Anything that produces linear damage such as you described isn't a disease.

It does sound like exhaust, or a least heat of some kind.

At what height is the damage?
And is the hedge located where vehicles could cause that?

If so, the damage may appear to get worse on the affected leaves and those nearby. What happened is that the nearby leaves weren't as seriously damaged, so they took longer to show the effects.

But the good news is that it won't go to new parts of the plants.

So, in the meantime, you get to continue to scratch your head while trying to figure what damaged the plants. Then you might be able to avoid a re-occurrence.

All that said, new growth will eventually mask the damaged area.

Did you make any of your neighbors mad? Nature rarely makes straight lines or regular shapes. It sounds like damage from roundup or something similar to me.

Here is a link that might be useful: Midnight Gardener Forum

It would surely help us help you if you would post an image.

Beyond that, how far from the ground is the brown line? Damage from an exhaust would be rather low.

Does "middle of the hedge" refer to the center 3 plants? Or does it mean halfway up the 6-foot tall plants -- that is, about 3 feet?

If the latter, that's a tad high for vehicle exhaust.

Further, vehicles would have to have access. Do they?

Any construction in the area?

Doubt that Roundup would make a line when applied to a hedge. But other weed killers could.

Here is a link to some photos. Thank you for your help and responses.

Here is a link that might be useful: Home Blog

It appears similar to the damage I get from my garden tractor when I get the exhaust too close to my yews. Does anyone park a large riding lawnmower/tractor in this area?

The only exhaust that the hedge is exposed to is minimal as we pull in and out of the garage. I just find it hard to believe that after so many years the hedge is only now having issues with car exhaust. Also the exhaust comes from the opposite side of the vehicle as the hedge and the initial damage appeared much higher than exhaust would have caused.

It should be noted that we live in Atlanta and went through a drought last summer with watering restrictions. I am not sure that this is the cause either though because the damage only appeared over the past few months.

Yes, I agree that exhaust didn't do it.

Your description of "the leaves have turned brown and fallen off along a horizontal line in the middle of the hedge" mislead us.

Instead of a brown line, you have a swath of brown/missing leaves, the damage broader at the garage end and narrowing at the far end.

I suggest you look very closely for critters. Both mealybugs and scales are common pests of gardenias.

It could also be a disease -- but I don't know what's common in your region.

The damage from drought is generally different than what your shrubs show. The tops go out first, sometimes only random branches.

That said, the worst damage is near the garage -- that appears to be elevated more than at the left of the picture. If so, it could be drier there.

But again, the damage isn't typical of drought.

If you don't see any critters, locate a branch which has *both* damaged and healthy growth. Take the sample to a knowledgeable person for advice/suggestions & advice.

After you collect the sample, secure it in a large clear plastic bag, and take to either your county's University Extension office or a nearby large independent garden center.

When you learn what's wrong, come back and tell us. That way, we all learn something new.

Good luck.

In the Southeast, the rule of thumb for gardenia is that you WILL have a problem with whitefly. Large plantings are often avoided because of this fact. Have you seen any hoards of these little critters?

Also very common in the deep South are root knot nematodes, often cited as one of the worst pests of gardenias. It would be worth your while to have someone examine the root system of your shrubs, as this type of nematode is fairly easy to diagnose by the signature 'knots' associated with the roots.

I see that these plants have been 'topped' regularly in order to keep them of a certain size, I presume. This practice may have weakened them. I am also wondering what kind of soil they are planted in.

This may seem a strange question, but...how old is the house? Your answer may help to diagnose the problem.

It was said: "... how old is the house? Your answer may help to diagnose the problem."

Why keep us in the dark? Please explain.

To jean001, it makes no sense to write a long post that may not have anything to do with this situation. Take a look at the fourth picture posted of the hedge. Can you figure out why I am asking the age of the house? My guess is that it is 8-10 years old. Until I have that answer I cannot suggest what might be the problem or suggest a 'fix'.

The house is 22 years old. We moved in two years ago, and do not know how long the hedge has been there, but my guess is a while.

The damage close to the garage started out as about a 6 inch line then spread upwards downwards along the bush. I do not see any bugs whatsoever. I am going to clip a branch and take it to a landscape company and see what they say.

We have sheared the tops, but haven't done so yet this year, especially since this problem arose. The soil is Georgia clay.

The only thing I can think is we had some unseasonably cold nights. Could they be frostbitten? There are black nubs at the end of many of the branches that have no leaves. There are also new green buds.

Is there a lawn on the other side of the driveway, were Weed n'Feed type lawn fertilizers recently used on the lawn?

Gardenias certainly can be damaged by frost but that doesn't look like frost damage to me.