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Outbreak of Potato Late Blight and First Report of Mating Type A2 and Metalaxyl Resistance of Phytophthora infestans in Iceland

Late blight caused by Phytophthora infestans (Mont.) de Bary was endemic on potato in Iceland from about 1890 until 1961. Probably because of a cooler climate in the 1960s, the disease disappeared and there were no epidemics until 1990 and 1991. Late blight was not found in the period 1992 to 1998 even though the weather conditions were favorable for epidemic development in some of the years. Lack of initial inoculum was probably the limiting factor for an epidemic in Iceland some years. During the summer of 1999, late blight reappeared in Iceland. In August 1999, potato leaves with single lesions of late blight were sampled from two locations and sent to the Norwegian Crop Research Institute. Isolates were trapped by placing infected leaves under tuber slices in petri dishes. Pure cultures were made by picking small pieces of mycelium from the upper side of the potato slice after 7 to 10 days incubation at 15°C and plating them onto modified rye B agar with antibiotics (1). Sixteen isolates maintained on rye B agar were paired with reference isolates of mating types A1and A2 and examined for oospores at the hyphal interfaces between the isolates (1). One isolate was typed as A2 and 15 isolates were A1. Response to metalaxyl was determined for 13 isolates in vitro (1) on rye B agar amended with 5 mg/l metalaxyl-m. One of the A1 isolates was metalaxyl-resistant and 12 isolates, including the A2 isolate, showed intermediate resistance to the fungicide. These data show that the “new” population of P. infestans is now present in Iceland as in the other Nordic countries (1). Iceland imports some seed potatoes each year, mainly from the Netherlands. Imported seed potatoes were probably the source of initial inoculum in 1999 in Iceland. In the future, oospores in the soil might be an additional inoculum source in Iceland since both mating types are now present.

Reference: (1) A. Hermansen et al. Plant Pathol. 49:11, 2000.