JOURNAL OF NEMATOLOGY Article | DOI: 10.21307/jofnem-2020-063 e2020-63 | Vol. 52 Reproduction of Meloidogyne enterolobii on selected root-knot nematode resistant sweetpotato (Ipomoea batatas) cultivars Janete A. Brito1,*, Johan Desaeger2 3 and D.W. Dickson Abstract 1Florida Department of Agriculture and Consumer Services, Division The ability of Meloidogyne enterolobii to reproduce on selected of Plant Industry, Gainesville, FL sweetpotato (Ipomoea batatas) cultivars (Beauregard, Covington, 32614-7100. Evangeline, Hernandez, and Orleans (LA 05-111)) was evaluated in two greenhouse experiments, each with 10 replicates. All cultivars, 2 Entomology and Nematology except Beauregard (control) and Orleans, were reported previously Department, Gulf Coast Research as moderately resistant or resistant to M. incognita, Fusarium and Education Center, University of oxysporum f. sp. batatas, and Streptomyces ipomoeae. Plants were Florida, Wimauma, FL 33598. inoculated with M. enterolobii (5,000 eggs/plant) and arranged in 3Entomology and Nematology a completely randomized design in a greenhouse with an average Department, Institute of Food and daily temperature of 24.8°C. Galls and egg masses per root system Agricultural Sciences, University of (0-5 scale), eggs per egg mass, eggs per gram of fresh root (gfr), Florida, Gainesville, FL 32611-0620. and reproduction factor (RF) were determined. Meloidogyne enterolobii infected and reproduced on all the sweetpotato cultivars. *E-mail: [email protected] The nematode induced galls on both fibrous and storage roots, This paper was edited by regardless of the cultivar, as well as induced necrosis and cracks Horacio Lopez-Nicora. on storage roots. The lesions and cracks on the storage roots were more visually pronounced on Hernandez than those on other Received for publication cultivars. Cultivar Orleans sustained less root galling and egg masses April 20, 2020. than other cultivars (p ≤ 0.01), and both Orleans and Beauregard cultivars had less eggs per gfr and a lower RF than Covington (5,683 eggs/gfr; RF = 16.92), Evangeline (7,161 eggs/gfr; RF = 30.01), and Hernandez (6,979 eggs/gfr; RF = 22.6). The latter two cultivars sustained the largest amount of reproduction of M. enterolobii. The number of eggs per egg mass ranged from 462 to 503 and was similar among all cultivars. In summary, M. enterolobii reproduced well on all sweetpotato cultivars; however, differences were observed among cultivars (p ≤ 0.001). The host status as previously reported for other root-knot nematode species was not a good predictor of host status to M. enterolobii. Some sweetpotato cultivars that were reported as resistant or moderately resistant to M. incognita race 3, such as Evangeline and Hernandez, were among the best hosts to M. enterolobii. Root growth of Evangeline and Orleans, but not of the other cultivars, was negatively correlated with nematode eggs per gfr. Keywords Guava root-knot nematode, Meloidogyne enterolobii, Pacara earpod tree root-knot nematode, Reproduction, Root-knot nematode, Susceptibility, Sweetpotato. © 2020 Authors. This is an Open Access article licensed under the Creative 1 Commons CC BY 4.0 license, https://creativecommons.org/licenses/by/4.0/ Host status of selected sweetpotato cultivars: Brito et al. Sweetpotato (Ipomoea batatas (L.) Lam) is a major these cultivars, except Beauregard and Orleans, are crop in the southeastern USA and a staple food in reported at least moderately resistant to M. incognita many tropical countries around the world. Root-knot race 3 and two other important pathogens to nematodes (RKN) (Meloidogyne spp.), particularly sweetpotato, Fusarium oxysporum f. sp. batatas M. incognita (Kofoid and White, 1919) Chitwood, 1949, and Streptomyces ipomoeae (Rolston et al., 1987; cause significant suppression in yield and root quality LaBonte et al., 1992, 2008, 2012; Yencho et al., 2008). of sweetpotato (Lawrence et al., 1986; Clark et al., Nematode isolate, M. enterolobii (DPI: N01-00283), 1992). Most recently, there has been an outbreak of was previously identified using both morphological M. enterolobii (Yang and Eisenback, 1983), Pacara and molecular analyses (Brito et al., 2004). The earpod tree RKN (Yang and Eisenback, 1983), species was reared on tomato Rutgers in steam- observed in the major sweetpotato producing states pasteurized soil. Sweetpotato cultivars used in this in the USA. This nematode species was reported study were Beauregard, Covington, Evangeline, causing severe yield and root quality suppression, Hernandez, and Orleans. Tomato Rutgers was used and in at least one case, total crop loss in North as a control to determine the viability of the inoculum. Carolina (Ashby, 2017). Other production states with Eggs were extracted from infected tomato roots severe outbreaks of damage include Louisiana (Strain, using the 0.5% NaOCl (Hussey and Barker, 1973) 2018a, 2018b) and South Carolina (Rutter et al., method as modified by Boneti and Ferraz (1981). 2019). In addition, M. enterolobii has been reported An individual sweetpotato stem (20-cm long) from causing damage on sweetpotato in Africa (Fargette, the appropriate cultivar was planted directly into 1987; Fargette and Braaksma, 1990; Karuki et al., 27-cm-diameter clay pots containing pasteurized soil 2017) and China (Gao et al., 2014). The first report (89% sand, 3% silt, 5% clay; pH 6.1; 1.1% organic of Meloidogyne enterolobii in the continental USA matter). Thirty days after planting the slips, each occurred in 2001 in Florida, where it was first found seedling was inoculated with 5,000 eggs/plant (Pi) infecting ornamental plants and since then numerous at 1,000 eggs/ml in five equal holes 3.5 to 4.5 cm other plant species (Brito et al., 2002, 2004, 2008, deep surrounding the root system. Plants were set up 2010). Furthermore, in 2013, this nematode species in a completely randomized design in a greenhouse was also found infecting cotton and soybean in North with 10 replications. The average temperature in the Carolina (Ye et al., 2013). Meloidogyne enterolobii greenhouse was 24.8°C (ITWatchDog, Austin, TX). is one of the most damaging RKN species noted Plants were watered daily and fertilized once a week today because of its wide host range, high degree of with Peter’s fertilizer (20-20-20 with micronutrients) virulence, and its ability to overcome RKN-resistant according to the manufacturer’s instructions (United genes in several important agricultural crops, namely Industries Corp., St. Louis, MO). The experiment Mi-1, Mir1, Me, N, Rk, and Tabasco genes in tomato, was repeated following the same methods and in the soybean, bell pepper, cowpea, and sweet pepper, same greenhouse. respectively (Carneiro et al., 2006; Brito et al., 2007; The tomato and sweetpotato root systems Cetintas et al., 2008; Kiewnick et al., 2009; Pinheiro were collected 60 and 90 days after inoculation et al., 2013; Gonçalves et al., 2014; Rosa et al., 2015). (DAI), respectively. In both tests, all the sweetpotato Currently, in the USA, this nematode species is of cultivars (treatments) were harvested later (90 DAI), great concern to producers of sweetpotato, cotton, when symptoms of nematode infection were clearly soybean, as well as many other agricultural crops. As visible in storage roots. At harvest, root systems a result, regulatory officials in Louisiana, Arkansas, from each experiment were removed from the pots and Mississippi have imposed an external quarantine and carefully washed to remove the soil. Storage for M. enterolobii. Several sweetpotato cultivars have roots when present were separated from the roots, shown moderate to good resistance to the major then root systems were weighted and rated for root Meloidogyne spp. including M. incognita, M. javanica galling and egg masses on a 0 to 5 scale, where 0 = 0 (Treub, 1885) Chitwood, 1949 and M. arenaria (Neal, galls or egg masses; 1 = 1 to 2 galls or egg masses; 1889) Chitwood, 1949 (Cervantes-Flores et al., 2002; 2 = 3 to 10 galls or egg masses, 3 = 11 to 30 galls Olabiyi, 2007; Marchese et al., 2010). Nevertheless, or egg masses; 4 = 31 to 100 galls or egg masses; very little is known about the ability of M. enterolobii and 5 = ≥ 100 galls or egg masses per root system to reproduce on sweetpotatoes in the USA. The (Taylor and Sasser, 1978). Eggs were extracted from objective of this study was to determine the ability of roots as stated above, except that 1% NaOCl was M. enterolobii to reproduce on selected sweetpotato used. Final number of eggs (Pf) for each plant was cultivars including Beauregard, Covington, Evan- calculated and eggs per gram of fresh roots (gfr) and geline, Hernandez, and Orleans (LA 05-111). All the reproductive factor (RF = Pf/Pi) were determined. 2 JOURNAL OF NEMATOLOGY Host suitability was designated as follows: RF ≥ 1 = Hernandez followed by Covington, Orleans and good host; 0.1 < RF < 1.0 = poor host; RF ≤ 0.1 = non- Beauregard. Nonetheless, the cultivar Orleans sus- host (Sasser et al., 1984). Fecundity was measured tained less root galling and egg masses than the by extracting eggs (1% NaOCl) from four egg masses other cultivars (p ≤ 0.001) (Table 1). Eggs per gram chosen randomly from each plant, totaling 80 egg of root were greatest for Evangeline and Hernandez mass/cultivar. and lowest for Beauregard and Orleans, and total Statistical analysis was performed on both egg production per plant was greatest for Evangeline tests combined, as no significant interaction was and lowest for Beauregard and Orleans (Table 1). The observed between cultivars and experiments (minor number of eggs per egg mass ranged from 462 to differences in gall index and egg mass index only) 504 and was similar among all cultivars. However, (Table 1). M. enterolobii infected and reproduced on Orleans no egg masses were found on half of the well on all the sweetpotato cultivars, including those plants (10 out of 20 plants in both tests). Root growth of known to have resistance to M.