A new disease of coast live oaks (Quercus agrifolia) in California (US) is associated with Geosmithia pallida and Pseudopityophthorus pubipennis
Since 2012, declining coast live oak (Quercus agrifolia) trees have been observed throughout urban landscapes in the counties of Los Angeles, Orange, Riverside, Santa Barbara, Ventura, and Monterey in California (US). Affected trees showed multiple insect entry holes (0.95 mm diameter), branch dieback, and eventually died. Symptoms also included a wet discoloration around entry holes on the trunk and main branches, followed at first by the production of reddish sap and then of whitish foam (hence the common name of the disease: ‘foamy bark canker disease’). Infested trees can produce a large amount of foamy liquid that can run up to 2 feet (60 cm) down the trunk. Beneath the outer bark phloem and xylem, necrotic tissues could be observed. Recent studies have indicated that this new disease is associated with a bark beetle, Pseudopityophthorus pubipennis (Coleoptera: Scolytidae - Western oak bark beetle) and a fungus, Geosmithia pallida. The bark beetle is not new to the USA, but the fungus is recorded for the first time in the USA. Little information is available on G. pallida, it is noted that this fungus has been shown to inhibit root growth of Lepidium sativum (Brassicaceae) by 25% during laboratory experiments conducted in the Czech Republic and that it appears to have affinities with a range of subcorticolous insects. Other studies have also found that a strain of G. pallida obtained from wilting Ulmus in Italy possessed a cerato-ulmin toxin, the protein involved in Dutch elm disease. More studies are needed to better understand the epidemiology, geographical distribution and potential impacts of this disease on oak trees.
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