Removing plum stains requires a good bit of elbow grease, but can be removed effectively when you implement the correct stain removal routine. There is a large selection of retail products available that effectively remove stains such as this, but these items can be costly, and no more effective than a simple homemade solution.
First, and most importantly, drench the stained area with cold water. Because the plum is a fruit with a dark pigment, the stain can set in fast, and sometimes permanently when in contact with hot water. Continue drenching the area with cold water until no remaining plum residue is rinsing off.
Lemon juice is a natural ingredient that has a slight acidic quality that will help to break up the plum stain. After the cold water application, pour a layer of lemon juice over the stain. When possible, place in direct sunlight, because the sun mixed with the lemon juice will work as a natural bleaching agent, bleaching out the plum stain. When sunlight is not available, simply pour the lemon juice onto the stain and scrub with a soft brush or cloth. Repeat as necessary until most of the stain has been removed, then launder as usual on a cold water wash cycle. Pour an optional cup of lemon juice into the washer with your normal detergent. If the stain has been removed completely, dry as usual. If any stain remains, air dry the item, then repeat this process as needed until the stain has been fully removed. Do not place in a heated dryer cycle until the stain is gone, as the heat will set the stain in for good.
If the lemon juice has not effectively removed the stain, move on to an application of distilled white vinegar. Do not use apple cider vinegar, because this is composed differently and can stain fabrics even more. Flush the stain with cold water until the water is rinsing clear, then pour a thick coating of distilled white vinegar onto the stain. Work the area with a soft cloth or brush, and repeat as necessary until the entire stain has been removed. Wash in a cold water cycle, adding your normal laundry detergent, as well as 1 cup of the white vinegar. If any stain remains after the wash, repeat this cycle again.