Powdery Mildew

Disease Identification

Description

Powdery mildew is easily distinguished from other diseases by the white appearance of the leaves it infects.  The entire plant is rarely affected by the mildew.  Instead, a group of leaves, usually around the same area, will begin showing signs.  The mildew may spread to adjacent leaves depending on if the conditions of growth are correct.  If not, the disease will not take hold in other areas.

First indications of a potential powdery mildew infection is the appearance of pale yellow spots on both the tops and bottoms of plant leaves.  This may go unnoticed as this type of leaf coloring looks similar to many other issues.  Only when the disease progresses to the point where the powdery white color spreads to the entire surface of the leaf does it become apparent. 

A plant that has been infected with powdery mildew will rarely die.  The main issue with the disease is that it looks unsightly and it reduces the photosynthetic ability of the plant which could reduce the ability for the plant to grow and produce fruit.

  • Leaves: Leaf surface takes on an appearance of white powder
  • Stems: Not affected
  • Fruit: Not affected

Images

Summer Squash

Powdery mildew results in leaves that look like they have been coated in baby powder

Affected Plants

Disease Control

Life Cycle

Powdery mildew is caused by a fungus that is spread by spores blowing through the air.  If a spore from the surrounding area blows into your plot and lands in an area with favorable growth parameters, then it will start spreading.

Like many diseases, powdery mildew favors cool to warm (60 – 80F), moist environments like what is found in many gardens in early and late summer.  However, this disease rarely makes an appearance in the early summer because plants are still small which allows air to easily blow around their leaves.  As plants get bigger, air has more difficulty moving through the plants which results in more humid conditions increasing the likelihood of disease.

 

Management

Follow management suggestions in order

  1. Improve Air Flow: Powdery mildew thrives in the warm, humid conditions found among crowded plants in late summer.  These crowded plants restrict air from blowing around their leaves increasing the likelihood of disease.  The first step is to selectively prune your plants to allow for adequate separation.  By removing infected and/or crowded leaves, you can eliminate most if not all of the problem. 
  2. Improve Watering Methods: When watering your plants, you want to remember two important rules: always water in the morning and always water the ground and not the plant.   By watering in the morning, you allow time for the sun and wind to dry off any leaves that happened to be sprayed with water.  If you water in the afternoon or evening, water sits on plant leaves increasing humidity and likelihood of disease.  If you cannot water in the morning, taking great care to only allow water to hit the soil and not the leaves will go a long way for your garden’s health.
  3. Chemical Sprays: Copper fungicide is an effective organic control that is readily available in stores.  Multiple applications may be necessary for effective control.  Addressing mildew early can reduce the need for further controls.