.png)
The last two particularly wet growing seasons here in Vermont have shown each season brings a brings new challenges for the cannabis cultivator. Septoria or Leaf Spot Fungus especially is on the rise and for those growing in conditions where this disease becomes rampant, it can cause serious issue and crop loss.
Septoria is a plant pathogen that infects vegetative leaves of plants, destroying the plants cells and causing chlorosis, and eventually decay. There are over 1,000 species in the Septoria classification each of which specializes in a particular species of plant as a host. Common especially with Nightshades and Squash species, a species exists that specializes in infecting cannabis as well. Septoria Cannabis is only known to infect cannabis plants unlike other species of septoria that may infect multiple species. The infection will spread plant wide if conditions are correct, killing most vegetative growth. This drastically reduces the plant ability to produce sugars and reduces yield in maturity.
Understanding The Septoria Life Cycle is important to addressing its management as a disease. Septoria is a polycyclic disease which means that each fungal cell is capable of producing many sporulations per season. A typical life cycle of septoria lasts 15-18 days but can have a latent onset period of up to 30 days without showing symptoms. At maturity septoria produces Pycnidium a flask shaped fruiting body which ejects conidia, asexual spores and an exact clone of the parents DNA, and ascospores that spread for sexual reproduction. Septoria thrives in temperatures between 68-77 degrees fahrenheit and in the first months of summer this fungus spreads very quickly. In high humidity conditions fungal life cycles are much shorter and larger numbers of spores are produced. In lab controlled studies the splash dispersal of a single rain droplet on septoria spore suspension showed that with a windspeed of 3m/sec, (6.7 mph) droplets 200 microns wide were carried as far as four meters (13ft) downwind and the majority of spores were carried in droplets larger than 1000microns wide, 70% of which contained more than 100 spores. (1) The spores carried this way germinate at new sites and are capable if creating new infections with leaf spots visible as soon as 7-10 days from the time of infection. It is worth noting that moisture collected on vegetative growth capture windblown spores and are a part of the mechanism by which this fungus spreads.
The Pycnidium also acts as protection for the spores as they overwinter on dead plant material of the previous growing season. Pycnidia can remain viable in debris for as long as nine months in cold weather. This allows the disease to remain present in any growing area until the seasons conditions are favorable to thrive again. In contrast dormant Pycnidia exposed to warmer temperatures or buried in soil survive less than a month. These spores are dispersed with rainfall by the same mechanism as infected vegetative growth, initiating their lifecycle over again.
Managing the spread of this disease in the garden requires removing the survival structures that the species depend on. Some practices to implement as an Integrated Pest Management system are as follows.
Sanitation of your growing area and finding any possible vectors for contamination is a must.
When growing outdoors, row cover or mulch can be used to bury last years plant material, reducing the likelihood of a dispersal being triggered by rain droplets.
Since this fungus also depends on the close proximity of suitable hosts, increasing the spacing of your planting, increasing airflow, rotating crops, and encouraging biodiversity in your grow space can help.
Avoid using overhead irrigation especially before dusk when the sun is not shining to evaporate droplets.
Keeping vegetative growth as dry as possible, hoophouse plastic is useful in avoiding morning dew especially which catch and trap windblown conidia spores on the leaf surfaces.
As plant canopies become denser the growth within the plant becomes a perfect environment for disease. Defoliate lower branches that don't get sunlight before septoria begins to spread. Any contaminated material should be buried or removed.
Fungicides such as Sulpher or Biological products like bacillus may be applied as a foliar spray on a schedule as a means of management. Products that contain surfactant compounds can reduce surface tension and droplet formation and provide a more even foliar application are very beneficial.
Maintaining healthy plants is an important part of bolstering their immunity against diseases. Nitrogen especially is important to resistance of septoria. Keep a diverse microflora in your soil and address any deficiencies your plant may have.
Selecting Resistant Varieties also plays a roll in a successful harvest. Select your strongest most resistant varieties for next season and buy seeds bred for disease resistance.
Septoria is generally considered non harmful to humans but can considerably effect the outcome of a cannabis product. Although ingestion of septoria spores and fungus may be relatively harmless, it is never advised to smoke fungus or their byproducts. For many farmers septoria renders their crop unusable for smokable flower. Many then turn to extraction as a means to make other products which can be an added expense. There are many types of extraction that could potentially accumulate and concentrate toxins produced by fungi. However unless the mycotoxin is specifically tested for it will not show in chemical analysis. Ice Water Extraction seems to be the safest means of washing away microscopic contaminants while capturing trichome heads.
Spraying topical fungicides should always be avoided if possible, however there are many organic and safe options available for the consumer that do not accumulate on cannabis plants. Studies have also shown that terpenes and essential oils could be an effective treatment to reduce mycotoxins in cannabis products. One particular study exemplified this effect by mixing the essential oil of Heliotrope, containing 32.9% a-pinene and 9.4% B-myrcene with the mycotoxin Aflatoxin B1 from Aspergillus mold, resulting in 82.6% of the mycotoxin being degraded. This is a great testament to the antimicrobial and antibacterial properties of terpenes produced by plants.
It makes sense then that with the wide array of terpenes produced by cannabis plants, that resistance to septoria could be attributed in part to certain cannabis profiles. Alpha-Pinene, Alpha-Bisabolol, Eugenol, Geraniol, Humulene, Thymol, Menthol, Eucalyptol, and Terpinolene are all terpenes known to have antimicrobial properties.
The recent increase in septoria occurrence is no doubt a result of many contributors. Similar increases have also been reported in other states "Growers in Michigan weren't seeing it 5 years ago, but now it seems to be everywhere" says @terpsiah an agronomist working in a similar parallel and climate to Vermonters. "warmer winters and wetter springs" He says are probably contributing factors. Climate change aside, recent decriminalization and rollout of the adult use market here in Vermont, and the amount of cannabis plants being grown, sold and traded has increased exponentially, and cannabis borne diseases with them. Tilla Hills Cannabis in Fairfield Vermont recalls septoria becoming prevalent as early as 2017, the same period that hemp production was becoming widespread in the state. "It doesn't touch anything in the greenhouse" they say. Just a single branch protruding from the plastic was soon infected on thier farm, showing the stark difference that plastic cover can make. Life Arises Farm in Wolcot Vermont who cut down 20 or so plants last year as a result of leaf spot says they mulch heavy and add lots of compost from their sheep, doing their best to cover any infected material in their fields. Like the other cannabis cultivators featured here they have taken the necessary steps to adapt to the ever changing challenges of the industry.
For home growers who have less experience with cannabis this disease can be very disheartening. Hopefully the information provided here can inspire some confidence to face this challenge and reap the great satisfaction of growing your own.
While climate shift may be in someways beyond our control there is much that we can do in our own practices to ensure that the cannabis that we distribute is free of contaminants and substantially resistant for this region.
Buy locally sourced genetics especially when transporting live plants to reduce the spread of any pathogens.
Find reputable breeders that are breeding specifically for septoria resistance in the climate you grow in.
Communicate and collaborate with your fellow growers to bring awareness to the issue.
You can find products by all all the great Vermont growers featured here in Vermont dispensaries
Dispersal of Septoria nodorum Pycnidiospores by Simulated Rain and Wind, R. M. Brennan, B. D. L. Fitt, G. S. Taylor, J. Colhoun First published: April 1985
Ranjith, A., Srilatha, C. M., Lekshmi, P. C., and Rameshbabu, N. (2021). Antiaflatoxigenic potential of essential oils of spices—a review. World Mycotoxin J. 14, 463–475. doi: 10.3920/WMJ2020.2636 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10620813/
https://www.healthvermont.gov/sites/default/files/document/DSU-CannabisDataReport2023.pdf