Myoporum is very susceptible to root and crown rot caused by soil born fungi including phytophthora. There is no way to eliminate the fungus from the soil, but you may be able to reduce the damage it causes with careful irrigation. Frequent irrigation maintains a high soil moisture content which favors water molds like phytophthora.
Myoporum is very drought tolerant and established plantings do not require frequent irrigation. The ground cover may only need to be irrigated once every 2 to 4 weeks depending on weather conditions. Watering deeply and allowing the soil to dry out as much as possible between irrigations should help control the phytophthora and keep your ground cover healthy.
When you irrigate, apply enough water to wet the soil at least two feet deep. Run your sprinklers a while, for example, 15 minutes, then check how deep water has penetrated into the soil. If the soil is wet only 8 inches deep, you would need to run the sprinklers another 30 minutes to wet the soil 2 ft. deep. If water begins to run off the surface before soil is wet to the desired depth, turn off the sprinklers for at least an hour to allow the water to soak in. Then turn the sprinklers on again and repeat the cycle as needed until soil is wet to the desired depth.
For more information, see UC Pest Note #74133 Phytophthora Root and Crown Rot in the Garden at http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn74133.html
By V. Lazaneo, Urban Horticulture Advisor, Emeritus, UC Cooperative Extension, August 2012