diff --git a/module/task/ui/edit.html.php b/module/task/ui/edit.html.php
index 7b6f43a2b2..0b6265d312 100644
--- a/module/task/ui/edit.html.php
+++ b/module/task/ui/edit.html.php
@@ -254,6 +254,7 @@ detailBody
                     set::name('parent'),
                     set::value($task->parent),
                     set::items($tasks),
+                    set::disabled(!empty($task->mode)),
                     on::change('getParentEstStartedAndDeadline')
                 )
             ) : formHidden('parent', $task->parent),