Despite their similar names and the fact that  they both produce powdery growth on leaves, the pathogens that cause powdery mildew and downy mildew diseases are very different. So why does it matter which mildew is causing disease on  the plants? It is  because different fungicide chemistries are required to manage downy and powdery mildew, and failure to accurately identify and treat the disease can result in significant economic losses. While both diseases are common on a wide range of plants, diseases  caused by the downy mildew pathogens are generally more destructive and more difficult to manage.

Contrary to popular belief, downy mildews are not fungi. The downy mildew pathogens are in a different taxonomic group and are more similar to algae than fungi. They are grouped with the water molds, and this group includes two other economically important plant pathogens: Phytophthora and Pythium. The downy mildews produce brownish-gray, lavender or white spores on the underside of leaves. On the upper leaves, angular yellow spots form. And as the disease progresses, the spots often turn brown. On some hosts, including some varieties of basil, spots are not observed but rather the entire leaf turns yellow. Downy mildews are favored by cool temperatures (45-75°F) and high humidity (>85%). Infection and spore production require the presence of liquid water.

Powdery mildews are caused by true fungi. And in most cases they form white circular spots on the upper surfaces of leaves. Exceptions include strawberry powdery mildew, which produces spots on the lower leaf surface, and cucumber powdery mildew, which forms spots on both the upper and lower surfaces of leaves. Powdery mildews produce a range of symptoms such  as leaf discoloration (yellowing, reddening, browning), leaf cupping or curling, and leaf distortion. Powdery mildew is favored by moderate to high temperatures (68-85°F) and high humidity (>95%). However, free water is not required for the pathogen to initiate infection or reproduce.

Accurate Identification

Accurate and rapid diagnosis is essential for successful management of downy and powdery mildew diseases. Diagnosis without confirmation can be costly and result in 100% crop losses. To the experienced eye, the type of mildew-causing disease can accurately be identified by observing the color, location and pattern of the pathogen on the leaves. However, disease manifestation is variable, and both types of pathogens can cause disease on the same plant at the same time. Therefore, microscopic confirmation (see figures) is strongly recommended. While fresh tissue samples are always preferred for disease diagnostics, tape mounts along with images can also be used.

For downy or powdery mildew disease confirmation with fresh tissue samples:

1. Collect 10-15 symptomatic (but not dead) leaves.
2. Place the leaves in a sealable bag with a slightly damp paper towel.
3. Seal the bag and label the outside of the bag with the collection date and time.
4. Mail the sample via next-day or second-day delivery. Do not send samples using regular post. Samples for downy or powdery mildew confirmation can be sent to:

Dr. Melanie L. Lewis Ivey
LSU AgCenter Department of Plant Pathology & Crop Physiology
Horticultural Crops Pathology Laboratory
302 Life Sciences Building
Baton Rouge, LA 70803

For downy or powdery mildew disease confirmation using tape mounts:

1. Take a 1- or 1.5-inch piece of transparent tape (glossy finish) and press the sticky side of the tape over the areas on the leaves with mildew.
2. Transfer the tape sticky side down onto a clear glass microslide.
3. Collect 2-3 tape mounts.
4. Place the tape mounts in a small bubble envelope or secure the mounts so that they do not break while in the mail.
5. Email images of the diseased leaves to Dr. Ivey.
6. Mail the tape mounts via next-day or second-day delivery to:

Dr. Melanie L. Lewis Ivey
LSU AgCenter Department of Plant Pathology & Crop Physiology
Horticultural Crops Pathology Laboratory
302 Life Sciences Building
Baton Rouge, LA 70803

Disease Management Strategies

Whether you are dealing with downy or powdery mildew diseases, an integrated disease management (IDM) program is  recommended. While the chemistries used to manage the two types of mildews differ, the cultural and sanitation practices used to prevent introduction and minimize spread of the pathogens are similar. Below is an example of a generalized IDM program for downy and powdery mildews.

• Start with high-quality seed or transplants.
• Select varieties with genetic resistance to each disease. Most seed catalogues or plant labels will indicate whether the variety has resistance to downy mildew or powdery mildew.
• Use high-quality water for irrigation and other agricultural purposes. 
• Scout routinely for signs and symptoms of disease. Remember to look on the underside of the leaves for downy mildews.
• Minimize humidity within the canopy by increasing plant spacing or providing continuous airflow (greenhouse environment). Avoid overwatering and overhead irrigation. Water early in the day so that plants have an adequate amount of time to dry during the day.
• Remove and destroy plant debris and weeds.
• Use registered biorational protectants (i.e. Serenade, Cease, Plantshield, Actinovate).
• Use appropriate registered chemicals. The mildew pathogens are very prone to chemical resistance development; therefore, fungicides should be used in a manner that minimizes the risk of resistance development.
   
  Chemicals for downy mildews -Protectants such as mancozeb or copper can be alternated or mixed with fungicides with systemic activity such as mefenoxam, strobilurins, phosphorus acid salts, dimethomorph and phosphonates. 
  
  Chemicals for powdery mildews -Contact-protectant fungicides such as potassium bicarbonates, sulfur and neem oils are effective during early and mild infections. For more severe infections, systemic fungicides can be applied, including, but not limited to, strobilurins, triflumizole, myclobutanil and propiconazole.

A list of selected chemicals with known efficacy against these pathogens on specific hosts is available in the 2013 Louisiana Plant Disease Management Guide.