Phytophthora cactorum (Leb. and Cohn) Schröeter has a long history in the plant pathological literature, having first been described as Peronospora cactorum - a pathogen of cacti - in 1870 (Lebert and Cohn, 1870) and subsequently found worldwide on well over 250 plant species in at least 150 genera (Erwin and Ribeiro 1996). With respect to forest trees, the potential for P. cactorum to cause disease across a wide range of hosts became evident early on inasmuch as Hartig (1876, 1880, 1882, 1883) recovered the organism from dying seedlings in the genera Abies, Acacia, Acer, Cedrus, Fagus, Larix, Picea, Pinus, and Robinia. Since then, P. cactorum has been reported from 21 additional genera of forest trees and from some commercially important roseaceous fruit trees.
Mycelia grow on a wide array of common laboratory media including PDA, CMA, LBA and V8A. P. cactorum readily fruits in culture or on plant tissue used as bait, producing caducous, usually terminal, pear-shaped sporangia, each on a short (< 4μm long) pedicel. The generative pedicels may be simple and well-spaced, but more often they are clustered and may even have a sympodial arrangement. Sporangia have prominent papillae and are typically 30 μm long (+/- 5μm) and 26 μm (+/- 4 μm) wide. Some, but not all, isolates produce chlamydospores, usually at the tip of generative hyphae but occasionally via intercalary formation. When they do occur, chlamydospores have moderately thick cell walls (1-1.5 μm) and range from 25-40 μm dia. Sexual reproduction in P. cactorum is uniformly homothallic with paragynous antheridia.