Poinsettias in greenhouse

All growers experience some level of crop loss or ‘shrinkage’. Whether it’s a small percentage across a whole crop or more specific to certain varieties that are a little more troublesome to grow, on average growers lose about 5% of their crop from root diseases. Often, fungicides are applied after the plant disease shows its ugly head, or you can apply fungicides as a preventative, however, this can be costly.

Then there is always the shrinkage at retail, which can vary from 5-30% plant loss. Growers have little control at retail and this loss is very costly since it includes the cost to deliver the plants. The bottom line is that plant loss shrinkage reduces profits. To reduce losses, biological active ingredients can be added to growing media to enhance plant growth, reduce crop shrinkage and provide long-term protection for plants.

Where Did They Come From?

Growing media used in greenhouses and nurseries are ‘constructed’ with organic-based materials, such as peat moss, bark, coconut coir and/or other materials. Ingredients, such as aggregates, fertilizers, limestone, etc., are added to achieve specific physical and chemical properties of growing media. However, growing media contains low levels of microbes making them virtually free of the biological component.

In mineral soil, there is a diverse biology. Some microbes convert organic matter into nutrients for plants, some fix nutrients in the soil and others interact to protect plant root systems. In mineral soil, there can be any combination of microbes consisting of bacteria and/or fungi. Of these microbes, there are saprophytes, which feed on dead material; pathogens, which feed on and damage plants; symbionts that assist plants; and competitors, which can attack or compete with other microorganisms.

Over the years, scientists have isolated and cultured various beneficial microbes for use in agriculture and horticulture. These microbes are produced in laboratories to be sure that strains remain true and continue to provide the benefits desired. If we review the various microbes commercially available to growers, we can place them into two general categories by the benefits they provide: Biocontrols and Growth Enhancers.

Biocontrols are biological additives that reduce the incidence of plant disease or control insects. They may directly attack the pathogen or insect, or indirectly control it by creating barriers, blocking food sources or excluding a pathogen. Compared to Growth Enhancers, these microbes assist plants by stimulating plant growth or acquiring nutrients and/or water to improve the overall growth of plants.

Beneficial Microbes/Biocontrols Advantages

One of the main advantages of using beneficial organisms is that they are safe to use. They have little to no plant, human or animal toxicities. Most have been discovered in mineral soil and are natural, not genetically modified. Because they are part of the normal checks and balances of soil biology, there is less potential for pathogen resistance.

However, most beneficial microbes are preventative, not curative; therefore, they prevent the onset of plant disease before it has a chance to affect crops. An added benefit is that most beneficial microbes remain in the root zone of the plant for an extended period, while some will remain with the plant for life. If you are growing organic-certified crops, many biological active ingredients are certified organic with OMRI and offer an option to control plant disease where chemicals cannot be used.

Beneficial Microbes/Biocontrols Disadvantages

A disadvantage is that these beneficial organisms are live microorganisms, and they may require special storage and use conditions. Almost all beneficial microbes have a shelf-life for their optimal use, so users need to respect the expiration date. If you recall your basic biology, fungi are more fragile than bacteria. Keep this in mind since fungal organisms are sensitive to warm temperatures and often require cool storage temperatures before use. Fungal organisms may also desiccate easily reducing their effectiveness. Bacteria are more robust and forgiving when handling, storing, and applying due to the thick cell walls of their spores.

When incorporated into growing media, microbes work best when growing medium temperatures are above 50F. Unlike chemical fungicides that are wide spectrum, biological controls generally interact with specific pathogens, therefore their range of pathogen control can be narrower than their chemical counterparts. If used in combination with chemical controls or other biological additives, be sure to check the compatibility of the microorganism with the chemistry and other bioadditives used. Some chemicals and other bio-additives may reduce populations, while others may eliminate the beneficial microbes.

Impatiens basket with biocontrol
The plant on the right was treated with a biocontrol to reduce root disease. The results show normal growth compared to the plant on the left which suffered from minor root disease which suppressed plant growth.

Cost to Add Biocontrols to Growing Media

The cost of biological additives is generally inexpensive, compared to some chemical controls. If we compare the costs of various fungi and bacteria available, the cost to apply to a cubic yard is in the range of $8.50 to $26.00 for the active ingredient. If we use an average cost of $17.25/cubic yard, we can compare the cost for various crops.

One cubic yard of growing media will fill approximately 1 944 azalea pots of 4.5 inches or 141 hanging baskets of 10 inches. Using our determined average cost ($17.25/cuyd) for the active ingredient, the cost to add the active ingredient is $0.0089/azalea pot or $0.1223/hanging basket. Based on the 2021 USDA Floriculture Survey, the average wholesale selling price for a ‘less than 5 inches’ potted flowering plant sold for an average of $1.56 for the top producing 15 states. A hanging basket of flowering and foliage plant had an average wholesale selling price of $8.70. If we take the cost of adding an active ingredient to these pots, it represents less than 0.6% for an azalea pot and less than 1.4% for a hanging basket.

Table 1. Cost of Active Ingredients based on an average of $17.25/yd3.
  Azalea pots
(4.5 inch)
Hanging baskets
(10 inches)
Number of pots 1944 141
Cost/pot $0.0089 $0.1223
Selling price $1.56 $8.70
Active ingredients cost (%) 0.57% 1.41%

Saving Money by Using Biological Active Ingredients

The average plant loss due to root disease is about 5% for most growers and 20% at retail. Based on a cubic yard of growing medium as in our previous example, this is equal to approximately 97.2 azalea pots, a $151.63 value, or 7 hanging baskets valued at $60.90.

If a biological active ingredient reduces root disease and plant losses by two thirds, the cost of $17.25/cuyd for the biological additive added to growing media would be offset by the savings of $82.83 for plants sold ($151.63 x .66=$100.08 savings minus $17.25/cost/yd3 = $82.83 reduction of loss). For 10” hanging baskets, the reduction of plant losses would give an additional revenue of $22.94 per cubic yard ($60.90 x .66 =$40.19 savings minus $17.25/cost/yd3 = $22.94 reduction of loss).

Table 2. Reduction of loss for 1 cubic yard (at growers).
  Azalea pots
(4.5 inch)
Hanging baskets
(10 inches)
Crop value $3,032.64 $1,226.70
Plant loss without Active ingredients (5%) $151.63 $60.90
Plant loss with Active Ingredients (1.66%) $51.55 $20.71
Active Ingredients Investment $17.25 $17.25
Reduction of loss / Additional revenue $82.83 $22.94

If we apply the plant loss reduction at retail, these savings can be up to 4 times greater. These are averages, actual savings and return on investment (ROI) can be much higher depending on the biological active ingredient used, its effectiveness in controlling root disease and the wholesale selling price of crops you are growing.

The use of biological active ingredients provides multiple advantages and is becoming a popular option and addition to chemical controls for plant disease. They are readily available, provide predictable results, cost cost-effective and provide long-term benefits for crops, offering growers a safe alternative and an additional tool in the integrated pest management toolbox.