End of Project ReportPhytophthora infestans (Mont.) de Bary, the causal agent of potato late blight is
best known for its role in the great Irish famine of 1845-1849 which resulted in the
deaths of over 1 million people. Since then, the disease has become established in
all potato growing countries and is the most important pathogen of potatoes
worldwide. The appearance of resistance to the phenylamide fungicides in the late
1970’s indicated that populations of P. infestans were changing. An antiresistance
strategy was developed for growers in an effort to reduce the spread of
resistant strains. Subsequently the A2 mating type of the fungus was discovered in
1989 promoting fears that a super strain of the fungus could evolve through sexual
reproduction. Populations of the fungus have been monitored from 1981 to 1998
for levels of phenylamide resistance and since 1988 for the A2 mating type.
Physiological race surveys were conducted in 1983 and 1996. Prior to the 1980s
no reliable methods were available for adequate identification of genotypes.
Development of molecular markers specific to P. infestans has made this possible
and a survey was conducted on isolates from the 1996 population.
Results confirm that the anti-resistance strategy for phenylamide based fungicides
has been effective in preventing the build up of metalaxyl resistant populations of
P. infestans. During the 1990’s the distribution of phenylamide resistance has
remained stable at about 50% of crops tested compared to a high of over 80 % in
1981. The level of A2 in the population has also fallen from a high of 35% of
isolates tested in 1989 to a static level of 3-4 % in the 1990’s. Physiological race
composition has become much more complex since 1983 and 16 different
physiological races were found in Ireland in 1996. The population was dominated
by race 3.4.7.10.11 which accounted for over 54% of isolates tested. This change
has taken place without a corresponding change to varieties with a complex Rgene
base.
Twelve different genotypes of the fungus were uncovered using the multilocus
probe RG57. Races of the fungus were independent of genotype. One particular
genotype IE-2 was predominantly associated with phenylamide resistance. The
low population diversity discovered suggests that sexual reproduction between A1
and A2 types has not been a major factor in disease epidemiology to date. Super
strains similar to those identified in the USA could not be confirmed.
The overall level of variation in the Irish isolates of Phytophthora infestans would
confirm that the population has become progressively more diverse during the last
forty years. However, the population is much less complex than that found in the
highland tropics of central Mexico.Teagasc Walsh Fellowship Programm
