Abstract
Samples of blighted potato leaves were collected from fields in six potato-growing districts of the Punjab Province of Pakistan in 2017–2018. A total of 149 isolates of Phytophthora infestans were obtained from six potato cultivars (Asterix, Barsenna, Burana, Caroda, Mazika, Sante). Of these isolates, 73% were A1 mating type, 23% were A2 mating type and 4% were self-fertile. Both mating types of P. infestans occurred in all six districts sampled, but in every case, the A1 mating type predominated. The foliar aggressiveness of 104 of these isolates (weakly pathogenic isolates were excluded) was tested on detached leaflets of potato cv. Caroda, and a composite aggressiveness index (CAI) calculated from lesion area (measured after 10 days), latent period and infection frequency was used to compare isolates. There was no difference in CAI between isolates obtained from different districts or cultivars. The A2 mating type isolates had significantly greater CAIs than A1 isolates but this does not imply a genetic linkage nor a general association between mating type and aggressiveness. It may be that the A2 isolates belonged to an aggressive clonal lineage such as 13_A2 that has been reported in other countries in the region. While it was not possible to test the isolates collected in 2017–2018, genotyping of samples collected in 2019–2020 showed the widespread occurrence of the EU_13_A2 clonal lineage in the same districts of Pakistan and supported this hypothesis. This is the first report of EU_13_A2 from Pakistan. The implications for potato late blight management in the Punjab are discussed.
Highlights
Potato (Solanum tuberosum L.) is the most important vegetable crop worldwide and the third most important food crop after wheat and rice (Jansky et al 2019); its tubers are a globally important dietary source of starch, protein, antioxidants and vitamins (Burlingame et al 2009)
A total of 149 isolates of P. infestans were obtained from samples of blighted potato leaves collected from fields in the potato-growing districts of the Punjab Province of Pakistan during 2017–2018 (Table 1)
Isolates were obtained from six potato cultivars in total; there did not appear to be any association between mating type and cultivar as A2 isolates were obtained from all six cultivars sampled (Table 4)
Summary
Potato (Solanum tuberosum L.) is the most important vegetable crop worldwide and the third most important food crop after wheat and rice (Jansky et al 2019); its tubers are a globally important dietary source of starch, protein, antioxidants and vitamins (Burlingame et al 2009). Late blight of potato caused by Phytophthora infestans (Mont.) de Bary is the most devastating disease affecting the crop worldwide in terms of both quality and quantity of yield (Nowicki et al 2012) It is one of the main limiting factors for potato production in the world, and if the disease is not controlled, losses can reach 100% (Ghorbani et al 2004), and even with low infection levels, the crop may be unsuitable for storage (Fernández-Northcote et al 2000). The A2 mating type was first reported outside Mexico by Hohl and Iselin (1984), and both mating types occur in most potato-producing regions worldwide (Fry et al 2015) Where both mating types are present in the same field, sexual recombination may occur resulting in formation of sexual oospores which can survive in soil for several years (Turkensteen et al 2000), cause early infections and affect the population structure (Fry 2008). P. infestans may reproduce predominantly clonally even if both mating types are present: two or more separate clonal lineages may exist in parallel, surviving on potato tubers during the winter, as when there was only a single mating type (Yuen and Andersson 2013)
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