Susceptibility of Eight Potato Cultivars to Tuber Infection by Phytophthora Erythroseptica and Pythium Ultimum and Its Relations

Annals of Applied Biology ISSN 0003-4746 RESEARCH ARTICLE Susceptibility of eight potato cultivars to tuber infection by Phytophthora erythroseptica and Pythium ultimum and its relationship to mefenoxam-mediated control of pink rot and leak R.J. Taylor, J.S. Pasche & N.C. Gudmestad Department of Plant Pathology, North Dakota State University, Fargo, ND, USA Keywords Abstract Metalaxyl; Solanum tuberosum; water rot. In addition to cultural practices, the application of the fungicide mefenoxam is an Correspondence important disease management tactic used to control both pink rot and leak on N.C. Gudmestad, Department of Plant potato tubers grown in the USA. Mefenoxam resistance has been identified in Pathology, North Dakota State University, many of the potato growing regions, and therefore resistance management strat- Fargo, ND 58105, USA. egies are very important for retaining this fungicide as a tool to manage these Email: [email protected] storage rot diseases. The relationship between mefenoxam efficacy and cultivar susceptibility to pink rot and leak was assessed in post-harvest inoculation studies. Received: 28 February 2007; revised version accepted: 12 October 2007. Mefenoxam was applied to potato (Solanum tuberosum) cultivars known to express varying levels of susceptibility to pink rot and leak caused by Phyto- doi:10.1111/j.1744-7348.2007.00203.x phthora erythroseptica and Pythium ultimum, respectfully. Tubers harvested from plants treated with in-furrow and foliar applications of mefenoxam were inocu- lated with isolates sensitive to the fungicide. Incidence and severity of both diseases ranged widely among cultivars. Russet Norkotah was the most susceptible to infection by P. erythroseptica, while cvs Pike and Atlantic were the most resistant. Cultivars Dark Red Norland, Russet Norkotah, Goldrush and Russet Burbank were most susceptible to infection by P. ultimum whereas Snowden was most resistant. Control of pink rot differed significantly among cultivars following mefenoxam treatment, ranging from 28% (cv. Goldrush) to 67% (cv. Snowden) and generally provided the greatest level of disease control on susceptible and moderately susceptible cultivars such as Russet Norkotah and Snow- den, respectively. In contrast, the impact of mefenoxam on leak development was minimal and disease control did not differ significantly among the cultivars. The fungicide failed to control leak in the susceptible cvs Atlantic and Pike and control ranged from 1.7% to 5.2% in cvs Goldrush, Russet Norkotah, Dark Red Norland, Russet Burbank and Kennebec. The greatest level of leak control was achieved with the moderately resistant cv., Snowden, at 12.7%. Cultivars most likely to benefit from mefenoxam treatments should be targeted as part of a pink rot management programme. Judicious use of the fungicide, when matched with the level of cultivar susceptibility, may prove to be an efficient and effective approach to reduce infection rates and possibly manage mefenoxam resistance thereby maintaining longevity of the compound. a world food staple (Thurston, 2001). Like other root- Introduction bearing and tuber-bearing species, many of the most Potato (Solanum tuberosum L.) is a cool climate crop rank- important potato diseases affect the below ground starch ing only behind wheat, maize and rice in importance as storage organ. Two of these, soil-borne diseases pink rot Ann Appl Biol 152 (2008) 189–199 ª 2007 The Authors 189 Journal compilation ª 2007 Association of Applied Biologists Susceptibility of potato cvs to infection by P. erythroseptica and P. ultimum R.J. Taylor et al. and leak, can cause severe losses in the field prior to har- however, mefenoxam remains the most reliable method vest and after tubers are placed in storage facilities of controlling pink rot and, to a certain extent, leak in (Blodgett, 1945; Powelson et al., 1993; Secor & Gudmes- areas where the pathogen populations predominantly tad, 1999; Lambert & Salas, 2001; Salas & Secor, 2001). remain sensitive to the fungicide. Both diseases may become particularly serious in low- The efficacy of mefenoxam against the water rots is lying areas of fields, locations with poorly drained soils probably one reason why traditional breeding pro- as well as during periods of heavy rainfall. Pink rot and grammes have not directed efforts towards developing leak often are collectively referred to as ‘water rot’ resistance to these diseases. As a result, genetic resistance diseases because of the watery breakdown of infected has not played a significant role in any integrated tuber tissue. The diseases are, however, differentiated by approach to manage pink rot and leak. Limited informa- unique symptomatology. tion is available from earlier studies that attempted to The oomycete Phytophthora erythroseptica Pethyb. is the quantify resistance to pink rot (Cairns & Muskett, 1939; primary cause of pink rot. The pathogen infects potato Goss, 1949; Jones, 1945; Fernandez-Northcote et al.,1972; tubers through stolons, eyes or lenticels via zoospores Lennard, 1980; Stack et al., 1994) and leak (Hawkins, and through cuts, cracks or other wounds made during 1916; Jones, 1945; Priou et al., 1997) in potato genotypes. harvest and handling operations (Pethybridge, 1913; These studies demonstrated that cultivars vary greatly Lonsdale et al., 1980; Secor & Gudmestad, 1999; Salas in susceptibility to P. erythroseptica and P. ultimum and, et al., 2000; Lambert & Salas, 2001; Taylor et al., 2004). although some are less severely affected, most cultivars In contrast, the oomycete Pythium ultimum Trow, the commonly grown in North America are susceptible to principal leak pathogen (Jones, 1935; Blodgett & Ray, one or both of these pathogens. Although limited studies 1945), cannot penetrate into the undamaged periderm assessing pink rot resistance of currently grown geno- tissue, therefore infection occurs exclusively through types have been conducted (Peters & Sturz, 2001; Peters abrasions, cuts or other wounds (Hawkins & Harvey, et al., 2004), only recently has a more concerted effort 1919; Salas & Secor, 2001; Taylor et al., 2004). P. erythro- been made to undertake large scale evaluations of potato septica and P. ultimum are widely distributed throughout cultivars for resistance to the two pathogens (Salas et al., potato growing regions of the world. They are endemic 2003). to most soils and are able to survive for many years as Disease aetiology (Taylor et al., 2004), and the level of oospores in the soil and in the infected plant debris. Both mefenoxam resistance present in the pathogen pop- pathogens are capable of infecting many other solana- ulation (Taylor et al., 2006), impact efficacy of the fungi- ceous as well as non-solanaceous species (Lambert & cide. Cultivar resistance may also have an impact upon Salas, 2001; Salas & Secor, 2001). These factors make these diseases, but it is not known whether the extent of effective control of water rots difficult to achieve. mefenoxam efficacy is influenced by the level of suscep- Strategies used to manage water rots generally target tibility of a particular genotype. Observations made in conditions that favour infection and disease development. earlier work with cvs Russet Burbank, Shepody and This approach commonly involves implementing agro- Yukon Gold suggested that control of pink rot or leak nomic practices, such as crop rotation, planting in well- by mefenoxam may be cultivar specific and related to drained soils and avoiding excessive late season irrigation. the local growing conditions; however, the results of Any action that promotes periderm development, such as these single-year studies were inconsistent (Platt et al., allowing sufficient time between vine killing and harvest, 2003a,b, 2004). A similar 3-year study found that mefe- or limits wounding, also is beneficial (Powelson et al., noxam applications provided greater suppression of pink 1993; Secor & Gudmestad, 1999; Lambert & Salas, 2001; rot in Kennebec than in Russet Burbank (Peters et al., Salas & Secor, 2001). Although growers can generally 2003a), but only these two cultivars were evaluated. manage pink rot and leak successfully by implementing Mefenoxam is largely used indiscriminately by potato such techniques, field applications of mefenoxam (Rido- producers without regards to cultivar susceptibility. mil Gold EC and Ultra Flourish EC) continue to be the Because of the development of resistance to mefenoxam most widely used means of controlling these diseases in P. erythroseptica and P. ultimum, it would be beneficial under high disease pressure conditions (Wicks et al., to limit its use to those cultivars in which treatment is 2000; Peters et al., 2001, 2003a; Platt et al., 2003a,b, necessary to adequately manage pink rot and leak to 2004; Taylor et al., 2004). Resistance to mefenoxam has reduce exposure of the pathogen to unnecessary applica- been detected in the P. erythroseptica and P. ultimum popu- tions of the fungicide. Our hypothesis was that mefe- lations across North America (Sanders, 1984; Lambert & noxam would provide little or no added benefit in the Salas, 1994; Goodwin & McGrath, 1995; Taylor et al., control of pink rot and leak in resistant or moderately 2002a,b; Hamm et al., 2004; Moorman & Kim, 2004); resistant cultivars. The current research was undertaken 190 Ann Appl Biol 152 (2008) 189–199 ª 2007 The Authors Journal compilation ª 2007 Association of Applied Biologists R.J. Taylor et al. Susceptibility of potato cvs to infection by P. erythroseptica and P. ultimum

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