Powdery mildew is an easy pathogen to recognize on Cannabis, forming white colonies on the upper surface of leaves. This is a common field and greenhouse pathogen and infects many different crops. Powdery mildew is unlikely to kill plants, but it can severely reduce growth and yield.
Growth and lifecycle
Powdery mildew is an obligate pathogen that requires living tissue to grow and reproduce. PM forms airborne spores called conidia that are released from structures called conidiophores. The spores land on tissue and germinate, producing an appendage that penetrates plant cells to absorb nutrients and water. Powdery mildew can reproduce relatively quickly, producing more spores in as little as 7 days. Favorable conditions for growth include warm, humid, poorly ventilated spaces. Outdoors, powdery mildew can be found when plant spacing prevents adequate airflow or when conditions are warm and humid.
Management
The easiest and most effective management strategy to protect against the spread of PM is to prevent infection. New plants brought into a facility should be quarantined and inspected for signs of infection. If an infection is found in your indoor facility, try and segregate the infected plants and remove diseased tissue as soon as possible to curb the spread. In an outdoor setting, removal of infected tissue may help to curb the spread and minimize yield loss. Maintaining proper nutrition status in your plants can also help them to better tolerate PM.
In terms of chemical management, there are few organic fungicides that have been shown to be effective at treating PM. There are many options currently being studied so it is anticipated that more options will be available for Cannabis soon. Currently, available chemical options include tea tree oil, neem oil, and mineral oil.
Other options can include better control of temperature and humidity when growing indoors. If infections get severe it may be worth considering chemical options. When considering conventional chemistry, it is important to make sure the product is labeled for use in Cannabis and that the fungicide won’t be present in the final product.
While we think chemical control should be a last-resort, we understand the economic importance of pathogen control. We urge you try a combination of the above, non-exhaustive list. No one understands your system as well as you do so be sure to carefully consider all management plans before making large scale decisions.
