Methods to Mind Mildew Matters in the Greenhouse

Integrated Pest Management for mildews is a little harder than it is for some other diseases. Good greenhouse sanitation practices, for example, may not help at all, because the inoculum is airborne through the vents or introduced on the crops brought in. Because there are systemic fungicides designed to keep downy and powdery mildews in check, there is some temptation to let the fungicides do all the work. But using a multifaceted approach still pays.

Choose Your Plants Carefully
Which plants you grow matters a lot. To avoid downy mildew losses on Impatiens walleriana, for example, the smartest route is to switch over to the Imara XDR (see Figure 1a in the photo gallery above) or Beacon Impatiens series. In the presence of the impatiens downy mildew (see Figure 1b), these plants go on flowering while the old standards melt down. Or you can stick to New Guinea impatiens for the time being, because these Impatiens hawkeri are naturally resistant to the disease.


To avoid powdery mildew losses on verbena, similarly, you will find your spring easier if you grow cultivars that are not prone to powdery mildew. In a trial we conducted in 2018, we saw that no powdery mildew was visible on upper leaf surfaces (thus earning a rating of 6 out of 6 on our 6-point quality scale) for ‘Firehouse Lavender;’ some Superbenas: Violet Ice, Stormburst, Large Lilac Blue, Pink Shades, Royale Peachy Keen, Raspberry, Coral Red, Royale Plum Wine, Royale Iced Cherry, Royale Cherryburst, Red, and Whiteout; and ‘Lanai Deep Purple.’
In the same trial, with the same exposure to inoculum, some verbenas were rated only 2.2 to 4.2 out of 6, meaning they showed an abundance of powdery mildew: ‘Firehouse Velvet;’ some Empress verbenas: Imperial Blue, Violet Blue, Sun Violet, and Sun White Blush; ‘Superbena Purple;’ ‘Lascar Dark Violet ’15;’ and ‘Lanai Blue Denim.’ Veering away from verbenas that are mildew prone can reduce your mildew management headaches.

Just remember that plants in the same series often may have a very different propensity to develop powdery mildew.

Control Your Climate
To avoid a greenhouse climate that fosters either of the mildews, you want to use your environmental control programs to prevent condensation on plant surfaces and minimize overall humidity. Keep air circulating in the greenhouse, which requires that plants must not be spaced too tightly to allow that beneficial air movement.

Walk Your Crops
Scouting your crops is important for noticing a powdery or downy mildew before it takes off and affects most of the plants. Watch carefully for signs of disease and inspect suspicious leaf areas with a hand lens or a dissecting microscope to know whether there might be a downy mildew or a powdery mildew involved. Powdery mildew fungi produce chains of conidia (tiny egg-shaped spores) that most often are seen projecting upwards on the top surface of leaves (see Figure 2 in photo gallery above); whereas, downy mildews, which are oomycetes, produce branched sporangia with spores at their tips that are most often seen on the undersurface of leaves. Often there is no dead tissue in the case of a powdery mildew, while downy mildews, in many cases, form angular lesions that progress from chlorotic to brown. The exceptions to these general rules (see Figure 3) can cause you to make a mistake, so if you aren’t sure which one you are dealing with, get a diagnostic lab to help you figure out which kind of mildew is on your plants.

Weigh Your Treatment Options
Biofungicides work well against powdery mildews, but downy mildews have not yet met a biocontrol they respect. For powdery mildew management, biocontrols with active ingredients that are Bacillus or Streptomyces species can be very helpful, especially if you begin the treatment program before the disease shows up. Use these alone if the crop is not very susceptible or alternate biofungicides with chemical materials if you have a harder time keeping the powdery mildew in check.

Fungicides are valuable for fighting both of the mildews. For downy mildews, you’ll need to use one of the following options:

  • strobilurins (FRAC 11)
  • the new FRAC 11+7 (e.g., Orkestra, Pageant Intrinsic, Mural, Broadform)
  • 11+3 (e.g., Trigo, Fame + T) materials
  • cyazofamid (FRAC 21)
  • phosphorous acids (FRAC 33)
  • dimethomorph- or mandipropamid-containing fungicides (FRAC 40)
  • fluopicolide (FRAC 43)
  • mefenoxam (FRAC 4)
  • oxathiapiprolin (FRAC 49)

You need to use these ingredients in a rotational program, and mancozebs and coppers are useful as contact materials in those rotations.

Powdery mildews are true fungi, and thus have a number of chemical controls different from those used for downy mildews. Strobilurins (FRAC 11) are effective against powdery mildew (along with FRAC 11+7 and 11+3 materials). DMI fungicides (FRAC 3), piperalin (FRAC 5), polyoxin D (FRAC 19), and thiophanate-methyl (FRAC 1) are useful against powdery mildew. Again, rotations are required for effective management. Potassium bicarbonate is particularly helpful against powdery mildew (FRAC NC, non-classified). Use oils (FRAC NC) cautiously, under very good drying conditions, because plant safety is an important factor with these.