I have two 3-4 year old rosemary bushes that have always been healthy. Last fall, I noticed the tips is the branches were defoliating, leaving the top 3-4 inches bare sticks. One recovered, one I just removed because it died completely. I am replacing it with a new one. The lavender next to it died also What could have caused this? The first photo shows the dead bush. The second shows the one remaining stick on the plant that seems to have recovered.
We are unable to diagnose the problem with your plants from the digital images you posted. However, since you have three plants and two species of plant, we suspect an abiotic problem such as irregular watering or winter injury. We are getting reports of winter injury from clients on both sides of the state this year. Although we did not have a harsh winter, the onslaught of winter was rather abrupt this fall. Sensitive plants in the more exposed areas of the home landscape may be impacted. Tip dieback is a common feature of winter injury on the woody plants.
I would review your watering schedule and irrigation system, just to eliminate this possibility. Make sure you compensate for the heat of the summer by increasing watering frequency. If you have an irrigation system, check for plugged emitters and observe the watering pattern to make sure the water is reaching the plants. The ultimate test would be to dig down to the root zone near the plants and check to moisture of the soil, Dry and crumbly soil indicates not enough water; if you squeeze the soil in your hand and water drips from your hand; too much water. Remember both overwatering and underwatering can cause similar symptoms of plant wilting a stress.
Finally, I would encourage you to contact the local Master Gardener volunteer program in your county. This type of question would interest them and they may be better able to help you identify the true problem with your plants. Better yet they would have a better idea on the possibility of winter injury in your area.
Hope this helps!