Phytophthora spp. are oomycetes grouped into the kingdom Chromista. Both P. citrophthora and P. parasitica are soil-borne fungi. They complete their life cycles in the soil
Foot rot or gummosis is caused by two Phytophthora spp.. Symptom development often begins near the soil line; dark water-soaked areas are formed in the bark and a sour smell may occur in wet conditions. Water-soluble gum exudes from affected tissues and is particularly noticeable in dry weather. Longitudinal cracking of bark, accompanied by profuse gumming, usually is positive evidence of infection. Soil removal around affected trees reveals bark that is water soaked, slimy, reddish-brown, or black in late stages. Brown necrotic areas can be seen extending to the cambium and wood. Advanced stages of infection will result in yellow, sparse foliage.
At later stages, the dead bark dries, shrinks and cracks and patches may fall off, leaving an open canker. Trees may later collapse and die due to the girdling action of the fungal infection. Infected fruits develop a soft brown rot eventually with a characteristic pungent odor. P. citrophthora thrives under cooler conditions with seasonal rainfall and favor Mediterranean climates. It attacks aerial parts of the trunk and major limbs.
Under suitable environmental conditions, chlamydospores germinate and produce vegetative hyphae or sporangia which initiate asexual reproduction. Sporangia are also formed by mycelial hyphae in the presence of water (flooding), which is essential for this process. Both pathogens produce papillate sporangia. Sporangia of P. citrophthora are extremely variable in shape; ellipsoid, broadly ovoid, globose, limoniform or extremely distorted (27-65 x 45-90 µm). Sporangia of P. nicotianae are predominantly spherical and average 30-40 x 38-50 µm. In water, sporangia are usually borne singly or in very loose sympodia of 2-4 sporangia. Zoospores are produced inside these sporangia, and are liberated via the sporangial papilla which dissolves. The reniform zoospores are highly motile.
They then round up and encyst, casting off their flagellae. After attachment they germinate and infect cortical root tissue. Oospores are produced only by P. nicotianae which is heterothallic. They are spherical, pale yellow when mature, 24 µm in diameter, with a wall 2 µm thick, and contain irregularly shaped subcellular inclusions. The frequency with which oospores are formed under field conditions and their role in survival and disease transmission is unknown.