Images
of Phytophthora ramorum
From the PDF 'How to recognizing symptoms
of the diseases caused by Phytophthora ramorum, causal agent of
Sudden Oak Death', by Matteo Garbelotto, UC Berkeley, David
M. Rizzo, UC Davis, Jennifer M. Davidson, USDA Forest Service, Susan
J. Frankel, USDA Forest Service, State & Private Forestry.
http://www.cnr.berkeley.edu/garbelotto/english/sod_diagnostic_report_final.pdf
There are about 60 known species
closely related to P. ramorum, and only specialists
can differentiate amongst them. The reproductive, dispersal
and survival structures of the pathogen are only visible
under a microscope. Sporangia (Fig. 1) are reproductive structures
known to play an important role in spreading the disease.
They will release zoospores, known from other species of Phytophthora
as key infectious structures. Zoospores will swim in free
water and efficiently infect plant tissue. The sporangia of
P. ramorum
may also germinate directly and infect plants.
|
Fig. 1. Sporangia of P. ramorum. (Rizzo)
|
Chlamydospores are thickwalled
and can probably survive several months. |
Fig.
2.
Chlamydospores
of P. ramorum.
(Rizzo) |
|
Fig.
3.
Sporangia growing on a rhododendron
leaf |
Under
moist conditions, sporangia, and at times chlamydospores, can
be found on the foliage of hosts such as rhododendron and bay
(Fig.3). Sporangia formed on leaf surfaces may be rainsplashed
and spread aerially to infect new hosts. Thus, foliar hosts
of P. ramorum may allow for the rapid establishment
of the disease in an area, and for the natural spread of the
pathogen across the landscape. |
All images
by Rizzo.
|
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