• Fallow syndrome may occur in fields left unplanted or where soils are flooded for long periods of time and deprived of oxygen which may cause a decline in populations of soil fungi.
• Vesicular arbuscular mycorrhiza (VAM) are symbiotic root-like fungi that aid roots in intercepting soil nutrients such as phosphorus and water.
• When fields are left fallow, soil populations of vesicular arbuscular mycorrhiza (VAM) may decrease and affect water and phosphorus uptake.
• Some weed and crop species in the Brassica family contain naturally occurring fungicides and are considered non-hosts of VAM.

After a wet spring and a record number of prevent plant acres in parts of the Midwest, fallow syndrome has been on the minds of many. Fallow syndrome may occur in fields left unplanted or where soils are flooded for long periods of time and deprived of oxygen. These conditions may result in lower populations of soil living fungi, specifically Vesicular-arbuscular-mycorrhiza or VAM. To better understand fallow syndrome a complete understanding of the cause is needed.

Vesicular Arbuscular Mycorrhiza (VAM) are naturally occurring soil fungi that produce mycelial growth (hyphae), that branches out into the soil profile.These hyphae are long thread-like branches of fungi that have symbiotic relationships with most plant species and act as extensions of the plant roots, helping the plants access water and nutrients.Specifically, early in the season, VAM helps with accessing soil phosphorus when it is less available in the soil solution. Some species of VAM can start to grow at temperatures of 32°F; lower temperatures than those needed for corn and soybean germination. Unfortunately, there are certain factors that can actually lower the populations of VAM in the soil. For instance, certain crops and weeds of the brassica genus are considered non-hosts of VAM and so populations of VAM may be lower when these crops are grown. Examples of such crops include radishes, cauliflower, brussel sprouts, cabbage, canola/rape seed, wild turnip, wild radish and mustards. Other factors include lack of vegetation and flooding. Once a field experiences fallow or flooding it can take time for VAM to repopulate.This can result in limited access to crop nutrients such as phosphorus and zinc which may in turn cause visual symptoms of deficiency or yield loss.

The 2019 growing season presented itself with many challenges. Early season rains resulted in wet/flooded fields that sat idle and while some were planted to corn or soybeans, others turned into “prevent plant” acres. The risk of fallow syndrome on these prevented plant acres can be estimated by the specific time period the field was fallow, the height of any vegetation growing in those acres before frost and the plant species planted the previous season.The longer the fallow or flooding the lower the VAM population.Various scenarios could exist where VAM presence could be low and phosphorus deficiency could occur.

• Field vegetation consisted of weeds that were greater than six inches in height and covered the entire field. >40% of the weed species were brassicas that were very uniformly distributed in the field.Soil VAM population questionable.
• Field planted to cover crop containing radishes or other Brassica crop (>40% total population) that grew to greater than six inches in height and covered the entire field, with fall termination.Soil VAM population questionable.
• Fields kept clean of weeds and devoid of a cover crop the entire season, until frost. VAM population questionable.
• Spotty weed growth or cover crop establishment (< 10% Brassica weeds or crop).VAM population absent in areas of the field without plant growth.

Phosphorus, being a major crop nutrient, makes fallow syndrome worth paying attention to, since deficiency of this major nutrient often happens in fields with low VAM populations and adequate to low soil P-tests.Phosphorus’ main role in plant growth and development is energy transfer.The carbohydrates produced through photosynthesis are trapped where phosphorus deficiency occurs.Lack of assimilation of carbohydrates results in slowed growth, purpling of plant tissue as unused sugars are converted to an anthocyanin pigment to reduce the toxic buildup of carbohydrates in the plant.

Commercially available VAM products are limited in the marketplace with Valent USA MycoApply EndoPrime being the only Agricultural product available at this time. MycoApply EndoPrime is a commercial blend of mycorrhizal fungi that promotes root growth, nutrient uptake efficiency and drought tolerance in corn. For maximum effectiveness, VAM products should be placed close to growing roots for the symbiotic relationship to develop.MycoApply EndoPrime requires in-furrow application with or without starter. In 2018, research done by Dr. Fred Below and Logan Woodward at the University of Illinois, on non-fallowed corn ground, found MycoApply EndoPrime to be 3-4 bu/A better than the control.In 2016, Valent contracted research trials to independent testing parties across 10 states, including IL, IA and WI, and found this product to have a 60%-win rate and 5 bu/A corn yield increase on non-fallowed ground. Data from 2018 and 2019 across some of these same trials (IA, IL, IN, ME, MI, OH and WI), found mycorrhiza root colonization percentages on strips treated with MycoApply to average 15.9%, with a range of 5-20.5%. Sampling was done between growth stages V8-R5, with most taken from VT-R5.Mycorrhiza root colonization by native populations in untreated strips was found in 11% of the corn trials sampled.Some of the reasons attributed to these levels of colonization in the untreated trials include the intensity of tillage, soil compaction, and high available soil phosphorus.

Developments in the area of soil biostimulants and biologicals are occurring at a rapid pace. The GROWMARK System and cooperating FS Member Companies continue to screen new products and will keep you updated on their performance.