diff --git a/src/_pytest/pytester.py b/src/_pytest/pytester.py
index d474df4b94e..aeab5794eef 100644
--- a/src/_pytest/pytester.py
+++ b/src/_pytest/pytester.py
@@ -500,6 +500,8 @@ def __init__(self, request, tmpdir_factory):
         mp.delenv("TOX_ENV_DIR", raising=False)
         # Discard outer pytest options.
         mp.delenv("PYTEST_ADDOPTS", raising=False)
+        # Do not load entrypoint plugins by default.
+        mp.setenv("PYTEST_DISABLE_PLUGIN_AUTOLOAD", "1")
 
     def __repr__(self):
         return "<Testdir %r>" % (self.tmpdir,)
@@ -790,6 +792,14 @@ def inline_genitems(self, *args):
         items = [x.item for x in rec.getcalls("pytest_itemcollected")]
         return items, rec
 
+    def _get_isolated_env(self):
+        tmpdir = str(self.tmpdir)
+        return (
+            # Do not load user config.
+            ("HOME", tmpdir),
+            ("USERPROFILE", tmpdir),
+        )
+
     def inline_run(self, *args, **kwargs):
         """Run ``pytest.main()`` in-process, returning a HookRecorder.
 
@@ -811,8 +821,8 @@ def inline_run(self, *args, **kwargs):
         try:
             # Do not load user config (during runs only).
             mp_run = MonkeyPatch()
-            mp_run.setenv("HOME", str(self.tmpdir))
-            mp_run.setenv("USERPROFILE", str(self.tmpdir))
+            for k, v in self._get_isolated_env():
+                mp_run.setenv(k, v)
             finalizers.append(mp_run.undo)
 
             # When running pytest inline any plugins active in the main test
@@ -1035,26 +1045,38 @@ def collect_by_name(self, modcol, name):
     def popen(self, cmdargs, stdout, stderr, **kw):
         """Invoke subprocess.Popen.
 
-        This calls subprocess.Popen making sure the current working directory
-        is in the PYTHONPATH.
+        This calls subprocess.Popen, making sure the current working directory
+        is in the PYTHONPATH by default.
 
-        You probably want to use :py:meth:`run` instead.
+        Optional keyword arguments:
+
+        :param env: OS environment to be used as is (no PYTHONPATH adjustment,
+                    nor isolation for HOME etc).
+        :param env_update: OS environment values to update the current
+                           environment with.
+                           PYTHONPATH gets adjusted if not passed in explicitly.
 
+        You probably want to use :py:meth:`run` instead.
         """
-        env = os.environ.copy()
-        env["PYTHONPATH"] = os.pathsep.join(
-            filter(None, [os.getcwd(), env.get("PYTHONPATH", "")])
-        )
-        # Do not load user config.
-        env["HOME"] = str(self.tmpdir)
-        env["USERPROFILE"] = env["HOME"]
-        kw["env"] = env
+        if "env" in kw:
+            env = kw.pop("env")
+            if "env_update" in kw:
+                raise ValueError("env and env_update are mutually exclusive")
+        else:
+            env = os.environ.copy()
+            env.update(self._get_isolated_env())
+
+            env_update = kw.pop("env_update", {})
+            if "PYTHONPATH" not in env_update:
+                env["PYTHONPATH"] = os.pathsep.join(
+                    filter(None, [os.getcwd(), env.get("PYTHONPATH", "")])
+                )
+            env.update(env_update)
 
         popen = subprocess.Popen(
-            cmdargs, stdin=subprocess.PIPE, stdout=stdout, stderr=stderr, **kw
+            cmdargs, stdin=subprocess.PIPE, stdout=stdout, stderr=stderr, env=env, **kw
         )
         popen.stdin.close()
-
         return popen
 
     def run(self, *cmdargs, **kwargs):
diff --git a/testing/acceptance_test.py b/testing/acceptance_test.py
index 408fa076e1f..9269244153e 100644
--- a/testing/acceptance_test.py
+++ b/testing/acceptance_test.py
@@ -157,6 +157,7 @@ def my_iter(group, name=None):
 
         monkeypatch.setattr(pkg_resources, "iter_entry_points", my_iter)
         params = ("-p", "mycov") if load_cov_early else ()
+        testdir.monkeypatch.delenv("PYTEST_DISABLE_PLUGIN_AUTOLOAD")
         testdir.runpytest_inprocess(*params)
         if load_cov_early:
             assert loaded == ["mycov", "myplugin1", "myplugin2"]
diff --git a/testing/test_assertion.py b/testing/test_assertion.py
index 330b711afb7..fcc446e4979 100644
--- a/testing/test_assertion.py
+++ b/testing/test_assertion.py
@@ -155,7 +155,7 @@ def test_foo(pytestconfig):
     @pytest.mark.parametrize("mode", ["plain", "rewrite"])
     @pytest.mark.parametrize("plugin_state", ["development", "installed"])
     def test_installed_plugin_rewrite(self, testdir, mode, plugin_state, monkeypatch):
-        monkeypatch.delenv("PYTEST_DISABLE_PLUGIN_AUTOLOAD", raising=False)
+        monkeypatch.delenv("PYTEST_DISABLE_PLUGIN_AUTOLOAD")
         # Make sure the hook is installed early enough so that plugins
         # installed via setuptools are rewritten.
         testdir.tmpdir.join("hampkg").ensure(dir=1)
diff --git a/testing/test_helpconfig.py b/testing/test_helpconfig.py
index 9c7806d5489..a9b06b00989 100644
--- a/testing/test_helpconfig.py
+++ b/testing/test_helpconfig.py
@@ -7,6 +7,7 @@
 
 
 def test_version(testdir, pytestconfig):
+    testdir.monkeypatch.delenv("PYTEST_DISABLE_PLUGIN_AUTOLOAD")
     result = testdir.runpytest("--version")
     assert result.ret == 0
     # p = py.path.local(py.__file__).dirpath()
diff --git a/testing/test_pytester.py b/testing/test_pytester.py
index 2e4877463a8..1ffabeb90c9 100644
--- a/testing/test_pytester.py
+++ b/testing/test_pytester.py
@@ -21,6 +21,11 @@
 from _pytest.pytester import SysModulesSnapshot
 from _pytest.pytester import SysPathsSnapshot
 
+try:
+    import mock
+except ImportError:
+    import unittest.mock as mock
+
 
 def test_make_hook_recorder(testdir):
     item = testdir.getitem("def test_func(): pass")
@@ -482,3 +487,36 @@ def test_pytester_addopts(request, monkeypatch):
         testdir.finalize()
 
     assert os.environ["PYTEST_ADDOPTS"] == "--orig-unused"
+
+
+def test_popen_env(testdir, monkeypatch):
+    monkeypatch.delenv("PYTHONPATH", raising=False)
+    popen_args = (["cmd"], None, None)
+
+    with mock.patch("subprocess.Popen") as m:
+        testdir.popen(*popen_args)
+        env = m.call_args[1]["env"]
+        assert set(env.keys()) == set(
+            list(os.environ.keys()) + ["PYTHONPATH", "USERPROFILE", "HOME"]
+        )
+        assert env["PYTHONPATH"] == os.getcwd()
+
+        # Updates PYTHONPATH by default.
+        monkeypatch.setenv("PYTHONPATH", "custom")
+        testdir.popen(*popen_args)
+        env = m.call_args[1]["env"]
+        assert env["PYTHONPATH"] == os.pathsep.join((os.getcwd(), "custom"))
+
+        # Uses explicit PYTHONPATH via env_update.
+        testdir.popen(*popen_args, env_update={"PYTHONPATH": "mypp", "CUSTOM_ENV": "1"})
+        env = m.call_args[1]["env"]
+        assert env["PYTHONPATH"] == "mypp"
+        assert env["CUSTOM_ENV"] == "1"
+
+        # Uses explicit env only.
+        testdir.popen(*popen_args, env={"CUSTOM_ENV": "1"})
+        env = m.call_args[1]["env"]
+        assert env == {"CUSTOM_ENV": "1"}
+
+    with pytest.raises(ValueError, match="env and env_update are mutually exclusive"):
+        testdir.popen(*popen_args, env={}, env_update={})
diff --git a/testing/test_terminal.py b/testing/test_terminal.py
index d0fdce23eb6..e66fc9a4cf6 100644
--- a/testing/test_terminal.py
+++ b/testing/test_terminal.py
@@ -568,6 +568,7 @@ def test_method(self):
         assert result.ret == 0
 
     def test_header_trailer_info(self, testdir, request):
+        testdir.monkeypatch.delenv("PYTEST_DISABLE_PLUGIN_AUTOLOAD")
         testdir.makepyfile(
             """
             def test_passes():
@@ -677,6 +678,7 @@ def test_verbose_reporting(self, verbose_testfile, testdir, pytestconfig):
     def test_verbose_reporting_xdist(self, verbose_testfile, testdir, pytestconfig):
         if not pytestconfig.pluginmanager.get_plugin("xdist"):
             pytest.skip("xdist plugin not installed")
+        testdir.monkeypatch.delenv("PYTEST_DISABLE_PLUGIN_AUTOLOAD")
 
         result = testdir.runpytest(
             verbose_testfile, "-v", "-n 1", "-Walways::pytest.PytestWarning"