My rescued collie cross had mild to moderate persistent colitis when I first got her at 18 months and I tracked it down to intolerance to wheat, soya and maize - all of which are common ingredients in dog food. It was a matter of experimenting with different foods until I found one that suited her. I used Burns for a while, then found she did better on Skinners hypoallergenic. Another dog I know gets colitis from poultry. The trigger can be anything. Mine has improved with age, she can now eat small amounts of wheat (like snatching a dropped bit of bread off the street) without any noticeable reaction. The yogurt (use goats milk yogurt not cows) could help, and you might also try digestive enzymes before meals. The help the digestive process, breaking the food down better so it doesn't irritate the colon. Some dogs (and people) don't naturally produce the enzymes for themselves. They can't do any harm, as the enzymes should be there anyway. You can get them from health food shops - Quest and Solgar make good ones.