Garden air is imbued with pleasant floral scents, photosynthetic-derived oxygen, and life-giving moisture from evaporation. It is also rife with fungal spores, both benign and pathogenic.
Unless extreme clean room measures have been taken, it can be assumed that there are already pathogenic fungal spores in any garden, awaiting the opportunity to germinate and contaminate. This is not mere hyperbole. In general, the air of any temperate indoor or outdoor space will contain some level of fungal spores due to the reproductive strategy of fungi.
To contrast, higher life tends to heavily invest in the quality and care of each of their offspring. They have comparatively few offspring, and each child represents a substantial cost in resources and time. Small litter or single child birthing habits are common to more complex life. A benefit to this is that the developmental head start and additional nurturing gives each individual child a better chance at successfully maturing to adulthood.
Fungi use the opposite approach. Instead of investing in a small number of well-cared-for offspring, they reproduce by ejecting (sometimes forcibly) individual cells called spores and use phenomenally large numbers to stack the odds in their reproductive favor.
Since the cost to make single-celled children is substantially less than making multi-celled complex babies, fungi can afford to make orders of magnitude more of them. The sheer number of spores released make up for a lack of development and an excess of mortality.
Fungal Reproduction in the Garden
Fungal reproduction relies on a huge number of spores produced to ensure at least a few eventually wind up in a hospitable environment to develop in. Here’s an analogy: Instead of going through childbirth, the hair and skin cells humans shed throughout the day became more people.
While in the animal world rabbits are legendary for their ability to procreate, a sporing fungus may release millions, billions, or even trillions of spores every cycle, each with the potential to start a new colony. The spores are small enough to be invisible with the naked eye and light enough to stay suspended in the air for extended periods of time, traveling long distances on the slightest of wind currents.
With so many fungal colonies spewing so many spores into the air, it isn’t surprising to find that an air sample with “only” 500-1,000 spores per square meter is considered clean and in the normal and healthy range. The same amount of air in an area with fungal contamination may have several thousand to a few hundred thousand spores.
A wet towel wadded into a ball and left in the corner of a humid room will develop a mildew smell as random mildew spores from the air land and germinate, wild yeasts can be collected simply with a medium in an open jar, and where there is wet greenery debris in the wild, composting fungi will find it. Fungal spore pervasiveness is an integral part of life (and death).
Even with some spores in the area occurring naturally, once a colony is established, it increases the risk of nearby plants by being a spore source. The higher the spore count in the area, the more likely any suitable space is to be found and exploited by an opportunistic spore. Infected plants and any growing medium that is suspect should be quarantined or disposed of to prevent increasing the local number of spores of the fungal pathogen.
Preventing Fungal Growth in the Grow Room
Since eliminating all spores from the air isn’t practical, reducing the number of hospitable sprouting sites should be the first line of defense. Gray mold (Botryotinia fuckeliana), damping off, and powdery mildew are all examples of fungal indicators showing a possible environmental problem.
If plants are overwatered, kept in a warm location, and exposed to high humidity, the possibility for a fungal problem is increased, not because of any new introduction of spores, but rather by suppling a hospitable environment for them to become established.
Fungal issues are easiest to address before they become issues. Keeping humidity from getting too high removes some of the moisture fungal spores need (but not too low or you may invite spider mites).
Allowing the tops of growing mediums (in non-hydroponic settings) to dry slightly between watering can not only help keep the root zone aerated, but interrupts the development of some types of fungi. Green waste should be dealt with and areas kept clean to avoid creating havens for fungal exploitation. Proper air circulation evaporates moisture from surfaces that otherwise might act as fungal safe harbors.
Once a plant has become noticeably infected with a fungus, it may require the use of a fungicide or fungistat, although caution should be taken and any fungicide that is used should be carefully researched, particularly if the plant produce will be consumed. If possible, environmental conditions should be adjusted at the same time as the treatment to dissuade reoccurring or one may be faced with a continuous ongoing problem.
Because of human sense thresholds, we can’t easily see, taste, feel, hear, or smell fungal spores in the air, and most folks either are unaware or ignore their existence. To be fair, for most it doesn’t make much difference, but for gardeners to know the habits of fungal pathogens can be helpful in combating them in defense of the plants in one’s charge.