Watch this video to understand what ph is http://www.youtube.com/watch?v=1YOeqRiG-3Y
Ph is simply a measure of H+ ions in the soil solution. If you have a lot of H+ ions, then you have an acid soil. If you have a lot of H+ ions, then you have little Ca++, Mg++, K+, NH4+, etc, etc because H+ has displaced them. So, acid soil really means deficient soil.
It is true that having a low ph in the soil can discourage some root diseases (particularly in potatoes), but also having low ph means you have little nutrients for plant growth unless the plant has evolved in an acid soil (blueberries, azaleas, etc).
If you have the proper amounts of Ca++, Mg++, K+, etc in the soil, then ph is not something you'd ever have to worry about. Ph is the effect of cation combinations in the soil, not the cause.
Here are a couple good articles explaining why the ideal soil should be around 70% Ca, 10% Mg, 5% K
As to how to maintain the ph at a certain level... if you wish to lower ph, you would add H+ ions. Organic matter does this, as does many fertilizer salts. If you wish to raise ph, then you use lime or simply add the Ca++, Mg++, and K+ back to the soil that the H+ ions displaced.
Calcium is used by plants to build cell walls. Potassium is associated with disease resistance. Having leaves which look as those do, one may question the availability of calcium and potassium in the soil.
That disease on your leaves looks similar to the fire blight disease on my pear trees which I've successfully used calcium to treat. I am of the opinion that most diseases are caused by nutritional deficiencies in the soil, though the mechanism for decay may be bacterial or fungal, the cause is actually something missing from the soil.