POTATO LATE BLIGHT - The Epidemiology of Plant Diseases - page 450

grown all year round, there can be a constant source of inoculum. Even if potatoes
are not grown all year round, there are other solanaceous plants in the Andes that
harbour P. infestans (Adler et al ., 2004) and these can be a source of the pathogen.
(b) Oospore survival
Because sexual reproduction in P. infestans is a recent phenomenon in most of the
world, there are still many unknowns and much interest in deciphering the details of
the role of oospores in the epidemiology of late blight. To date, it is known that
oospores certainly survive in soil from one season to the next (Strömberg et al .,
1999; Mayton et al ., 2000; Fernández-Pavía et al ., 2004). Experiments conducted
under field conditions in the Toluca Valley, Mexico, suggest that oospores can
survive for at least two years in the absence of potato plants (Fernández-Pavía et al .,
2004). The direct relationship between late blight intensity in the beginning of the
epidemics and the number of oospores per gram of soil together with the presence of
stem lesions at the soil interface indicate that oospores are important initial inoculum
for late blight epidemics in the field. Similar observations were made by others
(Andersson et al ., 1998; Medina and Platt, 1999; Turkensteen et al ., 2000). Survival
of oospores is also reported during the summer when air temperatures reached up to
44°C (Singh et al ., 2004). This is of particular interest for potato production areas in
tropical and subtropical countries.
Many additional questions remain. We do not yet know whether oospores might
give rise to epidemics earlier than infected seed tubers, whether the resulting diverse
pathogen population will be more difficult to suppress than clonal populations and
whether oospores in soil might give rise to infected tubers regardless of the
occurrence of foliar late blight. These questions need to be addressed.
(c) Alternative hosts
The host range of P . infestans is somewhat difficult to describe accurately because
many studies have identified host responses or lesions in response to inoculations.
Erwin and Ribeiro (1996) listed 89 host species but a large minority was included on
the basis of artificial inoculation experiments. Throughout most of the world, the
two most important crop hosts are potatoes and tomatoes.
While many species have been listed as susceptible to P. infestans , the number of
weed hosts that might serve as reservoirs of inoculum varies according to regions. In
north temperate regions, Solanum sarachioides , S . nigrum , S . dulcamara , and S .
sisymbriifolium are reported as hosts (Platt, 1999; Cooke et al ., 2002; Flier et al .,
2003). Plants infected with two strains, one A1 and one A2, may allow oospore
formation, and may affect the dynamics of initial inoculum for late blight epidemics.
Because S. sarachioides (hairy nightshade) is an increasingly important weed in North
America, it might serve as either a trap for P. infestans or as a source of inoculum. In
tropical and subtropical areas, many species of solanaceous as well as of other families
are reported to be hosts of P . infestans . In South America, a potentially large number
of species of Solanum are sources of this oomycete pathogen. For example, pear melon