Implementing imclose(IM, SE) in opencv

Implementing imclose(IM, SE) in opencv

I want to detect the background of the following image whose foreground is
always lots of black dots:
img.png

Someone performs morphological closing on the image with disk-shaped
structuring element and obtain a good result:

Matlab code:
img = imread('c:\img.png');
bg = imclose(img, strel('disk', 15));
figure('name', 'bg'), imshow(bg);
So how to implement imclose(IM, SE) in opencv to replace the work in
MATLAB or there is another better way to detect such background using
opencv method?