Disease symptoms caused by Phytophthora
are difficult to distinguish from diseases by other microbes. Phytophthora
species may infect all plant parts (fruits, flowers, leaves, stems,
and roots) and cause root rots, wilting, bleeding stem cankers,
shoot blights, fruit rots, and foliar lesions. In our recent survey
of woody hosts in Minnesota nurseries we found many different species
of Phytophthora causing severe damage. Some of the species
found include P. citricola, P. citrophthora, P. cambivora, P.
cactorum, P. fragariae var. rubi and P. nicotianae.
The development and spread of Phytophthora
diseases are favored by wet conditions. The fungus-like organism
produces zoospores (motile spores) that are propelled by flagella
in wet environments. Other spores, such as chlamydospores and oospores
may also be produced allowing the organism to survive adverse conditions.
In nurseries, if Phytophthora diseases are problematic,
there are several control procedures that can reduce the impact
of these diseases including chemical control, sanitation, changing
cultural practices and host resistance.
Phytophthora root rot must be prevented
as chemicals are often ineffective in controlling this disease.
Keeping nurseries free of diseased plants is very important in disease
prevention. Planting on raised beds and using porous potting media
may help to reduce excessive moisture and development of Phytophthora
diseases. Adding composted bark to the potting soil has also been
shown to reduce disease. If recycled irrigation water is used it
is essential that Phytophthora propagules are not present
in the water or additional cultural or chemical control strategies
may be needed. Environmental stress on woody plants also makes them
more susceptible to infection by Phytophthora species.
For example, drought stress, high soil salinity, and excess nitrogen
fertilization may promote disease.
For more information see our recent publications:
Schwingle, B. W. and R. A. Blanchette. 2008. Host range investigations of new, undescribed and common Phytophthora spp. isolated from ornamental nurseries in Minnesota. Plant Disease 92:642-647. Reprint
Schwingle, B. W., J. A. Smith and R. A. Blanchette. 2007. Phytophthora speices associated with diseased woody ornamentals in Minnesota nurseries. Plant Disease 91:97-102. Reprint
Schwingle, B. W., J. A. Smith, R. A. Blanchette,
S. Gould, and B. L. Blanchette, J. Pokorny and S. D. Cohen. 2006.
First Report of Dieback and Leaf Lesions on Rhododendron sp. Caused
by Phytophthora hedraiandra in the United States. Plant Disease
90:109. Published on-line as DOI: 10.1094/PD-90-0109A. Reprint
For information on Sudden Oak Death caused by Phytophthora ramorum follow this link
Foliar symptoms of Phytophthora on
Wilt and leaf lesions caused by Phytophthora