diff --git a/plone/app/discussion/browser/captcha.py b/plone/app/discussion/browser/captcha.py
index 6baa87b..bc32fc2 100644
--- a/plone/app/discussion/browser/captcha.py
+++ b/plone/app/discussion/browser/captcha.py
@@ -25,69 +25,45 @@ from plone.app.discussion.interfaces import IDiscussionSettings, ICaptcha
 
 from plone.app.discussion.browser.validator import CaptchaValidator
 
-HAS_CAPTCHA = False
-try:
-    from plone.formwidget.captcha import CaptchaFieldWidget
-    HAS_CAPTCHA = True
-except ImportError:
-    pass
-
-HAS_RECAPTCHA = False
-try:
-    from plone.formwidget.recaptcha import ReCaptchaFieldWidget
-    HAS_RECAPTCHA = True
-except ImportError:
-    pass
 
 class Captcha(Persistent):
+    """Captcha input field.
+    """
     interface.implements(ICaptcha)
     adapts(Comment)
     captcha = u""
 
 Captcha = factory(Captcha)
 
-if HAS_CAPTCHA or HAS_RECAPTCHA:
-    # Extend the comment form with captcha, if a captcha solution is installed.
-    class CaptchaExtender(extensible.FormExtender):
-        adapts(Interface, IDefaultBrowserLayer, CommentForm) # context, request, form
 
-        fields = Fields(ICaptcha)
+class CaptchaExtender(extensible.FormExtender):
+    """Extends the comment form with a Captcha. This Captcha extender is only
+    registered when a plugin is installed that provides the 
+    "plone.app.discussion-captcha" feature.
+    """
+    adapts(Interface, IDefaultBrowserLayer, CommentForm) # context, request, form
 
-        def __init__(self, context, request, form):
-            self.context = context
-            self.request = request
-            self.form = form
+    fields = Fields(ICaptcha)
 
-            registry = queryUtility(IRegistry)
-            settings = registry.forInterface(IDiscussionSettings)
-            self.captcha = settings.captcha
-            portal_membership = getToolByName(self.context, 'portal_membership')
-            self.isAnon = portal_membership.isAnonymousUser()
+    def __init__(self, context, request, form):
+        self.context = context
+        self.request = request
+        self.form = form
 
-        def update(self):
-            if self.captcha != 'disabled' and self.isAnon:
-                # Add captcha field if captcha is enabled in the registry
-                self.add(ICaptcha, prefix="")
-                if HAS_CAPTCHA and self.captcha == 'captcha':
-                    # If Captcha is installed and Captcha is enabled,
-                    # use the CaptchaFieldWidget
-                    self.form.fields['captcha'].widgetFactory = CaptchaFieldWidget
-                elif HAS_RECAPTCHA and self.captcha == 'recaptcha':
-                    # If ReCaptcha is installed and ReCaptcha is enabled,
-                    # use the ReCaptchaFieldWidget
-                    self.form.fields['captcha'].widgetFactory = ReCaptchaFieldWidget
+        registry = queryUtility(IRegistry)
+        settings = registry.forInterface(IDiscussionSettings)
+        self.captcha = settings.captcha
+        portal_membership = getToolByName(self.context, 'portal_membership')
+        self.isAnon = portal_membership.isAnonymousUser()
 
-    # Register Captcha validator for the Captcha field in the ICaptcha Form
-    validator.WidgetValidatorDiscriminators(CaptchaValidator, field=ICaptcha['captcha'])
+    def update(self):
+        if self.captcha != 'disabled' and self.isAnon:
+            # Add a captcha field if captcha is enabled in the registry
+            self.add(ICaptcha, prefix="")
+            if self.captcha == 'captcha':
+                from plone.formwidget.captcha import CaptchaFieldWidget
+                self.form.fields['captcha'].widgetFactory = CaptchaFieldWidget
+            elif self.captcha == 'recaptcha':
+                from plone.formwidget.recaptcha import ReCaptchaFieldWidget
+                self.form.fields['captcha'].widgetFactory = ReCaptchaFieldWidget
 
-else:
-    # This is necessary, otherwise the zcml registration of the CaptchaExtender
-    # would fail if no captcha solution is installed.
-    class CaptchaExtender(extensible.FormExtender):
-        adapts(Interface, IDefaultBrowserLayer, CommentForm)
-
-        def __init__(self, context, request, form):
-            pass
-
-        def update(self):
-            pass
\ No newline at end of file
diff --git a/plone/app/discussion/browser/configure.zcml b/plone/app/discussion/browser/configure.zcml
index e22aed8..02e8bb5 100644
--- a/plone/app/discussion/browser/configure.zcml
+++ b/plone/app/discussion/browser/configure.zcml
@@ -1,6 +1,7 @@
 <configure
     xmlns="http://namespaces.zope.org/zope"
     xmlns:browser="http://namespaces.zope.org/browser"
+    xmlns:zcml="http://namespaces.zope.org/zcml"    
     i18n_domain="plone.app.discussion">
 
     <include package="plone.app.registry" />
@@ -81,18 +82,23 @@
         />
 
     <!-- Comment form extender -->
-    <adapter
-        factory=".captcha.Captcha"
-        provides="plone.app.discussion.interfaces.ICaptcha" />
-
-    <adapter
-        factory=".captcha.CaptchaExtender"
-        provides="plone.z3cform.fieldsets.interfaces.IFormExtender" />
-
-    <!-- Captcha validator -->
-    <adapter
-        factory=".captcha.CaptchaValidator"
-        />
+    <configure zcml:condition="have plone.app.discussion-captcha">
+        <!-- 
+          Register the Captcha form extender and validator only if there are
+          plugins installed that declare to implement a Captcha solution for
+          plone.app.discussion (e.g. plone.formwidget.captcha and 
+          plone.formwidget.recaptcha). 
+        -->
+        <adapter
+            factory=".captcha.Captcha"
+            provides="plone.app.discussion.interfaces.ICaptcha" />
+        <adapter
+            factory=".captcha.CaptchaExtender"
+            provides="plone.z3cform.fieldsets.interfaces.IFormExtender" />
+        <adapter
+            factory=".captcha.CaptchaValidator"
+            />
+    </configure>
 
     <!-- Comment view -->
     <browser:view
@@ -125,4 +131,4 @@
         permission="cmf.ManagePortal"
         />
 
-</configure>
+</configure>
\ No newline at end of file
diff --git a/plone/app/discussion/browser/validator.py b/plone/app/discussion/browser/validator.py
index 65a5408..7e775ee 100644
--- a/plone/app/discussion/browser/validator.py
+++ b/plone/app/discussion/browser/validator.py
@@ -1,8 +1,4 @@
-# Captcha/ReCaptcha validators. We override the standard validators from
-# plone.formwidget.captcha and plone.formwidget.recaptcha, in order to
-# switch between the two. This is necessary, because the zcml registration
-# of the CaptchaValidator has to be there, no matter which captcha solution
-# is installed, or even when no captcha solution is installed.
+# -*- coding: utf-8 -*-
 
 from Acquisition import aq_inner
 
@@ -14,25 +10,24 @@ from zope.component import getMultiAdapter, provideAdapter, queryUtility
 
 from zope.schema import ValidationError
 
+from zope.interface import implements, Interface
+from zope.schema.interfaces import IField
+from zope.component import adapts
+
 from plone.registry.interfaces import IRegistry
 
-from plone.app.discussion.interfaces import IDiscussionSettings, IDiscussionLayer, MessageFactory as _
-
-from zope.interface import implements, Interface
-from zope.schema.interfaces import IField
-from zope.component import adapts
+from plone.app.discussion.interfaces import ICaptcha
+from plone.app.discussion.interfaces import IDiscussionSettings
+from plone.app.discussion.interfaces import IDiscussionLayer
+from plone.app.discussion.interfaces import MessageFactory as _
 
 try:
-    from plone.formwidget.captcha import CaptchaMessageFactory as _
-    class WrongCaptchaCode(ValidationError):
-        __doc__ = _("""The code you entered was wrong, please enter the new one.""")
+    from plone.formwidget.captcha.validator import WrongCaptchaCode
 except:
     pass
 
 try:
-    from plone.formwidget.recaptcha import ReCaptchaMessageFactory as _
-    class WrongReCaptchaCode(ValidationError):
-        __doc__ = _("""The code you entered was wrong, please enter the new one.""")
+    from plone.formwidget.recaptcha.validator import WrongCaptchaCode
 except:
     pass
 
@@ -40,6 +35,7 @@ from zope.interface import implements, Interface
 from zope.schema.interfaces import IField
 from zope.component import adapts
 
+
 class CaptchaValidator(validator.SimpleFieldValidator):
     implements(IValidator)
     adapts(Interface,IDiscussionLayer,Interface,IField,Interface)
@@ -53,7 +49,8 @@ class CaptchaValidator(validator.SimpleFieldValidator):
 
         if settings.captcha == 'captcha':
             # Fetch captcha view
-            captcha = getMultiAdapter((aq_inner(self.context), self.request), name='captcha')
+            captcha = getMultiAdapter((aq_inner(self.context), self.request), 
+                                      name='captcha')
             if value:
                 if not captcha.verify(value):
                     raise WrongCaptchaCode
@@ -61,9 +58,14 @@ class CaptchaValidator(validator.SimpleFieldValidator):
                     return True
             raise WrongCaptchaCode
         elif settings.captcha == 'recaptcha':
-            # Fetch recatpcha view
-            captcha = getMultiAdapter((aq_inner(self.context), self.request), name='recaptcha')
+            # Fetch recaptcha view
+            captcha = getMultiAdapter((aq_inner(self.context), self.request), 
+                                      name='recaptcha')
             if not captcha.verify():
-                raise WrongReCaptchaCode
+                raise WrongCaptchaCode
             else:
-                return True
\ No newline at end of file
+                return True
+
+# Register Captcha validator for the Captcha field in the ICaptcha Form
+validator.WidgetValidatorDiscriminators(CaptchaValidator, 
+                                        field=ICaptcha['captcha'])            
\ No newline at end of file