Scab of Irish potatoes, pox of sweet potatoes and black root rot of tobacco are favoured by

Potato scab is caused by Streptomyces scabies, which can survive indefinitely in slightly alkaline soils.
Pox in sweet potatoes is caused by a soil-borne filamentous bacterium, Streptomyces ipomoea and can persist in neutral to alkaline soil for many years.
Black root rot of tobacco is caused by the fungus Thielaviopsis basicola. This disease is less severe in soil with a pH below 6.5, but as the pH moves to the alkaline side, its severity increases.
Thus, all these three diseases are caused by different microbes but are favoured by Neutral to alkaline conditions.