My hydrangea develops what looks like mild dew on the leaves. Does anyone know precisely what it is and how to remedy it?
Powdery mildew is a very common problem in hydrangeas. You can spray a 1 part milk, 2-3 parts water (up to 1 part milk, 9 parts water) solution on the top/bottom of leaves every 10-14 days to prevent the problem. Good sanitation techniques should help too: space plants to improve air circulation; avoid overhead watering to slow/prevent the onset and spread of powdery mildew; discard spent blooms/leaves of shrubs with PM in the trash; water the soil early in the mornings from the root ball outwards; remove any fallen leaves and other debris from under the shrub.
Fungicides cleared for the control of PM in hydrangeas: azoxystrobin (Heritage; has the smallest application rate and largest repeat interval); fenarimol (Rubigan); fenarimol (Cleary's 3336). You can also use Sunspray Ultrafine Oil or apply ampelomyces quisqualis (aka "AQ 10", a biological control agent), iprodione (Chipco 26019), triadimefon (Bayleton), piperalin (Pipron), or myclobutanil (Systhane). Some of these are expen$sive so try the cheaper suggestions first (milk, Sunspray Ultrafine Oil, deny humidity to the fungi by watering the soil).
Thanks Luis. As a matter of fact, my hydrangea is placed at the corridor where the wind is pretty strong. Air circulation therefore shouldn't be an issue. I am thinking of cutting off all the stems and leave short stubs of 5" above the soil to restart the plant. Then I would periodically spray the new growth with fungicide to prevent mild dew from recurring. Do you think this approach ok?
I would consider not doing overhead waterings, disposing of the leaves when they dry out and following the other sanitation techniques. If the infestation is not severe, this and the fungicides should be enough. I converted my outside sprinkler system into a sprinkler and drip irrigation to prevent PM issues as the majority of the plants that the previous owners had, they all suffered from PM.
Snookie, all hydrangea are native to regions of the world with cool to cold winters. Without this annual dormancy of low temperatures, hydrangea plants slowly decline - with your tropical climate and high humidity you may be fighting a losing battle with yours. It's might be best to think of it as a temporary container or bedding plant and enjoy it for a few months as best.
Luis, the infestation is quite extensive. Practically, all the leaves, except a few young ones on each branch are infested. That's why I thought I might as well cut them all ad place my hope on the new buds.
Morz89, my hydrangea is 2 years old now. Other than the PM problem, the plant is growing quite well. It bloomed the second time only last month. The mother plant was violet but somehow I got pink. Will take note of what you said about the climate factor however.