Hoorman: Fungicide on Microbes
As spring approaches, farmers may be considering applying fungicides to wheat, corn, and soybeans. Fungicides are used to terminate fungus. Beneficial soil fungus like Arbuscular Mycorrhizal Fungus (AMF) are hurt by some fungicide applications, but not all. Some fungicides (Apron, Aliette, Ridomil, Metalaxyl) stimulate some AMF to grow better by terminating pathogenic fungus that compete with AMF. Here are some general tips about fungicide use and AMF.
In general, foliar applications of non-systemic fungicides to leaves and stems have much less impact on AMF which live around the roots. Non-systematic means the fungicide is not as mobile in the plant or soil or generally as long-lasting. Some fungicide can wash off the plant, but the soil organic matter and diluted spray tends to have minor effects on AMF long-term. Any effect is usually short term because the AMF can regenerate.
Soil drench applications of non-systemic fungicides are detrimental to AMF if applied before root colonization takes place. Once AMF spores start to colonized roots by getting inside the root, they tend to be less sensitive because they are protected inside the root. High fungicide soil levels are detrimental to exposed fungal tissue, but not fungal tissue embedded inside the root. The protected AMF will growth a new mycorrhizal network (like a spider web) out as much as 6-18 inches from the root, searching for water and plant nutrients (phosphorus, many micro-nutrients, and water) so the effect is usually only shortterm.
Some fungicides are systematic which means they are extremely plant and soil mobile and long lasting which means they can have a long-term effect on beneficial AMF. The detrimental effect can last up to 3 weeks. Some farmers now inoculate their seed and crops with AMF. The general rule is that foliar non-systematic fungicides can be applied any time without too much issue. Soil drenches should not be applied 2 weeks before (longer for systematic fungicides) or four weeks after AMF inoculation and planting.
The following is a list of fungicides that so far have not exhibited an inhibitory effect on AMF. In other words, these are the best ones to use if you want to build or maintain soil health. A fungicide is considered to be inhibitory if there exists a published report documenting inhibition. It should be noted that there are many chemical names for the same product and not all are listed.
Fungicides not known to hurt soil health include: Carbamate (many different fungicides, AMF effects vary); Carbendazim (Bavistan); Difolatan (Captafol, Difosan); Chlorothanil (All Pro, Dacnil); Fludioxonil (Maxim, Medallion, Dynashield, Celest, Agri Star); Mancozeb (Dithane M-45, Manzate, Fore, Junction); Manate (Dithane M-22, Maneb); Tricoderma sp (Root Shield, Root Shield Plus); Senator (Thiophanate); Sentinel, and Thiram (Tersan 75, Arasan).
Some fungicides that tend to hurt soil health and AMF populations include: Benomyl (Benlate, Tersan-1991); Captan; Tilt (CGA-65250, Banner, Propiconazol); Triadimefon (Bayleton, Strike, Strike Plus); Vitavax (Carboxin, DCMO); Strobilurins such as Azoxystrobin (Abound, Dynasty, Heritage, Headline, Protégé, Quadris, Quilt, Soyard, Uniform);
Kresoxim-methy (Cygnus, Sovran); Fenpropimorph (fungistatic effect); and Topsin-M (Easout, Fungo, Duosan).
Most fungicide treatments do not totally eradicate AMF; they only decrease development for a period of time after application. The time of this effect depends on the duration in which the chemical persists in the environment. Most insecticides or herbicides if applied at label rates do not directly affect AMF development. Herbicides may affect AMF fungi indirectly by harming or killing their host plant. Glyphosate (Roundup), however; is known to reduce soil microbial communities and soil health for a period of 2-6 weeks (longer if dry) due to its ability to chelate or tie up soil micro-nutrients. Since there are close to 200 beneficial AMF species that infect our agricultural crops, some AMF are more susceptible than others to pesticides (fungicides, insecticides, herbicides).
Several species of grass cover crops like oats and sorghumsudan promote many beneficial AMF species and should be grown often to increase AMF populations. Most legume (hairy vetch, winter peas) and clover (Balansa, crimson, red) cover crops are also good for increasing AMF populations. Avoid planting brassicas (Daikon radish, rape, kale, turnips) or buckwheat by themselves because they are non-AMF and tend to decrease beneficial AMF populations and may even decrease crop yields. Brassicas and buckwheat can be planted in cover crop mixtures at low rates with no issues because the other grasses, legumes, and clovers will keep the AMF alive for 9-10 months, until they reproduce and finish their live cycle. Fungicides can be used to reduce pathogenic fungus, but careful selection is required to promote AMF and soil health.