in Horoscope, Pitrudosha can be identified with position of Rahu Ketu and also by the study of 5th, 8th and 12th houses. It is also important to study Sun and Moon w.r.t to these Rahu, Ketu and above said houses. In certain books one can find the Rinas (Debts) and Shaaps (Curses) considering the Planatary Positions which causes problems to the Native. Ex. Matru Rina (Debt related to Mother/Maternal side), Pitru Rina (Debt related to Father/Paternal side) and Sarpa Shaap (Curse of the Snake), Brahma Shaap (Curse of a Brahmin) etc. Kaalsarpa is similar kind of yoga which has got tremendous popularity these days.
Pitrudosha is noting but problems created by the Spirits of Dead Relatives of the Native who have not got Mukti due to certain wish that remained unfulfilled before their death. In present time the Shraadh (Offering to Dead and Ancestors) is on decrease causing troubles like these (As said by old men). This in turn causes problem of Pitrudosha. In some cases the Native might had hurt that Person/Animal (In case of Killing Snake/Cow) in his/her previous Birth can causes troubles in this life.
Remedy - The main remedy as suggested by many Astrologers and Pundits is Tripindi Shraadh or Naarayan Nagbali (as known in some places). It is 2/3 days Puja done by Brahmins. It is nothing but Shraadh for all the Ancestors and also Rahu and Ketu Puja.
Places of Puja - There are many places like Tryambakeshwar (Near Nasik) for Western Region, Ujjain for Central Region, Gokarna for Southern Region, Kashi for Northern Region and Eastern Region. Each place has its own important in Hindu Mythology and all the places are Lord Shiva's worship places or Jyotirlingas. One can find the details of the Places, Pujas and Names of Pundits in the Astrology related Magazines.
