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Pathogen Biology

Phytophthora ramorum is an oomycete (or water mold). No longer considered true fungi, these fungal-like organisms are now classified with brown algae in the Kingdom Stramenopila (or Straminipila, or Chromista). P. ramorum is a relatively new Phytophthora species, first isolated in Europe in 1993 but not described until 2001. P. ramorum is placed in the family Pythiaceae, order Peronosporales, and class Oomycetes; molecular phylogeny shows it to be most closely related to Phytophthora lateralis, a root pathogen of Port-Orford-cedar, and P. hibernalis (which causes brown rot of citrus fruit).

P. ramorum has a unique set of morphological traits that can be observed well on certain laboratory media (PARP medium, V8-juice agar, corn meal agar, or carrot agar). Vegetative growth consists of delicate, highly branched hyphae that grow in culture best at approximately 20 C (Figure 17). No mycelial growth is observed over 35 C. Asexual reproduction in the form of sporangia or chlamydospores is the most common form of reproduction for this oomycete. Sporangia can germinate directly or release motile zoospores. Large numbers of zoospores can be produced, ideally between the temperatures of 15-20 C. At 25 C, sporangia are more likely to germinate directly, by producing a germ tube. Sporangia are elongate, semi-papillate (have a short, rounded tip), and are sympodial (where each successive sporangium develops on a branch behind and to one side of the previous apex where growth has already ceased) (Figure 18). The sporangia of P. ramorum are produced on a short (< 5 µm) to nonexistent pedicel (stalk), are caducous (fall off easily), and measure 25-97 x 14-34 µm (length x width). P. ramorum sporangia are significantly more elongate than those of its closest relatives. This oomycete also forms many chlamydospores in vitro, which are hyaline (clear) when young, and turn golden brown in older cultures (Figure 19). Chlamydospores of P. ramorum are larger than chlamydospores of P. lateralis, averaging 40-80 µm, and are mostly terminal and rarely lateral, although some can be intercalary (develop within the hyphal strand).

Figure 17

Figure 18

P. ramorum is heterothallic, requiring opposite mating types designated as A1 and A2, for sexual reproduction. The A1 mating type is most commonly found in Europe and was first described there. The A2 mating type is most common in the U.S., although the A1 type has occasionally been observed in U.S. nurseries. Oospores (spores that result from sexual reproduction) are formed when an oogonium of one mating type is fertilized by the antheridium of the opposite mating type. Terminal oogonia are almost spherical and range in size from 24-40 µm in diameter (Figure 20). Antheridia are amphigynous (the oogonium grows through the antheridium) and also rounded. Oospores have only been observed under laboratory conditions and their role in disease development in nature is not known.

Figure 19

Figure 20

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