How to disable disclosureClosedIcon on a tree for a particular node ?

Question

How can I selectively disable the disclosureClosedIcon in  front of a particular tree node and leave the other tree nodes unaffected. If I do myTree.setStyle("disclosureClosedIcon",null);
Then the whole tree is affected. Please look at the code below.
<?xml version="1.0" encoding="utf-8"?> 
<?xml version="1.0" encoding="utf-8"?>
<mx:Application
xmlns:mx="http://www.adobe.com/2006/mxml">
<mx:Script>
<![CDATA[
import mx.events.ListEvent; 
import mx.controls.Alert; 
import mx.events.TreeEvent;[
Bindable] 
public var selectedNode:XML; 
public function myTreeOpening(event:TreeEvent):void { 
//How to do the following selectively just for one node. For example if I want to just disable
//further opening of "Marketing" node and leave the other two
Alert.show(
"Trying to disable the disclosureClosedIcon just for the Marketing node"); 
//myTree.setStyle("disclosureClosedIcon",null);
]]>
</mx:Script>
<mx:XML
id="treeData">
<node label="Mail Box">
<node label="Inbox">
<node label="Marketing">
<node label="Professional"/>
<node label="Individual"/>
<node label="team"/>
</node>
<node label="Product Management">
<node label="Professional"/>
<node label="Individual"/>
<node label="team"/>
</node>
<node label="Sales">
<node label="Professional"/>
<node label="Individual"/>
<node label="team"/>
</node>
</node>
</node>
</mx:XML>
<mx:Panel
title="Tree Control Example" height="75%" width="75%" paddingTop="10" paddingLeft="10" paddingRight="10" paddingBottom="10">
<mx:Label width="100%" color="blue" text="Select a node in the Tree control."/>
<mx:Tree id="myTree" width="50%" height="100%" labelField="@label" showRoot="
false" dataProvider="{treeData}" itemOpening="myTreeOpening(event)"/>
</mx:Panel>
</mx:Application>

Answer 1

Thank you
but I don't want to modify the underlying xml data by adding isBranch on the node. I just want to modify the display of the data on Tree. The reason is that the xml data is retrieved from database and I can not  modify the XML.