible to determine with the info. given?

You’re right about the house water report. They do include a ton of test results. The part I pasted earlier was only about 1/3 of the report. I doubt your interested, but just in case, here is this years full report (for the house). The part I pasted earlier, was from an older report, but the water quality hasn’t changed much from year to year.

I bet anyone can go online and find the water report for their local water source. Never occurred to me until just now, when I started hunting for a water report for the farm.


Yes, presuming that the calcium and magnesium are listed as mg/L of Ca and Mg, respectively: multiply the result for calcium by 2.5 and magnesium by 4.12. Then sum the two to get the total hardness in mg/L as CaCO3. This gets you to a value that can be compared to the 250 mg/L as CaCO3 in the paper I posted.

So: 47.9 mg/L Ca x 2.5 + 6.55 mg/L Mg x 4.12 = 146 mg/L as CaCO3. Reporting things in mg/L as CaCO3 is stupid, but it is historical standard practice in environmental engineering.

Here is KC’s consumer confidence report.

At the very least the utility is required to mail the confidence report to customers yearly. Not all utilities in small towns have the info easily available online. That is somewhat understandable as they don’t have the resources to have great (or any?) websites. I have heard that utilities even in decent sized cities (100,000+) barely get calls on the info or results in the confidence reports.

Looking at the KC area confidence reports makes me wonder if the Kansas DEQ/DNR/EPA (whoever has jurisdiction) requires more info than most states. They all have much more info that ours in Iowa.

Sorry for the off-topic digression… This is my vocation (environmental engineering), and we have a small project at work taking results like these (particularly lead and arsenic) and hopefully making them into a map-type website for Iowa. The goal is to make these types of results more accessible to anyone with an internet connection (and also to hopefully understand what small towns can do to avoid having Pb in their water).


Thank you very much Levers. I appreciate you letting me pick your chemistry brain. I would have never known the AMS was not needed for my water sources. I plan to discontinue using it and watch to see if I see any difference.

One last question (I hope). Since I won’t be using the AMS, do you think I should continue using citric acid for the glyphosate to try to mitigate the small amounts of Ca/Mg? One thing the abstract you linked didn’t mention, was the pH of their water source.


I should say I’m not an agronomist… and they would probably still say to use the AMS. Looking into it more it might not just be the calcium in water that AMS is useful for enhancing glyphosate, but also mitigates calcium binding of glyphosate in the weeds. Velvetleaf apparently has lots of calcium in the fuzzy parts of the leaf and AMS helps overcome that leaf Ca to kill the weed.

Here is a calculator that you can plug your water results in, and it comes out with much less AMS than the 7 lbs/100 gallon that keeps coming up in my searches.

I’m not sure about the citric acid. Besides lowering the pH it is also a chelator, and it would likely have a similar effect of binding up Ca in the water or in the weed leaf surface.


Thanks again.

According to the calculator, I’d only need about a pound per hundred gallons anyway. You’re right that is much less.