It is difficult to answer since we do not know how the relationship between y'all is. You were asked to only water the plants so I would be careful when you expand your duties without first getting consent from the owner.

The wet soil problem does fall under the scope of your duties. If you know how her sprinkler controller works then I would reduce the time that the oscillating sprinkler is active provided that doing that does not affect any other plants growing in the owner's landscape.

As for the other changes that you mentioned, I would first obtain verbal or written approval from the owner first.

Regarding the plants showing signs of a Phytophthora Fungal Infection, I do not think there is anything that you can do once they are infected. You can prevent Phytophthora root rot by using raised beds with sphagnum peat moss or by choosing varieties that are resistant. But the improvements that you propose are outside of the scope of just watering plants; you need to discuss the plans with the owner first.

Thanks for taking the time to check your neighbor's landscape so well, Diane!
Luis

Plants wilt and die slowly when their roots become blocked. There are two causes of this:

1) Root strangulation. This is best prevented by proper root pruning when planting. If the plant is not too far gone, it might be rescued by digging and removing the soil. Then cutting any circling roots that may be strangling other roots. The roots need to be opened up. On larger plants, some of the top must be removed to compensate for the weak state of the roots. Any time the roots are exposed, they must be kept moistened. Roots that dry out will die.

2) Phytophthora crown rot or wilt. This root rot is the major killer of rhododendrons and azaleas. It develops when roots are growing in wet conditions. The rot is more prevalent in warm summer conditions. Plants infected with crown rot caused by the fungi Phytophthora have roots which become clogged with brown fungi internally. The roots get blocked and the plant wilts and dies. There is not much of any cure for crown rot. Some varieties of rhododendrons are vulnerable (Chionoides, Catawbiense Album, Nova Zembla) and some are resistant (Roseum Elegans, Scintillation, PJM). Sphagnum moss and bark dust combined with good drainage seem to prevent crown rot, but do not cure it.

Phytophthora crown rot does not kill single branches, it kill plants. When branches die, it is usually either borers or dieback.

1) Borers. Ends of branches die when rhododendron or azalea borer larvae tunnel in a stem. This affects the portions of the plant away from the roots from where the borer larvae is in the stem. Borers have done their damage when you see the wilting and usually it is best to cut off the affected region. The requires removing the damaged branch from the hole where the eggs were laid at the base of the dying branch to remove any larvae before they become adults and infect more plants. The parts that are cut off should be destroyed to kill the larvae that are in them. Borers are prevented by following a spray schedule (timing is very critical) for borers with chlorpyrifos or lindane. Your county agent can help you with this information.

2) Dieback. Terminal buds and leaves turning brown and leaves wilting is symptomatic of Phytophthora, Botryosphaeria, or Phomopsis dieback. All three diseases spread rapidly and may threaten an entire plant. They are more active in warm summer conditions.

Raised Beds. The wetness could be caused by over watering or poor drainage. Dig a hole about twice as deep as the pot or root ball and fill it with water. If the hole drains within a few hours you have good drainage. If the water is still standing 12 hours later, check the drainage in your yard, you may have to take corrective actions and probably use a raised bed. Turn the soil well and dig a hole twice as wide as it is deep. Add plenty of organic material, remove the plant from its container and loosen the root ball. Set the root ball into your prepared hole (making sure the top 1-2 inches of the ball is above the soil level), pull in your humus/organic soil around the plant, pack firmly and cover with mulch.

Here is a link that might be useful: How to Grow Rhododendrons