New Races and Novel Strains of the Spinach Downy Mildew Pathogen Peronospora effusa

Abstract

Downy mildew disease, caused by Peronospora effusa (=P. farinosa f. sp. spinaciae [Pfs]), is the most economically important disease of spinach. Current high-density fresh-market spinach production provides conducive conditions for disease development, and downy mildew frequently forces growers to harvest early owing to disease development, to cull symptomatic leaves prior to harvest, or to abandon the field if the disease is too severe. The use of resistant cultivars to manage downy mildew, particularly on increasing acreages of organic spinach production, applies strong selection pressure on the pathogen, and many new races of Pfs have been identified in recent years in spinach production areas worldwide. To monitor the virulence diversity in the Pfs population, downy mildew samples were collected from spinach production areas and tested for race identification based on the disease reactions of a standard set of international spinach differentials. Two new races (designated races 15 and 16) and eight novel strains were identified between 2013 and 2017. The disease reaction of Pfs 15 was similar to race 4, except race 4 could not overcome the resistance imparted by the RPF9 locus. Several resistance loci (RPF1, 2, 4, and 6) were effective in preventing disease caused by Pfs 15. The race Pfs 16 could overcome several resistance loci (RPF2, 4, 5, 9, and 10) but not others (RPF1, 3, 6, and 7). One novel strain (UA1014) could overcome the resistance of spinach resistant loci RPF1 to RPF7 but only infected the cotyledons and not the true leaves of certain cultivars. A new set of near-isogenic lines has been developed and evaluated for disease reactions to the new races and novel strains as differentials. None of the 360 U.S. Department of Agriculture spinach germplasm accessions tested were resistant to Pfs 16 or UA1014. A survey of isolates over several years highlighted the dynamic nature of the virulence diversity of the Pfs population. Identification of virulence diversity and evaluation of the genetics of resistance to Pfs will continue to allow for a more effective disease management strategy through resistance gene deployment.

Genetic diversity of the spinach downy mildew pathogen based on hierarchical sampling

“…Downy mildew of spinach remains a major economic constraint for both organic and conventional spinach production in all major spinach growing areas globally. Although virulence diversity of the downy mildew pathogen, P. effusa, has been examined extensively [4][5][6][7][8], little is known of the genotypic diversity of the pathogen at various levels of hierarchy. The genotypic diversity in populations of P. effusa has been evaluated with samples collected from CA and AZ [10].…”

“…For example, the spinach cultivar Lion is susceptible to races Pfs 10 and Pfs 17, but has resistance to races Pfs 1-9, 11-16 [6]. Thus, isolates of races Pfs10 or Pfs 17 may be recovered from cultivar the cultivar Lion, but it is not possible to recover isolates of other named races of P. effusa.…”

“…However, approximately 50% of U.S. spinach production is organic, which negates chemical control and makes host resistance the most practical management tool. Unfortunately, the pathogen evolves rapidly, For example, 14 of the 17 named races and 13 novel virulence strains of P. effusa have been identified in the past 30 years [4][5][6][7][8]. New races and novel strains of the pathogen may overcome newly released resistant cultivars, and result in disease epidemics and severe economic losses.…”

Spinach Growers Value Host Resistance and Synthetic Fungicides in the Fight against Downy Mildew

“…Most organic growers rely heavily on disease resistant varieties. As new races of the pathogen overcome the resistant varieties, growers need to heavily rely on new varieties being bred by a relatively small portion of seed producers (Feng et al 2018). Although some organic fungicides on the market are available and show some control of disease, they do not control to economic levels and need to be supplemented with resistant varieties for full effect (Choudhury et al 2015a;Choudhury et al 2017b).…”

Transcriptional analyses of differential cultivars during resistant and susceptible interactions with Peronospora effusa, the causal agent of spinach downy mildew

“…The movement of pathogen through infested seeds as oospores introduces novel recombinant isolates into spinach production sites and can spread the disease rapidly over large geographical regions 9 . Additionally, P. effusa produces tens of thousands of asexual spores per plant, which disperse aerially and initiate secondary infections within a field, or primary infections at another site, in distant fields 9,11 .Increasing numbers of races, or new virulent pathotypes of P. effusa, have been detected using a set of differential spinach cultivars and reported in recent years in the US and other countries 12,13 . The emergence of virulent isolates capable of overcoming resistance genes is more common in pathogens such as downy mildews since they produce many asexual spores and retain high recombination rates 14,15 , but their emergence is exacerbated by sexual reproduction, as noted by the presence of oospores in the population 10 .…”