diff --git a/libs/ardour/ardour/plugin_manager.h b/libs/ardour/ardour/plugin_manager.h
index 6239bd7cb0..31ab833508 100644
--- a/libs/ardour/ardour/plugin_manager.h
+++ b/libs/ardour/ardour/plugin_manager.h
@@ -127,7 +127,7 @@ class LIBARDOUR_API PluginManager : public boost::noncopyable {
 	void add_lxvst_presets ();
 	void add_presets (std::string domain);
 
-	void au_refresh ();
+	void au_refresh (bool cache_only = false);
 
 	void lv2_refresh ();
 
diff --git a/libs/ardour/ardour/rc_configuration_vars.h b/libs/ardour/ardour/rc_configuration_vars.h
index 20c3857537..2ae2b562eb 100644
--- a/libs/ardour/ardour/rc_configuration_vars.h
+++ b/libs/ardour/ardour/rc_configuration_vars.h
@@ -211,6 +211,7 @@ CONFIG_VARIABLE (bool, use_lxvst, "use-lxvst", true)
 CONFIG_VARIABLE (bool, show_plugin_scan_window, "show-plugin-scan-window", false)
 CONFIG_VARIABLE (bool, discover_vst_on_start, "discover-vst-on-start", false)
 CONFIG_VARIABLE (int, vst_scan_timeout, "vst-scan-timeout", 600) /* deciseconds, per plugin, <= 0 no timeout */
+CONFIG_VARIABLE (bool, discover_audio_units, "discover-audio-units", false)
 
 /* custom user plugin paths */
 CONFIG_VARIABLE (std::string, plugin_path_vst, "plugin-path-vst", "@default@")
diff --git a/libs/ardour/plugin_manager.cc b/libs/ardour/plugin_manager.cc
index 1791e2f10d..344b2ef9b6 100644
--- a/libs/ardour/plugin_manager.cc
+++ b/libs/ardour/plugin_manager.cc
@@ -243,7 +243,7 @@ PluginManager::refresh (bool cache_only)
 
 #ifdef AUDIOUNIT_SUPPORT
 	BootMessage (_("Scanning AU Plugins"));
-	au_refresh ();
+	au_refresh (cache_only);
 #endif
 
 	BootMessage (_("Plugin Scan Complete..."));
@@ -604,11 +604,23 @@ PluginManager::lv2_refresh ()
 
 #ifdef AUDIOUNIT_SUPPORT
 void
-PluginManager::au_refresh ()
+PluginManager::au_refresh (bool cache_only)
 {
 	DEBUG_TRACE (DEBUG::PluginManager, "AU: refresh\n");
+	if (cache_only && !Config->get_discover_audio_units ()) {
+		return;
+	}
 	delete _au_plugin_info;
+
+	// disable automatic scan in case we crash
+	Config->set_discover_audio_units (false);
+	Config->save_state();
+
 	_au_plugin_info = AUPluginInfo::discover();
+
+	// successful scan re-enabled automatic discovery
+	Config->set_discover_audio_units (true);
+	Config->save_state();
 }
 
 #endif
@@ -1050,9 +1062,9 @@ ARDOUR::PluginInfoList&
 PluginManager::au_plugin_info ()
 {
 #ifdef AUDIOUNIT_SUPPORT
-	assert(_au_plugin_info);
-	return *_au_plugin_info;
-#else
-	return _empty_plugin_info;
+	if (_au_plugin_info) {
+		return *_au_plugin_info;
+	}
 #endif
+	return _empty_plugin_info;
 }