Ever present in the environment, a causal agent of “Budrot” or “Gray Mold” is Botrytis cinerea, and its many strains and relatives are some of the most commonly encountered fungi in agriculture—though not always visibly or as damaging parasites.
Botrytis species are the most infamous, but there are many species documented infecting Cannabis belonging to groups such as Penicillium, Diaporthe, Sclerotinia, and Fusarium. They often look too similar to distinguish visually.
In Cannabis, it is an exceedingly common post-flowering, post-harvest pest group, and their rotten symptoms are collectively referred to as budrot, facilitated by high humidity and moisture near and on the surface of floral surfaces.
Treatments exist, and a wide range of preventative measures help shape the likelihood of infection to drive down incidence, from choosing plants with certain floral structures to assessing the environmental context of the grow space. I discuss some of these qualities, the specific pathogens that have been detected, and their treatment in this presentation.
Budrot is globally infamous as a highly destructive agent of mold, particularly associated with floral and fruiting structures. However, it is a much more complex organism. Like with other fungal research, a biased focus on pathogenic populations of Botrytis species reinforces a narrow perspective of their interactions with plants.
Additional assessments of diverse populations indicate a more flexible, strain-host-dependent symbiosis between Botrytis strains and their many hosts. If they are at all, people are often only aware of the overt, documented populations that damage crops; pest strains are the central focus of this evaluation, but simply knowing that some Botrytis strains are found to be non-parasitic, possibly even beneficial or effectively neutral helps contextualize the extreme adaptability brought to bare by the destructive iterations which some evidence indicates may have evolved from innocuous strains.
A typical model for Botrytis infection specifically starts with a spore developing on a stalk that releases it into the air, where it subsequently lands on a suitable host through air currents or carriers like Dark-winged Fungus Gnats (Bradysia sp.), equipment, or clothing. Spore production per square millimeter is high, but the success rate can be fractional without a nearby host and environment suitable for germination.
A vast host range, including symbioses with algae and primitive plants like ferns, helps with this. Close proximity to host tissue significantly increases the likelihood of colonization as not every spore is viable or germinates with the same microclimate on the plant’s surface. Too much moisture and spores can actually be harmed, essentially drowning.
Too little moisture, or possibly some strong ultraviolet radiation, and the spore may not germinate or become damaged post-germination. Colonization is successful when the spore’s hyphal strands penetrate the host tissue and secrete various compounds, which initiate the cell’s capacity to self-destruct, exploiting an antiviral defense to its own benefit.
After that, the fungus feeds on the liberated cell contents leaving necrotic tissues as mycelial threads grow larger. Eventually, necrotized foliage filled with hyphae and various kinds of spores fall to the ground and, in natural and cultivated settings, can be the source of additional infections in other plants with which they make contact. For more information on their biology and plant immune systems suppression, see my Botrytis Pest Primer video.
This is a major reason why eruptions in Botrytis and similarly-infecting fungi often occur suddenly and severely: Cannabis flowers can grow large and densely packed, trapping moisture and shielding some spores against the environment.
Those that get past plant defenses and establish start to produce copious spores. After the first colonization event in a cultivated space, spores are much more likely to encounter other plants that are also flowering. By the time the mold noticeably grows, there have been thousands of spores released. The cycle then continues.
There is also an alternative host colonization strategy as a seed endophyte. Endophytes are microbial organisms like bacteria and fungi that exist inside plant tissue, usually with a neutral or beneficial effect on the host.
With a penchant for reproductive tissues, it is fitting that seeds can often become colonized by budrot fungi internally as they develop, as well as on the exterior seed coat with hyphae and spores.
It is not currently known to what degree this occurs in Cannabis. Still, it is quite common for parasitic and even neutral strains to occupy seeds and only show symptoms like gray molding under general physiological stress, flowering, and senescence.
Management is difficult considering these capabilities. Preventative measures are easier to implement in greenhouse and indoor contexts where environmental regulation is possible to a greater degree.
In some warm and wet locations, the climate may be conducive to budrot population spikes and should be considered when assessing the logistics of cultivation. Of course, having some or any control over this parameter is not always possible.
Similarly, seasonal changes may preclude easy cultivation as budrot pressure may increase in some months and decrease in others making timing a critical aspect of preventative strategy.
Scouting for pest presence is extremely important because it confirms the status of crops and catches problems early in their establishment. Unlike some other pest damage, the mold’s presence by itself fouls the product, and most presentations will be concentrated at the inflorescence, meaning a certain level of loss is inevitable.
Cutting away damaged tissue carefully and sealing it for disposal can reduce the spore load immensely, especially if detected promptly.
Applying responsible products like microbial biopesticides such as Bacillus amyloliquefaciens, Gliocladium catenulatum, and Trichoderma asperellum, as well as potassium bicarbonate can curtail growth on the plant surface while retaining the integrity of the flower. Good coverage and regular applications are necessary to maximize their effectiveness.
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