Oospores of Phytophthora infestans within integrated control systems of late-blight of potato

Abstract

Oospores were produced in vitro and in planta in matings between four A1 and five A2 mating type isolates of P. infestans obtained from blighted potatoes in the United Kingdom in 1993. More oospores (maximum 11,700 oospores/cm² ) formed in leaf disks with, rather than without, illumination (photoperiod 9, 12 or 16 h/day). Light was necessary for oospore germination which ranged from 4 to 24% for different matings after 28 days on water agar. Oospores in suspension were infective producing blight lesions on floating leaf disks of cv. Home Guard. Sexual progeny of in vitro and in vivo matings were found to be segregants of the parental isolates with regard to mating type and DNA markers. Of 18 progeny tested , all except two were less aggressive than the parental isolates with one equalling and the other exceeding the aggressiveness of the parents.
Leaf disks of ten potato cultivars with a range of race non-specific resistance were inoculated with mixed A1 and A2 inoculum. Most oospores (approximately 12,600 oospores/cm² ) were generated in cultivars of medium resistance.
Oospores were not found in leaf disks (cv. Home Guard) floating on metalaxyl solutions (100µ1/ml a.i.) when inoculated with mixed inoculum of metalaxyl-sensitive Al and A2 isolates. When metalaxyl-resistant parents were used, more than 400 oospores/cm² were produced but fewer were produced if one parent was metalaxyl-sensitive. Metalaxyl added 7, 14 or 21 days post-inoculation reduced numbers of oospores generated and also reduced the subsequent germination of oospores produced between metalaxyl-sensitive parents when these oospores were extracted and plated on water agar.
Oospores produced in leaf disks and added to two plots (2 x 2m) in the field in November gave rise to lesions 7 and 10 months later on the stem bases and foliage of cv. Home Guard planted in the plots. Using various markers including mitochondrial and nuclear DNA polymorphisms, some isolates from plants in these plots were confirmed as segregants of the parental A1 and A2 isolates whilst others were contaminants. Evidence for infection by oospores was also obtained from repeated sampling of blighted cultivars in a polythene-tunnel which had been inoculated with mixed A1 and A2 inoculum.
Isolates of P. infestans taken from blighted plants and soil at a garden site with a history of A1, A2 and self-fertile phenotypes showed extensive polymorphisms for DNA markers. Active sexual reproduction was suspected at this site but could not be confirmed.

Date of Award	6 Jun 1997
Original language	English
Awarding Institution	University of Wales, Bangor