The Most Likely Disease

• The most common affliction to lilac bushes is a powdery mildew, which most frequently occurs during hot and humid weather spells. The earlier you treat the plan the more likely its survival. While there are chemical products available to hose down the plant or plants, ecologically there is an equally effective method to save the plant. As unusual as it may sound, mix a half a cup of milk in a gallon of water and spray your plants. Repeat the process until the mildew subsides.
  
  

A Word of Caution

• When mildew infection is prevalent, trim the infested branches and don't be shy about it. A word of caution: do not add the infected branches to a compost heap. Burn them if possible or dispose of them in another manner to keep the problem localized. The mildew can spread to young fruits and vegetables if composted. Although it is unlikely to cause long-term problems, it could damage the young harvest.
  
  

Rodents

• Rodents can kill lilacs quicker than any mildew. One good nights work by the rodents or a few weeks by bugs and your lilacs won't last very long. Mildew takes time to wreak its havoc. Rodents are the biggest threat to lilacs, particularly if they are unestablished and newly planted. Mice and moles are the biggest culprits. They love the young tender shoots which can be protected at planting by wire basket undergirding. To simplify, make a thin wire basket placed in the hole where you plan to plant and then plant as you normally would. It allows the young tendrils to become stronger and tougher before moles and mice can get to them. They don't like established roots as much when they are tougher and are deterred by the wire basket and are likely to just move on. It also allows the plant to establish itself so that nibbling at the edges of the shoot won't likely hurt the plant.
  
  

Pests

• Lucky for the lilacs, lucky for you. There aren't many insects that wantonly damage lilacs. They are less of a problem because there are just a few types that like lilac. Aphids and boring insects are the biggest problem but not with much frequency. The easiest way to keep insects off your plants is with an anti-insecticidal soap, available at most nurseries.
  
  

Pruning Tips

• Since mildew is the biggest threat to lilac bushes and trees, they become infected mostly in warm moist climates. You should prune in drier and cooler weather to prevent the spread of the mildew. While the pruning shouldn't be so severe as to threaten the tree or bush, it should be considerable in order to create as much air flow as possible throughout the plant. This will diminish the likelihood of mildew returning.