This species was originally collected on the island of Mindanao (Mt. Apo) in the Philippines in 1882 (type attributed to either Hieronymus (Meijer, 1997; Nais, 2001) or Schadenberg and Koch (Merrill, 1923). Despite concerted efforts by field botanists, R. schadenbergiana had not been seen and was presumed extinct. Previous reports of R. schadenbergiana on Mt. Matutum (e.g. Nais, 2001) are erroneous (J. Barcelona, pers. comm.). A bud from Mt. Matutum examined at the Central Mindanao University in Bukidnon Province in 2002 represents a small-sized Rafflesia, too deteriorated for proper identification.
After 112 years since it was first discovered by the German ethnologist Schadenberg, R. schadenbergiana, was rediscovered on the island of Mindanao. In 1994 Pascal Lays found buds of R. schadenbergiana in South Cotabato Province on the island of Mindanao. His paper reporting this result has only recently been published. Moreover, Dr. Julie Barcelona reports on the discovery of yet another population of this rare species in Bukidnon (Flora Malesiana Bulletin, submitted; )
The morphological features of these flowers fit well with the original descriptions of this species (see below). Moreover, this is the largest flower among all the species of Rafflesia present in the Philippines, ranging from 52 to 70 cm in diameter. For these reasons, there is little doubt that this taxon is R. schadenbergiana. The Bukidnon "population" contains several buds, flowers, and fruits, but all are apparently parasitic on a single individual of the host vine Tetrastigma. For this reason, these plants are under extreme threat of extinction and concerted conservation efforts are required to maintain them for future generations.
Description translated from the original German text (Koorders 1918) by Vanessa Ashworth
Warts of the perigone lobes medium sized, bumpy, often coalescing in a reticulate pattern. Perigone tube and the interior surface of the diaphragm covered almost to the margins with filiform ramenta with somewhat club-shaped swollen endings. A single annulus [coronal ring] protruding diagonally outward; the outer one represented by a level ring zone that is devoid of ramenta. Disc similar to previous species [R. hasseltii], as is the position of anthers in the male flower. Disc edge densely short-hairy above the anthers, as is the entire column. Anther cavities very flat, not extending half way down the column width, recessed on the upper side; ridges between them [the anther cavities] here narrow and pubescent at the edge. In the female flower the stigmatic ring forms a weak outer ridge; indented in front of the anther rudiments. Below each one of these [rudiments] there is a shallow area surrounded by hairs which is the rudiment of the anther cavity; column otherwise flat and everywhere short-hairy.
Description From Flora Malesiana treatment (W. Meijer, 1997) Flower buds prior to expansion 16-20 cm in diameter, cupule 10-14 cm in diameter, bracts up to 17-18 cm long, 12-13 cm wide. Open flower about 80 cm in diameter. Ramenta on the inside of the flower tube 7-10 mm long, filiform, somewhat thickened at the apex, or branched, partly in fascicles. Diaphragm 6-8 cm from insertion to opening, opening 13-14 cm in diameter, margin with pinkish zone, lower face except the marginal zone provided with ramenta ca. 4-5 mm long. Perigone lobes 25-26 cm in diameter, with yellowish whitish warts which are laterally stretched, irregularly shaped, partly connected and about 4 or 5 [in number] across the middle part. Disk 12-13 cm in diameter, processes 30-50. Male flowers with 26-38 (-40) anthers.