The Role of Soil Fungi in Medicinal Plant Conservation and Regenerative Agriculture: Soil Fungal Inoculants’ Impacts on Crop Yield, Nutrition, and Basil Downy Mildew

by Melody Wright

Research Beds

The cultivation of medicinal herbs using regenerative agriculture methods can be as vital as conservation and education in meeting United Plant Savers’ goals. Herb farmers have the ability to cultivate some of the UpS “At-Risk” or “To-Watch” species, potentially reducing pressure on wild populations, as well as grow and promote alternatives to these same plants of concern. Herb farmers, particularly those who sell directly to consumers through farmers’ markets or Community Supported Agriculture (CSA) models, can also play a vital role in educating consumers who may not be aware of herbalists’ and UpS members’ stewardship concerns.

In an ecosystem, such as a farm, just as in humans, the health of the individual depends on the health, support, and cooperation of the larger community. On farms the role of soil microorganisms is increasingly being studied and understood as vital. Peer-reviewed research has shown that a healthy population of soil microorganisms can help increase crop yields, promote disease resistance of plants, and provide tolerance to abiotic plant stressors such as drought.

Newhealth basilgrowth
New healthy basil growth

Folks drawn to herbalism often look beyond the individual health benefits of plants but also believe in the role of plant medicine in the healing of the community and planet. Soil microorganisms can be part of this larger healing, as they can help produce soil stabilizing humic compounds, act as natural fertilizers for crops, provide a significant role in carbon sequestration to help combat climate change, and also provide economic stability to farmers. Finally, research has shown that the right soil microorganisms can increase the nutrient density of foods and increase the levels of the active constituents in medicinal plants, potentially leading to increased medicinal benefits.

The relationship between plant roots and one part of the world of soil microorganisms, the mycorrhizal fungi, has been called the most important symbiosis on earth, essential to ecosystem function. The challenge for gardeners and farmers is that it is still unclear how to best work with these mycorrhizal fungi. The high level of interest of growers in the benefits of working with mycorrhizal fungi led to the development of a farm-based field research project looking at crop nutrient density, disease resistance, and crop yield in five medicinal herb crops: sweet basil (Ocimum basilicum), ashwagandha (Withania somnifera), parsley (Petroselinum crispum), fennel (Foeniculum vulgare), and onion (Allium cepa). This field research was completed on a small medicinal and culinary herb farmlet called Pleasant Valley Botanicals in Northwest Connecticut. It was completed by the farmer, with the assistance of volunteers (special thanks to Andrea Boneset!) and statistical analysis by Dr. Maura Bozeman (a local professor of Environmental Sciences).The methodology was designed to look at the crop yield of these five crops, the nutrient density of onions, and the disease resistance of sweet basil to basil downy mildew. Basil downy mildew was chosen because it is a devastating oomycete (Peronospora belbahrii) that can cause 100% crop loss, and there are no effective organic or conventional controls. Basil is the most econonomically profitable annual culinary herb crop in the United States, and crop losses due to basil downy mildew can have a significant negative economic impact on vegetable and herb growers.

This research used a block design method, where each of the five crops in the field received one of three treatments at time of transplantation: a control, a commercially purchased fungal inoculant, and a homegrown fungal inoculant called IMO (Indigenous Microorganism). IMO inoculant is based on a Korean Natural Farming method, designed to cultivate a native population of mycorrhizal fungi and hopefully provide a wide range of benefits to the plants through these symbiotic relationships.

As each crop was harvested throughout the growing season, the yield was recorded from plants in each of the different treatments; crops were then included in the farm’s Medicinal Herb CSA shares. After data analysis, no crops in the IMO or commercial inoculant treatments had statistically significant improvements in crop yield compared to the control treatment. We also looked at the total inter-species crop yield from each bed to see if there was any possible benefit from the common mycorrhizal network. The CMN concept has shown that fungi will share nutrients between different species of plants, such as one study showing nutrient flow between a Douglas fir, a paper birch, and a western red cedar. However, there was also no statistically significant increase in inter-species crop yield.

Ultimately, this researcher concluded the environment created in the field trial did not meet the conditions necessary to support the mycorrhizal fungi and ultimately the benefits to crops as measured by yield. Specifically, regenerative agriculture presents us with three key components to have more effective sustainable agriculture systems: cover cropping, crop diversity, and low or no-till practices. This particular field design only incorporated crop diversity, as there was no prior cover cropping, and the design incorporated significant prior soil disturbance. Other research strongly suggests that maximum benefits are achieved by the inoculation of plants at the time of seeding (germination) as opposed to at the time of transplantation, as done in this study.
Because of financial and lab access limitations, this study used a refractometer to look at the brix levels of one crop, onions, as an attainable proxy for nutrient density or active constituent levels. There were no statistically significant differences in the onion brix between each treatment.

Finally, additional sweet basil plants in each treatment protocol were monitored weekly for signs of basil downy mildew based on percentage of leaf area with sporulation. There was a delay in infection in basil plants inoculated with the Indigenous Microorganisms treatment, which is a potentially promising finding worth further investigation. Even more exciting were two basil plants in the IMO treatment protocol that were infected with basil downy mildew (which typically has 100% crop loss), but then made a significant recovery with new healthy leaf growth. These finding may be an indication of systemic acquired resistance, though such a conclusion is beyond the scope of this study. This resistance is when a plant is able to develop an immune response or resistance after exposure to a pathogen, in this case because of the benefits of mycorrhizal symbiosis. Plants growing as part of a healthy ecosystem that includes mycorrhizal fungi have improved access to the diversity of metabolites and are better equipped to combat pathogens. Remarkably, plants can down-grade their immune systems to allow for beneficial symbiotic relationships, which later can protect them from other pathogens.

IMO (Indigenous Micro-Organism) treatment

Setting this research project within a larger body of knowledge, the author would offer the following conclusions to gardeners and polyculture farmers: By adding fungal inoculants at the time of seeding, we improve the likelihood of seeing beneficial results. By exploring low cost, low tech methods of cultivating local indigenous microorganisms, we may find benefits well worth the effort. But most important is supporting growing conditions where soil microorganism can thrive, which can potentially help grow higher quality and more medicinally potent plants and offers significant environmental benefits. If used simultaneously, the three key tenets of regenerative agriculture: use of cover crops, crop diversity, and low or no-till practices potentially will create a habitat for the soil microorganisms and their associated benefits. These practices bring us back to farming methods more in keeping with natural processes than current conventional agriculture methods.

Author Melanie Wright
Author Melanie Wright

Melody Wright is the farmer/founder of Pleasant Valley Botanicals, a small farmlet growing vibrant medicinal and culinary herbs for the local community. Her commitment to growing medicinal herbs is based in her belief of herbs as an integral part of affordable and holistic health care and agricultural systems.

The author welcomes correspondence on this article and will gladly share the data and findings, a bibliography of helpful books and relevant peer-reviewed research, or information about the SARE program for interested medicinal herb farmers. or

This project supported in part by the Northeast Sustainable Agriculture Research and Education (SARE) program. SARE is a program of the National Institute of Food and Agriculture, U.S. Department of Agriculture.