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Downy Mildew vs. Powdery Mildew

August 14, 2008  By Sean Valk


March 18, 2008 

Downy mildew and powdery mildew are diseases that may look similar at first glance, but are actually very different.

Both usually affect only the leaves, but downy mildew can be identified from the fungal layer on the underside of leaf, that develops in moist weather and is accompanied by leaf spots on the top of the leaf.  Powdery mildew causes white, powdery, fungal growth in the absence of any leaf spotting.
Downy Mildew

Under humid, cool conditions, downy mildew spores appear in large numbers on the lower surfaces of leaves, growing in tree-like formation on branched fruiting structures.  In the presence of water from recent irrigation, rain or heavy fog, the spores will germinate within four hours.  In fact, sporulation on leaf surfaces may occur in three days under ideal conditions of 18°C temperatures.  Below 5°C the spores won't germinate, and they're killed by exposure to 27°C temperatures for 24 hours; dry, warm, clear days inhibit spore production. Roses, for example, are unaffected by downy mildew when humidity is less than 85% and unlike powdery mildew spores, which are spread by air movement; downy mildew is spread by splashing water.


Downy Mildew on Snapdragon


Symptoms of Downy Mildew
• During moist weather, the undersides of the spots are covered    with a layer of fungus that can be white to purplish to almost black.
• The spot is brighter on the upper leaf surface than on the lower leaf surface.
• The spots may turn brown or may remain yellow.
• The entire leaf dies quickly.
• Usually older leaves are affected first, then younger leaves. Petioles remain green after the leaf blade dies. 

Prevention of Downy Mildew
• Promote good air circulation and leaf drying; use wide spacing between plants.
• Avoid overhead irrigation.
• Water early in the day to allow leaves to dry thoroughly.

Control of Downy Mildew
NEW! Acrobat 50WP from BASF which contains the active ingredient dimethomorph, is now registered for the control of downy mildew on greenhouse ornamentals.  It is not registered for use on cut flowers.

Powdery Mildew
Powdery mildew is caused by the fungi Erysiphe cichoracearum and Sphaerotheca fuliginea.   It is favored by moderate temperatures. These fungi are unusual in that they do not require the leaves to be wet for them to infect the leaf, but rather the humidity must be high. They produce chains of spores when the humidity is low.


Powdery Mildew on Poinsettia


Symptoms of Powdery Mildew
• A whitish, powdery, fungal growth is present on the upper surface of plant parts.
• Fungal growth often starts on the shaded undersurface of the crown leaves.
• Severely affected leaves can become dry and brittle, or can wither and die.

Prevention of Powdery Mildew
• Employ good air circulation and low humidity.
• Separate new plantings from old plantings.
• Control weeds that can host the fungi.

Control of Powdery Mildew
NEW!  Pristine WG Fungicide from BASF which contains the two active ingredients boscalid and pyraclostrobin, has received a full registration for the control of powdery mildew in greenhouse tomatoes and greenhouse cucumbers.

Registered Control Products
(products in bold are marketed and distributed by Engage Agro)

Downy Mildew
• Acrobat 50WP – systemic

Powdery Mildew
• Actinovate
Banner Maxx – Outdoor only
• Compass O
Daconil Ultrex – contact
• Folpan
Funginex – systemic
• Meltatox
• Milstop
• Nova
Phyton 27 – contact and systemic
Pristine WG – (greenhouse vegetable only) – systemic
• Rhapsody
Senator 70 WP – locally systemic

For more general information about THE LOUPE, contact




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