Colony slow growing uniform, without a pattern, with limited aerial mycelium, margin regular and off white. Hyphae swellings common on some isolates, spherical and irregular, averaging 3.7 ± 1.1 (ranging 1.5–7.4) μm. Sporangia rarely observed on solid agar but produced abundantly in non-sterile pond water, papillate, caducous or persistent, borne terminally on simple sporangiophores, predominantly ovoid to rarely globose or limoniform in shape, averaging 31.7 ± 6.6 × 23.3 ± 6.7 μm (ranging 16.6–55.3 × 12.6–39.2 μm). Papillae averaging 3.0 ± 1.0 × 4.9 ± 0.8 μm and ranging from 1.1–5.5 × 3.2–6.6 μm. Sporangial exit pores 5.4 ± 0.8 μm diameter. Sexual reproduction is homothallic. Oogonia aplerotic, averaging 35.1 ± 3.6 (ranging 24–45.6) μm with 29–80% aborting within two weeks, before the formation of an oospore wall. Oospores averaging 29.8 ± 3.3 (ranging 20.8–39.1) μm, with thin walls and an oospore wall index of 0.19, hyaline to rarely pigmented yellow on maturity. Antheridia paragynous (65%) or amphigynous (35%), mostly club-shaped, rarely spherical, 14.9 ± 3.0 × 11.7 ± 2.5 μm. Growth between 5 and 25 °C, slow (optimum, 3.4 mm day-1, at 20 °C).

holotype: NZFS 4037 (dried culture), ex-holotype living culture NZFS 4037 = ICMP 22750

records of P. cactorum ex Pinus radiata in NZ may be based on misdientifications "Isolates of a Phytophthora species identified as P. cactorum by D. J. Stamps of the Commonwealth Mycological Institute, Kew (IMI 158986, IMI 185987), were obtained from diseased P. radiata seedlings in a nursery near Dunedin in 1971, although these isolates were lost and and could not be used in this study. Isoaltes identified morphologically as P. cactorum from P. radiata in New Zealand may therefore have been P. aleatoria; however, this cannot be confirmed."