50. ...   27-31!

This puts white in serious trouble, but there is still an escape:

51. 39-34 28x30
52. 19x26 22-27
53. 47-42 27-31
54. 26x37 30-34
55. 37-32 34-39
56. 32-28 39-43
57. 28-22 43-48
58. 14-09