Phytophthora cactorum

Description:  Phyophthora cactorum is an oomycete that is capable of infecting a large number of hosts and is problematic in low-lying or wet field conditions.  It can limit production for many economically-important crops such as apple, pear, rhododendron, azalea, and strawberry.  P. cactorum can cause root, collar, and crown rots, as well as foliar and fruit infections. 

Identification: On potato dextrose agar media, P. cactorum forms a white, loosely matted colony.  Sporangia are distinctively papillate, and are usually borne terminally.  Sporangia shapes vary widely, and can range from broadly ellipsoidal, obpyriform, or ovoid to spherical.  Sporangia are often clustered, and sporangiophores are normally simple.  Each sporangium may contain more than 50 zoospores.  Chlamydospores can develop in culture on v8 juice-carbonate broth when incubated for 20 days at 4º C, but may not be produced at other temperatures or on other media.  Generally, chlamydospores are terminal, but may occasionally be intercalary.  P. cactorum is homothallic, and oospores can be found as within the tissue of certain host crops and in culture.  All antheridia are paragynous and are generally spherical or club-shaped.  Oogonia are usually hyaline and smooth-walled, and oospores are plerotic.

Disease Cycle and Epidemiology: The specific epidemiology of this pathogen may vary with the crop that it infects.  The most important propagule for this pathogen is zoospores, which originate from hyphae or germinating oospores and sporangia.  In many cases, this pathogen may enter a field through infected transplants.  Infection by P. cactorum usually occurs during warm periods with prolonged wetness.  Motile zoospores are released from sporangia during saturated soil conditions and enter through wounds.  Once zoospore reaches a host, it infects and developing hyphae of the fungus colonize the host.  Development of symptoms may vary depending on host species, but are usually the result of conducive environmental conditions.

Hosts: Phytophora spp. can infect more than 200 species in 160 genera including strawberry and a number of woody ornamentals and fruit crops.  

Distribution: This pathogen occurs worldwide, but is most common in temperate regions.

General Information:

Sources:  

http://www.cals.ncsu.edu/course/pp728/cactorum/Pcactorum.html

http://www.ars.usda.gov/SP2UserFiles/person/31120/Bhat.pdf