How to Combat Powdery Mildew and Fungus

October 8, 2018
Category: Growing Tips

Fungus can be a major source of crop loss for growers. Learning how to combat powdery mildew and other fungi is well worth the time and effort.

What is a Fungus?

Fungi (that’s the plural for fungus) are a Kingdom of organisms that includes yeasts, rusts, smuts, mildews, molds, and mushrooms. Sometimes slime molds and oomycetes (water molds) are lumped into the fungi category, but they aren’t actually in the Fungi Kingdom.

While fungi play a significant role in both medicine and the environment, they aren’t so great to have inside your greenhouse.

The fungi you find destroying your crops are parasitic fungi. They feed on living organisms, causing disease and rot.

Fungi kill your plants by secreting digestive enzymes into the surface on which they are growing. These enzymes break down your plant into carbohydrates and proteins, which are then slurped up (or “absorbed,” if you want to be less dramatic) through the walls of the hairy-looking hyphae.

What About Powdery Mildew?

Powdery mildew is a mildew, which is a fungus. Powdery mildew gets a lot of attention from the greenhouse industry because it attacks flowers, woody ornamentals, and trees.  

In your greenhouse, powdery mildew will survive by spreading from one diseased plant to new plants of the same crop. If the fungus is deprived of its crop for several weeks, it will die out.

For powdery mildew to re-establish itself, new fungal spores would have to come in from an outside source.

That could mean it hitched a ride on another plant, or it just blew in from the woods and fields nearby.

Powdery mildew gets its name for the white, powdery fungus that grows on the upper leaf surface of lower leaves. Sometimes infected leaves may become twisted and distorted before they finally wilt and die. Other plants may get dry, scabby spots that make the fungal growth less obvious.

Powdery Mildew loves high humidity at night and low humidity during the day. The high humidity creates an ideal environment for spore formation while the low humidity creates an ideal environment for spore dispersal. (They can sail through the air with less resistance.)

Mild spring and fall temperatures also create a greater risk for powdery mildew.

How Can I Prevent Fungus?

Typically we see fungus develop in cloudy and rainy weather. However, sometimes we can create environments that encourage it even under warm sunny conditions. Either way, there are some things you can do to prevent it.

  1. Give the plants adequate light.

Sunlight is a good natural fungus destroyer. Just like some of the fictional monsters we’re familiar with, fungi also hate sunlight. The UV rays inactivate these parasitic organisms.  

  1. Airflow is very important.

Be sure that your plants are spread out so that they have adequate air flow between them. Also, adding horizontal air flow fans will greatly reduce outbreaks.

  1. Reducing nighttime moisture helps minimize fungus populations.

Since fungus grows under cool, dark, and humid conditions, we do not want the plants sitting wet at night. We want to water them first thing in the day so that they dry out sufficiently before dusk.

Combat Fungus with Fungicides

Even with all of these precautions, we will still see fungus outbreaks. Typically after a couple of days of cool and wet weather, those pesky spores will start popping up.

Once it gets started, fungus will spread rapidly. So if you see any signs at all, don’t give it a chance to get a foothold. It’s best to nip the situation in the bud.

To combat fungus, it is good practice to treat with a broad spectrum fungicide or one that is specifically labeled for your particular fungus.

And if the weather outside is dreary, do some extra scouting for this uninvited guest.



Fungus control is also important in unrooted cuttings. To get more information on how to handle unrooted cuttings, check out our blog post, here.