Disease cycle of potato late blight - MSpace at the University of ...

the late blight management strategies (Fry and Goodwin 1997). In orden to reduce the
pathogen populations (survival, dispersal and reproduction), cultural control measures
can be used to reduce pathogen. The cultural methods consist of: the use of clean seed,
elimination of volunteer plants, management of plant nutrition, and the use of crop
rotations (Garrett and Dendy 2001).
The most appropriate and practical way to manage late blight is using resistant
varieties. Nevertheless, developing potato varieties with stable resistance to late blight is
difficult, due to the ability of the funsgus to rapidly overcome specific resistance genes
(Fry and Goodwin 1997).
1.2.3 The disease and life cycle of Phytophthora infestans
Phytophthora infestans is considered a specialized pathogen limited to potato
(Solanum tubersum) and tomato (Solanum lycopersicum). However, natural infection of
plants outside these genera has been reported (Erwin and Ribeiro 1996). According to
van West and Vleeshouwers (2004), P. infestans infection generally starts in leaves and
occasionally on stems. Leaf symptoms consist of water-soaked lesions that turn dark and
expand rapidly, generating the total destruction of the plant. Infection typically begins
when a sporangiophore is formed on the infected leaves and a zoosporangium or
sporangium is released (Fig 1.1). Zoospores and sporangia penetrate the leaf surface
either through stomata or directly through the epidermal cell wall. Sporangia germinate
mainly at temperatures above 12°C. On the other hand, during wet conditions and
temperatures below 12°C, zoospores are released, encyst and produce germ tubes. The
germ tube is able to differentiate into an appressorium and a penetration peg is formed to
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