PITRU or PITHRU dosha is the combinaiton of certain Malefic planets in Lagna, Putra, Ayur, Dharma or Karma Bhavas. Also the Lords of those bhavas are afflicted with the natural Malefics like Sun, Rahu or Kethu, the effects of the bhava will be inauspicious causing this Pitru Dosha.
1) Aspect of Saturn, Rahu or Kethu on the Ascendant, of if the Ascendant lord is afflicted with these malefics.
5) Association of Natural Malefics like Saturn, Mars with 5th lord or in 5th house along with Papa-Kartari Yoga.
6) Apart from the above, combination of Natural Malefics (Sun, Mars, Saturn, Rahu and Kethu) with 1-5-9 lords and deposited in those bhavas, creates Pitru Dosha.
Vedic Astrology is very sensitive in these issues, and clearly indicates chanting Mantras of those Malefic Planets to weed off these impact of those malefic planets. Also doing Homan / Yagya to the Adhidevathas to those planets will reduce the Malefic impact of the planet and auspicious results can be obtained.
Nowdays, we could see lot of persons longing to weed off this malefic effect by visiting various temples and doing all sort of rituals as suggested by various astrologers / saints / temple priests / spiritual gurus, etc.
But the Power of God is so strong, that one cannot escape from the fixed karmas laid down as per the birth chart of the native. Hence, Vedic Seers have clearly elaborated that, the malefic effect of the planets can only be reducted by performing Mantra Japa, which creates the hole in the cosmic energy, thereby protecting the native from those malefic impacts of that particular planet.
GOD BLESS YOU ALL