Status of research, breeding and protection of cucurbits in relation to cucurbit downy mildew: their limits and perspectives

Knowledge of the recent distribution, host range, virulence, DNA profiles, and resistance to fungicides of Pseudoperonospora cubensis (Berk. & M.A. Curtis) Rostovzev, and sources of host resistance to this pathogen are reviewed. P. cubensis is widely distributed in nearly all continents, with a high capacity to adapt to changing environmental conditions and new regions. Its host-plant range includes more than 60 species and 20 genera of Cucurbitaceae. It exhibits clear host specialization, with shifts in host range and an increasing spectrum of new host species. Temporal shifts to higher virulence levels and more complex virulence patterning within pathogen populations have been recorded in the Czech Republic, Israel (in 2002), and the USA (in 2004). Pathotypes have been described, and a triple-tetrad code proposed for their denomination. A differential host set for racial determination and denomination is under development. Forty-five different reaction patterns were recorded from the interaction of 115 Cucumis melo accessions to 8 isolates of P. cubensis with different levels of virulence; most C. melo accessions were highly susceptible to all isolates; but PI 315410 (VIR 5682) was resistant to 5 isolates. Because of race-specificity in host-pathogen interactions, resistance breeding is rather difficult and resistance typically not stable. In vitro tests of fungicide efficacy in 2001-2014 documented high levels of efficacy in fosetyl-Al (Aliette 80 WP) and propamocarb (Previcur 607 SL) to Czech strains of P. cubensis, with increased resistance of some strains, inefficacy of metalaxyl, metalaxyl-M and cymoxanil (Ridomil Plus 48 WP, Ridomil Gold MZ 68 WP, Curzate K), and inefficacy of dimethomorph (Acrobat MZ) with a temporal shift from resistant to tolerant and sensitive isolates. Changes in the genetic structure of populations of 67 P. cubensis isolates collected in the Czech Republic and other European countries during 1995-2012 were detected by Multilocus Sequence Analysis (MLSA) of four mitochondrial and two nuclear DNA regions. These changes may be associated with recently reported shifts in virulence. Data on host-range changes and spatio-temporal variability in virulence and fungicide resistance confirm the high micro-evolutionary potential of P. cubensis populations and more detailed research is required for a better understanding of these host-pathogen interactions as well as for more efficient disease management.