Agriculture Reference
In-Depth Information
(
Solanum muricatum
) is an agricultural host that can harbour isolates of the A1 and A2
mating types in Ecuador (Adler
et al
., 2002). The main implication of this finding to
the epidemiology of late blight is to emphasize that plants other than potatoes or
tomatoes can be a source of the pathogen and can provide opportunity for formation of
oospores (Oliva
et al
., 2002). In Africa, solanaceous and species of Asteraceae were
reported as host of
P
.
infestans
(Fontem
et al
., 2004).
Despite many reports of potential hosts and reservoir species for
P
.
infestans
, the
epidemiological significance of such plants remains largely unknown. Assessing the
contribution of non-economical plants as inoculum sources is not trivial. Population
genetics tools such as those reported in a study to detect late blight foci in The
Netherlands may be useful to epidemiologists (Zwankhuizen
et al
., 1998). The
ability to trace genotypes will contribute to elucidate the role of weeds, ornamentals,
and cultivated plants as inoculum sources.
In north American greenhouse production, late blight is a consideration if
petunias and tomatoes are grown in the same house. In general, petunias are not
nearly as susceptible as are tomatoes, but they can be infected and they can also be a
source of inoculum to initiate an epidemic on tomatoes in the greenhouse (Becktell
et al
., 2005a). Infections on petunias are not nearly so visible as on tomatoes so
infected petunias might not be detected.
17.3.2 Dispersal of
P. infestans
Infected tubers can be transported kilometres, hundreds of kilometres, or from one
continent to another. Transport of infected tubers is almost certainly the most
important mechanism by which global migrations have occurred. Presumably, it is
also probable for oospores to be transported long distances. However, the relative
importance of oospores in global migration is difficult to ascertain. Oospores may be
present in tubers or in soil accompanying tubers. The low germination rates of
P. infestans
oospores (Andrivon, 1995) further challenges our efforts to investigate
the role of this spore form in epidemiology.
Although movement of infected seed tubers certainly transports
P. infestans
, the
efficiency of subsequent establishment has been reportedly very low (Van Der Zaag,
1956). Typically, many infected tubers die before producing plants and those that
produce plants may not be infected. However, a variably low frequency of infected
tubers may give rise to infected plants (Inglis
et al
., 1999). It now appears that
specific strains differ from each other in their abilities to infect tubers (Lambert and
Currier, 1997) and presumably also in their abilities to initiate new epidemics.
A major challenge in late blight epidemiology is to develop quantitative systems to
predict the probability and intensity of epidemics starting from infected seed tubers.
Airborne sporangia can also initiate epidemics. However, because free sporangia
do not survive long when exposed to sunlight, very long distance transport is rare
and highly unlikely. However, shorter range transport can be much more common
and very important, especially under cloudy and moist conditions. It is true that
while disease gradients indicate that most sporangia are deposited within meters of a
source (Paysour and Fry, 1983), some obviously escape the local area and can be
