Cause The fungi Erysiphe platani (formerly Microsphaera platani) has been reported from Washington and Oregon. Conidia are released in response to abrupt decreases in relative humidity. Wind blows conidia to healthy foliage where they start new mildew colonies. Too much water, such as flowing water or rain, destroys spores by causing them to burst. The fungus does not grow in a leaf but rather across the surface. Small anchor cells of the fungus, haustoria, remain in the leaf and take nourishment from the plant, but the main filament and the multitudes of spores it produces are outside the leaf. The disease can develop under relatively dry conditions in spring and summer so long as the air is humid. Multiple disease cycles occur during the growing season.
Symptoms White powdery patches on both leaf surfaces. Affected leaf areas may pucker or crinkle. Small, black, fruiting structures (chasmothecia) form within the patches in fall.
Chemical control Fungicides will do best when used before symptoms develop since few materials have good eradicant activity. Use at 7- to 14-day intervals; using shorter intervals when environmental conditions favor disease development. Alternate or tank-mix products from different groups that have different modes of action. Limit the use of any one group during crop production.
- Armada 50 WDG at 3 to 9 oz/100 gal water. Do not use a silicone-based surfactant. Not for nursery or greenhouse use. Group 3 + 11 fungicide. 12-hr reentry.
- Banner MAXX at 5 to 8 fl oz/100 gal water may be effective. Group 3 fungicide. 12-hr reentry.
- Bayleton 50 T&O at 5.5 oz/275 to 550 gal water. Landscape only, not for use on plants for sale. Group 3 fungicide. 12-hr reentry.
- Eagle 20 EW at 6 to 12 fl oz/100 gal water. Group 3 fungicide. 24-hr reentry.
- Myclobutanil 20 EW T&O at 6 to 12 fl oz/100 gal water plus spreading agent. May observe a PGR effect. Group 3 fungicide. 24-hr reentry.
- Spectracide Immunox Multi-Purpose Fungicide Spray Concentrate for Gardens at 1 fl oz/gal water. Group 3 fungicide. H
- Tebuject 16 is registered for tree injections. The number of capsules used is based on tree size. Group 3 fungicide.
- Torque at 4 to 10 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Tourney 50 WDG at 1 to 4 oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Trigo at 3 to 9 oz/100 gal water. Group 3 + 11 fungicide. 12-hr reentry.
Reference Glawe, D.A. 2003. First report of powdery mildew of Platanus occidentalis caused by Microsphaera platani (Erysiphe platani) in Washington state. Online. Plant Health Progress doi:10.1094/PHP-2003-0818-01-HN.