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Symptoms and Signs

Phytophthora ramorum causes two distinct sets of symptoms, depending on the host species. On certain members of the Fagaceae (oak family) such as tanoak (Lithocarpus densiflorus) and coast live oak (Quercus agrifolia), P. ramorum causes lethal bole (trunk) cankers resulting in the disease called sudden oak death (SOD). On other native plant species as well as many horticultural nursery crops, P. ramorum causes a foliar blight and shoot dieback. The disease on these hosts is referred to as ramorum leaf blight or ramorum shoot blight.

Sudden oak death

Bole and crown symptoms
P. ramorum infects the phloem and inner bark to cause bleeding cankers on susceptible members of the Fagaceae. The presence of these bleeding cankers is the most diagnostic symptom of SOD. Initially, individual spots exhibit a clear, reddish exudate on the main trunk of the host (Figure 1). As the disease progresses, this exudate may continue to seep through both cracked and intact bark (Figure 2). The affected area eventually stains a dark reddish-brown as the bleeding is diffused by rainfall (Figure 3). Removal of the outer bark reveals discolored areas (cankers) in the inner bark, often surrounded by a black line (Figures 4, 5). Cankers range from a few centimeters to 3 meters in length, developing most often in the lower part of the trunk but also as high as 25 meters from the ground. Cankers have only been found on mature coast live oak trees, but may appear on tanoak at any stage of development.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

P. ramorum may also infect sapwood, restricting water transport in xylem vessels. The crown of infected plants often appears off-color before individual branches or the entire crown turns brown and dies (Figure 6). Infected stump sprouts may exhibit foliar necrosis, stem lesions, and wilted shoot tips known as “shepherd’s crooks” (Figure 7).

Figure 6

Figure 7

Figure 8

Foliar symptoms
Foliar symptoms are not always observed on canker hosts, but tanoak leaves may exhibit mid-vein and petiole necrosis (Figure 8).

Ramorum leaf blight and ramorum shoot blight
Most hosts of P. ramorum exhibit symptoms chiefly on the foliage. On some hosts, lesions and vascular discoloration are associated with the stems. Plants susceptible to ramorum leaf blight or shoot blight (simply referred to henceforth as ramorum blight) include woody ornamental nursery stock as well as native herbs, shrubs, and trees other than the oak family hosts described above. On Rhododendron, necrotic lesions may appear anywhere on leaf surfaces (Figure 9) and are easily confused with lesions caused by other leaf-infecting species of Phytophthora. Leaf infection may occur through the petiole from infected stems, causing necrosis along the mid-vein (Figure 10). Foliar symptoms on other hosts vary, consisting of wedge-shaped or irregularly-shaped, darkened lesions that may appear watersoaked (Figure 11). Defoliation of affected camellia leaves is common (Figure 12). On California bay laurel (Umbellularia californica), necrosis is often confined to the leaf tip, with small yellow-brown spots scattered on other parts of the leaf (Figure 13). Conifers can also be hosts for P. ramorum, although disease incidence is usually limited to young trees beneath heavily infected bay laurel or tanoak (Figure 14). For a description of symptoms on various hosts, refer to the following sources: Davidson et al., 2003; Rizzo et al., 2002; Tjosvold et al., 2005; and The California Oak Mortality Task Force Website.

Figure 9	Figure 10	Figure 11
Figure 12	Figure 13	Figure 14

In greenhouse studies, P. ramorum infects the roots of several hosts, although it does not produce root rot symptoms typical of soilborne Phytophthora species. Root infection of nursery-propagated rhododendrons has been reported. It has been demonstrated that the pathogen can spread from the roots and up the stem to above-ground plant parts.

Signs
Although not detected on all hosts, sporangia (asexual fruiting structures) and hyphae of P. ramorum can be found on leaves, usually on the lower surface (Figure 15). Sporangia are only formed under very moist conditions. Chlamydospores (asexually produced survival structures) may also be found on or in infected leaf tissue of foliar hosts (Figure 16), or in the wood and bark of canker hosts.

Figure 15

Figure 16

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