Need to set fire to spores

Had the Askellon Sector’s military command been in any
fit state, formalised procedures would have been set in motion to
eradicate the Ork presence rising across the sector. Experienced
commanders with sufficient clearance would have known or been
able to ascertain that the Orks were the result of the bizarre manner
in which their species spreads across the galaxy. Each time the Orks
fight, their bodies shed microscopic spores which lodge upon the
ground and much later develop into subsequent generations. In
areas where the greenskins have won dominance, these creatures
are integrated into the pre-existing Ork culture. Where they appear
upon ground where the Orks have been defeated, they carry on
the war, ignorant of the earlier defeats of their forebears but still
driven by deep-rooted biological imperative to avenge them. The
only way to avert such an infestation in the aftermath of an Ork
invasion is to set the ground to fire with constant eradication
sweeps, a duty innocently neglected by the second line defenders
left on to hold their worlds.
Inevitably, the defenders were overwhelmed. Across each
of the affected worlds the beastly Orks rose up as one having
reached a critical mass of numbers and raw, savage bloodthirst.
The worlds fell in an orgy of death, a million and more defence
force troops slaughtered by the primitive savages. It would be
over a decade before the sector was able to mount an effective
campaign of reconquest after having restored something of its
previous strength, but by then those worlds had very little worth
reclaiming, and the effort of scouring the soil of alien spores meant
that any such campaign was judged highly unlikely to succeed.

- Dark Heresy 2E: Enemies Without, Page 23