Fungal and Microbial Infections

Plants would not have made it onto land half a billion years ago (500MYA), if not for the symbiotic partnerships that they have made with fungi[1].  Every single species of plant forms some form of symbiotic relationship with a fungus[2], which is proof that a healthy culture of fungal microbes is necessary for plants to thrive.

However, while most fungi are good or neutral, some are detrimental to your orchid’s health.  Slime molds, soft rots, and other fungi fill the niche in the environment of controlling the population of plants.  Most of the offending fungi, interestingly, are majority Oomycetes fungi, commonly known as the slime molds.  These fungi spread majorly through water splash transmission or contact transmission, and once established, cause major damage, if not death of your orchids.

Many fungal infections are commonly called “root rot”, as they do indeed rot your roots.  But as orchid growers, with a keen eye, if you can identify the fungus that’s affecting your orchids, then you can use the right fungicide and cultural methods, and completely purge it.

General Rules for Preventing and Getting Rid of Fungal Infections

Orchids typically grow in humid, wet environments, which are also environments that fungi/bacteria find favorable.  In greenhouses or outdoors, it’s nigh impossible to not get the leaves wet while watering.  Indoors, it’s much easier to keep the leaves dry.  Managing fungal/bacterial infections indoors is much easier than in greenhouses because of this.  Diseases spread primarily through contact or through water splash.  Less commonly, fungal spores and bacteria can travel through the air, but leaves/roots must be wet for an infection to happen.

Recommended Precautions to Prevent Infections

  • Space out plants so that their leaves are not touching can prevent contact and water transmission. 
  • Frequently cleaning up dead leaves off the floor and around the plants prevents sources of spores/bacteria
  • Regularly spray a fungicide, such as Physan or 3% hydrogen peroxide as a prophylactic on your plants
  • Sterilize tools between trimming/repotting plants
  • If reusing pots, soak overnight in a 10% bleach solution, then rinse
  • Do not reuse media

If you suspect you have an infection, keep the leaves dry, remove infected tissue, and spray plants down with Physan 20, 3% hydrogen peroxide, or a suitable broad-spectrum fungicide.  One word of caution regarding fungicides containing copper, some orchids, especially dendrobiums, are sensitive to copper and can be easily damaged or killed by fungicides containing it..

[1] Field, K. J., & Pressel, S. (2018). Unity in diversity: structural and functional insights into the ancient partnerships between plants and fungi. New Phytologist, 220(4), 996-1011.

[2] van der Heijden, M. G., Martin, F. M., Selosse, M. A., & Sanders, I. R. (2015). Mycorrhizal ecology and evolution: the past, the present, and the future. New phytologist, 205(4), 1406-1423.