February 23, 2017

Arbuscular Mycorrhizal Fungi

In the last few years, the University of Minnesota Fruit Research Group’s Strawberry Low-Tunnel Project has spurred several other strawberry projects.  One of them, the Strawberry AMF Inoculation Trial Project, looks at the viability of using commercially available arbuscular mycorrhizal fungal inoculant to improve strawberry yields.  This project is being conducted by Jared Rubinstein, advised by Drs. Emily Hoover and Mary Rogers at the St. Paul Campus of the University of Minnesota.

Arbuscular mycorrhizal fungi (AMF) are naturally occurring fungi that interact with the roots of most of world’s plants, forming what’s called an “association.”  These associations are evolutionarily ancient, and are thought to have helped facilitate the movement of plants from sea to land, before the development of true plant roots!  Plants and AMF form what are generally considered to be symbiotic relationships: the AMF provide the plants with increased access to nutrients like phosphorous and nitrogen, among other benefits, while the plant provides the fungus with an ample source of carbohydrates.

AMF naturally associate with many agricultural crops, including strawberries, but agricultural fields often have low natural populations of AMF due to the frequent disturbance of the soil.  Some growers try to stimulate AMF by adding it to the soil artificially. For strawberries, these artificial “inoculations” have been shown to increase plant nutrient content, disease and drought resistance, and in some cases, even yield.

Some of the most successful studies have found higher strawberry yields after inoculating June-bearing strawberry plants with AMF and then allowing them to grow in sterilized potting soil inside a greenhouse or other controlled environment.  After about a month, the plants were planted out into the field and allowed to produce flowers and fruit.  We decided to simulate this method to see if we could get similar results using our day-neutral strawberries in an annual system.  We also wanted to see if we could get similar results but also save our strawberry growers some time and money—our second trial involves inoculating bare-root strawberries with AMF and then directly planting them in the field, cutting out the sterilized soil and controlled environment.

We used bare-root ‘Portola’ strawberries from Nourse Farms as our plant material and Micronized Endomycorrhizal Inoculant from BioOrganics as a source of AMF.

Bareroot strawberry

Bareroot strawberry

The AMF inoculant comes as a mixture of AMF spores and a talc-like, chalky compound that helps the fungus grow.

1/10th teaspoon inoculant

1/10th teaspoon inoculant

To inoculate the bare-root plants, we first cut the roots down to about three inches and misted the roots with water.  We then dipped the damp roots into a petri dish containing the powder until all the roots were covered in the inoculant.

Bareroot strawberry dipped in inoculant

Bareroot strawberry dipped in inoculant

Bareroot strawberry covered in inoculant

Bareroot strawberry covered in inoculant

There are liquid AMF root-dipping solutions available that make dipping roots very easy, but none of them are certified organic so we went with the BioOrganics powder.  After inoculation, half of the plants went into trays of sterile soil, where they grew in the greenhouse along with a non-inoculated control.  They were planted in the field approximately one month later.

Bareroot strawberries planted into trays

Bareroot strawberries planted into trays

Planting from tray to field

Planting from tray to field

Planting into the field with white on black plastic mulch

Planting into the field with white on black plastic mulch

The other half was planted into the field immediately after inoculation, along with another non-inoculated control group.

Inoculating in the field

Inoculating in the field

Newly inoculated plants

Newly inoculated plants

Planting newly inoculated plants in the field

Planting newly inoculated plants in the field

Once the season is over, we’ll pull out some plants and examine the roots for signs of AMF colonization to rate how well our methods worked.  We’ll also be keeping track of yield, brix, leaf nutrients, and plant size throughout the season to see if the two methods differ in how well our strawberries are producing.  Stay tuned for results!

Day-neutral strawberry field

Day-neutral strawberry field