how do you come to the conclusion it is rust...???

DO NOT USE CHEMICALS .. until you diagnos the problem ....

a rust colored spot might be rust.. or it might not be ... dont jump to conclusions...

i would google lilacs.. and study all your options ....

you have to insure that water is getting to the root zone ... you may have lost one.. because it is NOT getting there ... it is not a timer thing..

you have to stick your finger in the soil after watering and find out just how much is getting to the roots ... on sand.. on a hill.. an hour of sprinkler may be insufficient .... you may have to build a small moat.. and give it a few bucketfuls once a week or so .. whenever your your finger tells you its dry and hot ....

in a year or two.. water wont matter to an established lilac ... they can take it all without your help .. but your babe needs help ..

lilacs are very susceptable to mildews .... especially cheap no-name types ... the leaves turn all grey from the mildew... this is from going into the night with wet leaves.. like from sprinklers .... the low humidity is saving you from that.. the other thing would be to water after sun up so that the plant goes into night with dry leaves ...

the injury to the leaf .. whatever it is/was ... may be the cause .. and the damage you see now a result ... meaning.. maybe a hail storm blew threw .. causing an injury .. which was later attacked.. causing the brown.. maybe a rust.. maybe just necrotic tissue [dead & brown]...

take a leaf to the best garden shiop around.. and ask their opinion.. or give us a picture...

but a brown spot really doesnt mean anything ....

lilacs are pretty hardy ... i really wouldnt waste much time or money on them.. other than watering as noted above ...

good luck!!!

ken

I'm guessing at rust, didn't know if lilac was susceptible to that or not, which is why there is a question mark after the title. It is a reddish brown spot, and looks similar to rust I've had in years past on snapdragons.

The one that is dead wasn't getting watered. Different location. This one is getting a good amount of water, and the sprinklers are timed to water at dawn, so there is no issue of water on leaves overnight. Believe me, at 7000 ft. the sun is intense enough there is no water left on the leaves. There actually is a moat to ensure that I get a good amount of water on the roots.

Glad you said what you did about what mildew looks like, cause I'd wondered about that. This is definitely not a gray color.

When you say injury to the leaves, we've had no hail. (Knock on wood). What else could cause injury? I have inspected carefully at different times of the day/evening for insects, and seen none. I tend to be really paranoid because of my hostas.

I wondered about sun scald, because it is in full sun all day long, but in my limited experience, the sun usually turns a leaf brown and dry from the edges first. This is round spots on the leaf.

I will try and get a picture loaded, me and that camera don't get along well. I work a 24/7 on site job, so I very rarely get to a garden center, but was sure you guys here would have some ideas.

gardinginut - new shrubs would do better to have a good ground soaking rather than an overhead sprinkling until established. I'm including a link below on lilac diseases (one possibility is a blight).

Here is a link that might be useful: Lilac Diseases