Question
Hi jen
  I have a white  lilac tree over 25 yr. old  that has decade away the off springs of that tree are 6ft wide and 30 ft. long forming a nice wind break on the south side of the property
  the problem is that I am loosing all of this to a white mildew. how can i stop this fungus and save the lilac trees. would putting ashes from the fire pit around the trees help?

Answer
Hi Kathleen,

Although your lilac may look unsightly, the plant is most likely fine.  The problem you describe is powdery mildew and it seldom will seriously threaten the health of the tree.  Most of the damage is cosmetic.  

The presence of powdery mildew on the plant tells me that your lilac suffers from high humidity.  This will favor the disease, so by lowering the humidity you can often reduce the amount of disease.  If the plant is very thick, it may help to prune out some of the branches to open the plant up and allow for greater air circulation.  You should avoid wetting the foliage if you are watering the plant.  It is best to water the soil/roots only and not wet the foliage.  If you can water the plant in mid-morning, this is the best time.

Many lilac growers find that the only way to keep their susceptible lilacs free of mildew is by using fungicides.  There are many products available for control that you can find at your local garden center.  I'm not sure of your state, so I will tell you several products.  Some of the products may not be labeled or available in your area.  You can look for any of these products and follow the label directions for mixing and treating the plant.  
Advanced Garden Disease control (Bayer)
Bi-carb old fashioned fungicide (monterey)
Dusting sulfur (ferti-lome)
Erase RTU (Monterey)
Fung-Away Systemic Fungicide (Green Light)
Garden Disease control (Ortho)
Garden Fungicide (Safer)
Immunox Multi-purpose Fungicide (Spectracide)
Orthenex Insect & Disease Control Concdentrate (Ortho)
Remedy (Bonide)

You may also find others in your area.  You will want to verify on the label that it is for ornamental plants and that it is effective for powdery mildew.  Products such as those containing sulfur or copper are actually often labeled for organic use, so if you desire to go this route, there are options.

The fungicide should be applied every 7-10 days until you see that you have good control.  This is when the new growth is normal and does not show the white mildew.  You may only need 2 applications and then can just watch for the symptoms to return.  Rake up and discard any fallen leaves in the trash.  

Good luck with your plants!