The bloggers for Ontario Greenhouse Floriculture Growers joined forces with Plant Pathologist Ann Zemke of the Chase Research Group and wrote a blog with the focus on monitoring, and which control products to use once you’ve found an infection.
Monitoring:
Assuming you are doing all the preventative measures possible to avoid powdery mildew (see the cultural controls in our previous post), it’s still necessary to monitor crops on a regular basis for infection. Scouting should occur on at least a weekly basis, or MORE if favorable conditions for this disease have occurred.
Make sure the person doing the scouting knows what to look for, as initial signs can be subtle. Epidemics that seem to develop overnight are often the result of undetected low level infections that have spread spores throughout the greenhouse.
The very beginning of a mildew issue often resembles spray residue when it first colonizes the plant. And, these first colonies are usually located on lower leaves; an area that often gets missed in a cursory walk-through.
Note that PM can look different on different plants. Not every species will develop the characteristic talcum-powder-like spots. Some plants simply develop a purplish discoloration, which can mimic nutrient issues. Know what PM infection looks like in YOUR crop.
Surprisingly, some plants like Kalanchoe often develop brown scabs where Powdery Mildew infection occurs, and never develop the characteristic talcum-like white spots.
Once you find an initial infection, immediately rogue infected plants or, if necessary, prune out diseased tissue. Always perform this operation when plants are wet or immediately place diseased material into a plastic bag to prevent spores from spreading.