How to manage and identify powdery mildew in cannabis

“On cannabis, powdery mildew appears as white colonies on the upper surface of leaves, making it one of the more easily recognizable diseases. Powdery mildew occurs in all production environments and may affect plants at all stages of growth. Although it will rarely kill plants, it can significantly reduce their growth.” Therefore, one section in the Handbook of Cannabis Production in Controlled Environments (in chapter 8, written by Cameron Scott and Zamir Punja, a book edited by Youbin Zheng) helps growers identify and manage this potential threat. “Cannabis plants infected with powdery mildew will be of lower quality and be less marketable due to the visible growth of the fungus.”

“Powdery mildew on cannabis is caused by several Golovinomyces species. These are obligate pathogens, which require living tissues to grow and reproduce,” Cameron and Zamir explain. “Infections on cannabis plants start when spores germinate on leaves, floral tissues, or on the stems of plants. The spores produce a germ tube that penetrates the host cells to form haustoria, structures that absorb nutrients and water from the cells without killing them. Further, mycelium and conidiophores develop as distinct white powdery patches on the upper surface of leaves and release more spores. This can occur within seven days of the initial infection. Cannabis plants infected with powdery mildew may appear stunted, leaves may appear brown and drop prematurely, and flower quality and yields may decrease.”

Cameron and Zamir give growers some advice on how to prevent the spread of powdery mildew. “Incoming plant material should be examined for symptoms, placed under quarantine, and tested for the presence of the pathogen. Shoe covers, gloves, hair and beard nets, and footbaths with disinfectant will minimize the introduction or spread of the pathogen. Equipment or tools should also be cleaned with a detergent and water. Reduce plant-to-plant spread through the use of air purification and filtration. Moreover, maintain relative humidity at appropriate levels to limit infection.”

It is, of course, also important to remove infected leaf material. “It also helps to apply products such as MilStop, ZeroTol, or Regalia Maxx when disease pressure is low. Sulfur-based products, biocontrols, or UV light exposure may also be used to manage powdery mildew. It is, of course, recommended to use cannabis strains that are less susceptible to powdery mildew, especially when seasonal disease pressure is high,” the researchers add.

Click here for the Handbook of Cannabis Production in Controlled Environments, in which you can read the full chapter 8: Management of Diseases on Cannabis in Controlled Environmental Production.

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