POTATO LATE BLIGHT - The Epidemiology of Plant Diseases - page 453

proportion of sporangia that produce zoospores declines as temperatures increase
above 15°C and some sporangia germinate directly by producing a germ tube.
However, the proportion of sporangia that germinate directly rarely rises above ca
20%. The influence of temperature on mode of germination was first described by
Crosier (1934). This relationship, described in the first part of the 20 th century,
appears to be accurate for the US-1 clonal lineage of P. infestans. It is presumably
this clonal lineage on which the experiments were performed. However, these results
do not apply equally to all clonal lineages. For example, the effect of temperature on
sporangial germination of US-8 (a clonal lineage introduced from Mexico to the
United States) appears to be rather different from the effect on US-1 (Mizubuti and
Fry, 1998). Crosier (1934) found that P. infestans sporangia (presumably US-1)

germinated reasonably well (directly) at 18 to 20 ° C. In recent experiments, sporangia

of US-1 also germinated well at 18 to 20°C, but the situation was different for US-8
(Mizubuti and Fry, 1998). This clonal lineage germinated well at 10 to 15°C, but not
at 18 to 20°C. The clonal lineage-dependent response to temperature suggests that
predictions concerning the effect of abiotic factors on growth and development of
P. infestans need to be made with some caution until we learn more about the
responses of different genotypes to various abiotic factors.

17.3.4 Interaction with the host: infection, colonization and reproduction

Host plants are of overwhelming importance to the population dynamics of
P. infestans . Pathogenic specialization within P. infestans occurs within a host
species as well as among species. Specialization for host genus has recently been
documented (Legard et al ., 1995; Adler et al ., 2004). Some isolates recently
introduced into the United States during the 1970s were especially pathogenic to
tomatoes, whereas others were especially pathogenic to potatoes. In competitive
fitness experiments, tomato-adapted strains rapidly dominated the total population
when growing on tomatoes (Legard and Fry, 1996). While many strains can cause
lesions on both potatoes and tomatoes, it is primarily the tomato-adapted strains that
occur commonly on tomatoes and these typically do not occur commonly on
potatoes (Oyarzun et al ., 1998; Lebreton et al ., 1999; Suassuna et al ., 2004).
The intraspecies host-specificity of P . infestans results from the compatibility of
isolates with resistance genes, R-genes. Some isolates are compatible with cultivars
containing diverse R-genes, whereas other isolates may be incompatible with such
cultivars. The observed plasticity in populations of P. infestans has caused most
potato breeders to avoid the use of R-genes (Umaerus et al ., 1983; Wastie, 1991).
The situation with the cultivar Pentland Dell seems unfortunately common. This
cultivar contained genes R1, R2, and R3 and was resistant to late blight when it was
first identified. However, during the 'adoption' phase of the cultivar, a compatible
pathotype of P. infestans appeared (Malcolmson, 1969), and the resistance broke
down before widespread adoption. This scenario has occurred many times (Van der
Plank, 1968). Goodwin et al . (1995a) have suggested that the compatibility loci in
P. infestans can be highly variable and perhaps are more readily mutable than other
loci. Variation in virulence was demonstrated for asexual progenies of P . infestans