There was very little action through the first two rounds, until Mulder landed a flurry of punches just before the bell at the end of the second round.
However, the fight developed into a sloppy affair in rounds three and four with both fighters lunging and missing badly.
In round five Karam (72.50 kg) picked up the pace and sent Mulder (71.84 kg) down for an eight count with a series of lefts and rights.
Mulder, who appeared to be tiring, did not come out at the bell for the sixth round.
It was later revealed that Karam will relinquish the title as he is relocating to Ireland in May.
On Friday night at the Dr Petrus Molemela Indoor Centre in Bloemfontein, Sphamandla Baleni (47.40 kg) retained his South African strawweight title with a 12-round points decision over Thembani Okolo (47.48 kg), and Tsiko Mulovhedzi (66.40 kg) won the vacant IBF International welterweight title when he outpointed Dario Socci (66.60 kg) from Italy over 12 rounds.
On the same card, former South African flyweight champion Makazole Tete (55.00 kg), fighting at junior featherweight, outpointed Cleutus Mbhele (53.48 kg) over six rounds.
Arnel Lubisi W pts 4 Layten Gloss – junior featherweight.
Luke Hendrikz W tko 2 Kwanele Mkhwanazi – featherweight.
Wade Groth W tko 3 Benjamin Thababetse – middleweight.
Clement Kamanga W tko 1 Andile Mabilisa – welterweight.
Fortune Muyumba W pts 6 Valentino Mutumba – welterweight.
Jarred Silverman W pts 6 Vusi Bilankulu – welterweight.
Xolani Mgidi W pts 8 Junior Assoya – junior welterweight.
|14||2017-04-06||159¾||Wynand Mulder||158¼||3 3 1||W||RTD||5/10|
|13||2016-12-08||162¾||Dido Mukanya||162¼||6 2 0||W||PTS||6/6|
|12||2016-10-30||159½||Dennis Mbath||154¾||0 4 0||W||PTS||6/6|
|11||2016-08-14||160||Harrison Johnson||157¾||0 1 0||W||TKO||6/6|
|10||2016-06-24||161¾||Christian Okeke||167¼||1 2 0||W||TKO||4/6|
|9||2016-05-28||160||Barend van Rooyen||159¼||21 3 1||L||TKO||10/10|
|8||2016-04-10||162½||Christopher Monyai||162½||1 6 1||L||PTS||6/6|
|7||2016-02-25||166¾||Christopher Monyai||167||1 5 1||W||PTS||6/6|
|6||2016-02-07||171¾||Fiston Kabamba||172¾||1 0 1||W||SD||4/4|
|5||2015-11-06||173¾||Fiston Kabamba||173¾||0 0 1||L||SD||4/4|
|4||2015-10-07||172½||Christian Okeke||195½||0 1 0||W||PTS||4/4|
|1||2015-05-27||178¼||Nkosinathi Malinga||180¾||0 1 0||W||TKO||1/4|