1. The common strain and nine specialised strains of
Phytophthora infestens were employed in testing seedlings
and seedling progenies, obtained from four different breeding
systems, for resistance to the disease.2. The resistance exhibited by S. demissum (CPC 2127) and
seedlings bred from it is due primarily to the hypersensitive
condition of the protoplasm. This condition is manifested
in the presence of one or more major resistance genes, of
which four have been identified, viz. R₁, R₂, R₃ and R₄.3. Each major gene confers resistance to the common strain
and to a particular group of specialised strains of the parasite.
The genes are inherited independently in simple
Mendelian fashion.4. A series of minor genes, associated with morphological
and physiological characters of the plant, modify the phenotypic
expression of the major gene system, and so determine
the degree of susceptibility in susceptible phenotypes and
the extent of necrosis in resistant ones.5. In the early generations of S. demissum-S. tuberosum
hybrids, the irregularity of chromosome behaviour and the
presence of unpaired chromosomes caused the ratios of
resistants resistants to susceptibles to vary widely from standard
Mendelian ratios.6. In certain progenies, particularly those obtained by
crossing S. tuberosum plants with resistant hybrid derivatives
of S. demissum, deviations from standard Mendelian ratios
were consistent in their trend, and appeared to be due to some
relationship between genes affecting disease resistance and
incompatibility genes.7. In certain parent seedlings with duplicate genes,
derived from self- fertilised plants or from recombination
crosses, partial auto-syndesis resulted in an excess of
resistant segregates in the progenies.B. The interrelationships of ten strains of blight (Phytophthora
infestans) and four major genes R₁, R₂, R₃ and R₄
controlling resistance to the disease in the potato are
examined.9. Since dominance of the genes is complete the maximum
number of phenotypic reactions which can be distinguished is
sixteen. This series represents the complete range of
differential hosts for the material in question and is
capable of distinguishing sixteen different strains of the
parasite.10. The reactions exhibited by the series form a concise
statement statement of the various relationships and provide the
necessary groundwork for the systematic classification of
strains. The classification will afford a basis for the
calculation of segregation- ratios to be expected from the
mating of any pair of genotypes when infected with any strain
or group of strains of the parasite.11. Each specialised strain, being adapted to a particular
Solanum genotype, is more prolific on it than on any other.
This genotype is regarded as the natural host of the strain.12. Specialised strains differ in their effect upon varieties
of S. tuberosum: Those with the widest host range
being least virulent. Also, they are less prolific and
cause less damage than the common strain.13. The exact mode of origin of strains is not clear but
mutation appears to play a significant part.14. Specialisation appears to progress in stages in certain
directions determined by the genetic constitution of the
various hosts. Examples of three progressive stages of
specialisation are apparent in the material reported on here.
If the fourth stage should be reached, the resulting strain
will be capable of attacking all sixteen genotypes.15. It is suggested that strains should be classified
according to the genetical constitution of their natural
hosts and that each should be known by the numeral pertaining
to the genes in question
