Controlling Powdery Mildew on cannabis with CleanLight UV-Light

Report by: Cameron Scott

Supervised by: Professor Zamir Punja

Date: April 30, 2019


Overall Conclusions

In two trials, the effects of CleanLight UV-light against powdery mildew were very clearly visible. Daily exposure of plants to CleanLight for 3-5 seconds each time proved to significantly reduce the severity of powdery mildew. In both trials it was observed that the effect of CleanLight against powdery mildew is more preventative than curative, although areas of very high infection were still observed to be suppressed.


Cuttings were taken from a cannabis strain which is susceptible to powdery mildew and rooted in a turbokloner, which had the dome on for approximately 5 – 7 days. Plants were then potted in a 75% coco coir 25% perlite mix and separated into groups of 4, with there being 2 CleanLight treatment groups and two control groups.

The CleanLight treatment group was treated daily with a CleanLight Pro unit, for approximately 3-5 seconds per day. This is approximately 3-6 mJ/cm2 as per emails with the manufacturer. Treatments continued daily for 4 weeks. Over this period of time 5 disease assessments were made by counting the % coverage of PM on each affected leaflet. The 30 most diseased leaflets were counted only.

Treatments were made in a space separate from the control treatments to avoid any unintended PM reduction on the control treatments. When plants were not being treated, they were kept together under 2 54watt 6400k Sunblaster T5HO lights.

Explanation of AUDPC

AUDPC means the Area Under the Disease Progress Curve. This calculation is done by finding the difference between two time points, such as between weeks 2 and 3 and multiplying it by the average disease rating for those two weeks.

CleanLight SFU Report 2 – April 30 2019

Summary and General Impressions

The second trial of the CleanLight treatment shows results similar to the first trial, with there being a reduction in the powdery mildew disease rating of the cannabis plants as the trial progressed over a 4-week period (see graph below). The leaves of the treated plants also appeared to be greener and have a shine to them compared to the untreated leaves, indicating possible physiological changes to the plant such as increased phenolics, increased chlorophyll, strengthening of leaf tissues, etc. (see attached figure).

Disease Progress Graph

Values are means ± 95% confidence intervals (n=8).

The graph above shows the progress of the disease ratings for the two treatment groups over the duration of the trial with each treatment starting at very similar disease ratings as of week 1. From this graph the Area Under the Disease Progress Curve values are calculated for the next graph. The CleanLight treatment again seemed to be much more effective as a preventative measure, although it did have a degree of curative value if the powdery mildew infection was not too intense at the start of treatments. The analysis of the two sets of AUDPC data proved that again the CleanLight treated group had a significantly lower AUDPC value, and therefore disease rating, showing the efficacy of CleanLight at managing powdery mildew on cannabis under these experimental conditions.

Values are means ± 95% confidence intervals (n=8).

This bar graph illustrates the difference between the two AUDPC values for these treatments. The small error bars also help to confirm the validity of these results as they show there was relatively little variability within each treatment, the CleanLight treated group was less diseased as a whole and there were similarly high disease levels within the control group itself.

Statistical Analysis

Analysis of the two sets of data for these treatments was done using ANOVA in R.

These results show that the CleanLight treatments had a significantly lower AUDPC (p<0.01) compared to the controls. For both the first and second trial the CleanLight treated groups had significantly lower AUDPC values than the control group, which are excellent results.

Note that when k=2 there is only one pair of (independent) treatments/ samples to be compared, so the Tukey HSD Test for pairwise comparison of multiple treatments/ samples is not conducted. In this case, the one-way ANOVA is equivalent to a t-test with the F ratio such that F=t2 .

Photos – CleanLight Treated and Untreated Leaves

These photos are of treated and untreated leaves at the end of the trial. These treated and untreated leaves offer a very good idea of the efficacy of CleanLight treatments at managing powdery mildew on cannabis (see photo below).

CleanLight SFU Report 1 – Feb 11 2019

Observations and conclusions based on the first trial:

  1. CleanLight UV seems to do a much better job of preventing powdery mildew than it does at treating it. There was an observable reduction in the size of established colonies of powdery mildew, but some areas were too diseased for the UV light to make a difference over the duration of the trial.
  2. New growth appeared to be darker green in color and possibly with thicker leaves than the control group.
  3. At the start of the experiment, when the cuttings were still relatively small, some negative effects of the CleanLight UV-light treatments were observed such as leaves that drooped towards the edges. This effect didn’t seem to be permanent and eventually the plants grew out of it and were able to better handle this dose of UV. This negative effect was only seen on maybe 7 leaflets in total out of 8 plants.


Values are means ± 95% confidence intervals (n=8).