Managing fungicide resistance buildup (for those who spray)

9 years ago

This was inspired from perusing older threads discussing fungicides for roses. In some of them resistance management was mentioned but in many of these the advice given was inaccurate.

Avoiding building up fungal resistance to fungicides is an important issue in fungicide use. AFAIK fungi have exhibited ability to evolve resistance to fungicide for most fungicide classes (especially organic fungicides) invented. Two things are important: Limiting fungicide use to the minimum and alternating fungicides that act differently. Below a few key points:

1. Get to know the name of your fungicide's drastic ingredient. Referring to fungicides by brand does not help much in this. Many different brands may contain the same or similar active ingredients. By alternating between two different brands without knowing the active ingredient you will probably end up alternating btween the same ingredient..

2. Get to know the drastic ingredient's mode of action GROUP (FRAC codes in the US). Resistance is more often than not exhibited to all drastic ingredients in a group. For example, one may be using myclobutanli and tebuconazole alternating between them thinking resistance will be managed in this way but both of these belong to the same triazole group 3. Thus, alternating between these two does not help much. Below a link to a fairly up to date list of fungicides per mode of action group. This I found with a quick google search. Better and more inclusive lists may exist on the net. In the chart all classes not signified by an M have a high chance of fungal resistance build-up.

Fungicides

3. Limit fungicide applications to the minimum possible per growing season

4. Limit use of ingredients of the same group to max 3 per season (or follow the label if it states otherwise but extend the restriction to all ingredients of the same mode of action group).

5. Alternate between ingredients of different groups and limit successive applications of ingredients of the same group to maximum 2.

6. Resist the urge to spray repeatedly with the same mode of action ingredients during serious outbreaks of disease since when fungal presence is high the possibility of a resistant strain being selected by evolution is higher. Applying an organic fungicide alternating with other chemicals (such as potassium salts like potassium bicarbonate or monopotassium sulfate or with oils) may be preferable.

7. Do not spray using less (or more, of course) than the recommended dose thinking that you will do less harm to the environment / yourself in this way. Use the correct dose depending on whether the fungicide is being used preventively or curatively or do not spray at all.

I hope the above helps. I do not encourage the use of organic fungicides but if one chooses to use them it's better to use them properly.

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michaelg

9 years ago

Just to point out that nik's OP uses the word "organic" in the scientific sense of "containing carbon" and not in the sense of "organic gardening" remedies. Actually the meanings are reversed in that "organic gardening" fungicides are not chemically "organic" fungicides and vice versa.

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Comments (10)

Buford_NE_GA_7A
9 years ago
It's a good idea to rotate different fungicides. I've always used the Bayer Advanced Disease Control, and it works well. However this year I am alternating with another fungicide and I have noticed that my roses look cleaner and one of my roses that gets these awful pink dots on its flowers doesn't have them. I think that is a sign of boytritis, so maybe this other spray is better at handling it. Many rosarians combine a contact fungicide and a systemic in the same spray.

I'm doing the Consulting Rosarian course and this is a huge part of that curriculum.
nikthegreek
Original Author
9 years ago
BADC contains tebuconazole as I read. Your other fungicide should not contain an active ingredient in the same group (e.g. myclobutanil, propiconazol etc).
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Buford_NE_GA_7A
9 years ago
You are right, it does have propiconazol. Maybe that is just more effective than tebuconazole? I am going to phase out the Bayer and then possibly add another non zole fungicide to alternate. I just wanted to use up the Bayer that I had left over.
nikthegreek
Original Author
9 years ago
last modified: 9 years ago
At least for PM I firmly believe in wettable sulphur despite its disadvantages (cosmetic issues and limited effective temperature range without tissue toxicity issues). Nothing else comes close IMO for its combination of preventive and curative action together with volatile effectiveness (i.e. no need for full coverage, some protection of shoots and leaves growing after the application) and lack of acquired resistance to it. For PM, the options to alternate between organic fungicides are not very many for roses I'm afraid. Strobilurins, which work well for PM fungi in other plants, seem not to be highly effective for rose PM.
nikthegreek
Original Author
9 years ago
last modified: 9 years ago
Yes indeed Michael. Good point, thanks. I use the word as in 'organic chemistry'. 'Organic' being a word coming from greek, I'm confused sometimes about its everyday use in english. In greek (and in other languages I believe) we use another greek word (of course..) 'biological' for this. The use of either word is problematic (another greek word lol) in my view. Not all 'organic' chemicals (yet another..) are supposed to be a no-no for organic growing and vice versa (this comes from latin though).

PS1 To add insult to injury, in the case of fertilizers the matter is reversed. Chemically organic ones are 'good' for organic growing while inorganic ones may be considered 'bad' regardless of the fact that plants like to eat mainly inorganic ingredients... What is OK for organic growing and what not is a matter of debate, confusion and as much of science as of pre- and misconceptions*.

*PS2. In some quarters for example, mined mineral fertilisers (inorganic compounds) are supposedly OK while the same chemical, if manufactured, is disallowed (the difference being the supposedly environmentally hazardous production process). Go figure.
michaelg
9 years ago
Yes, I over-generalized. Some chemicals derived from botanical and bacterial sources are "organic" in both senses. However, sulfur, copper, and bicarbonates are chemically inorganic but horticulturally "organic."

"Synthetic-organic" is a useful term for manufactured substances that contain carbon but do not occur naturally.
nikthegreek
Original Author
9 years ago
Yes, a funny thing about copper, given what copper overbioaccumulation can do...
michaelg
9 years ago
And horticulturally organic rotenone turned out to be a dangerous chronic toxin for mammals.
henry_kuska
9 years ago
last modified: 9 years ago
If you use one of the triazole class of fungicides to spray your roses, you should rotate with a non triazole class fungicide to avoid fungi found in the soil around the rose plants (that also infect humans) building up resistance to medical triazol drugs.

"Such strains probably arose in the environment, and, indeed, multiple studies have now demonstrated TR34/L98H triazole resistance strains of A. fumigatus from soil. Docking and other in vitro studies are consistent with environmental resistance induction through exposure to certain triazole fungicides, notably difenoconazole, propiconazole, epoxiconazole, bromuconazole and tebuconazole. This article addresses the potential implications of this issue for both human health and food security."

Please see the following published, reviewed scientific paper.

http://onlinelibrary.wiley.com/doi/10.1002/ps.3567/abstract

This paper, although recent (2014), has already been cited by 6 other papers.

http://scholar.google.com/scholar?cites=640167471732079514&as_sdt=5,36&sciodt=0,36&hl=en

Managing fungicide resistance buildup (for those who spray)

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