First Report of Meloidogyne morocciensis Infecting Hops (Humulus lupulus)

Hop (Humulus lupulus L.) is a climbing herbaceous plant (Fig. 1A) belonging to the Cannabaceae family, native to temperate regions. Flowers of this plant are mainly used in the brewing industry for conferring aroma and flavor to beer, but since it has secondary metabolites with significant biological activities (Astray et al., 2020), the plant is also applied in medicine and can be used in the treatment of diseases, such as insomnia, stress, and anxiety (Kyrou et al., 2017). Although Brazil is a country with a tropical climate, H. lupulus has adapted in regions with a mild climate and shows good plant development (Durello et al., 2019).

Figure 1

Seedlings of H. lupulus of the Cascade variety obtained through vegetative propagation of branches of a healthy hop plant were planted in the municipality of São Mateus, in the northern region of the state of Espírito Santo (18° 40′ 25″ S, 40° 51′ 23″ W; altitude of 77 m) in July 2019. Eight months after transplanting, the plants showed symptoms of leaf yellowing, reduced development evolving to leaf wilting, and senescence of the aerial part. After observation of the presence of root galls (Fig. 1B,D,E), soil and root samples were collected from the rhizosphere of five hop plants. Four soil subsamples were collected from each plant and homogenized in plastic bags to form five composite soil samples (n = 5). To sample the roots, five plants were collected from the soil, taking care to keep the root system intact, and placed in a plastic bag. The soil and root samples were taken to the Nematology Laboratory of the Universidade Estadual do Norte Fluminense Darcy Ribeiro, the Nematology Laboratory of the Universidade Federal de Lavras, and the Phytopathological Analysis Laboratory of the Universidade Federal do Espírito Santo for nematological analyses.

Nematodes were extracted from soil using the method of Jenkins (1964) and from roots using the method of Coleen and D’Herde (1972). For the identification of Meloidogyne species, the morphology of female perineal patterns was used (n = 10), morphometry of juveniles (n = 20), and by esterase phenotypes obtained by electrophoresis gel (n = 8). Eggs and juveniles were counted on Peter’s slides to quantify the population found in the samples.

According to the results of the analyses, it was possible to identify the species as Meloidogyne mococciensis Rammah and Hirschmann, 1990 and as a parasite of the hop plants. The estimated population of eggs and juveniles was 864/100 cm³ in soil and 2,763/g root. The perineal patterns found exhibit oval-to-square shapes, with moderately separated dorsal arches and continuous and separate coarse striations, similar to that found by Machaca-Calsin et al. (2021) (Fig. 1G).

The measurements of second-stage (J2) juveniles (n = 20) (Fig. 1C) were as follows: L = 408.84 ± 3.46 (380.98–434.31) μm; stylet length: 13.61 ± 0.17 (12.48–15.08) μm; dorsal gland opening: 3.67 ± 0.08 (2.99–4.26) μm; tail length: 48.62 ± 0.39 (45.91–51.03) μm; hyaline tail length: 15.96 ± 0.25 (14.73–17.85) μm; a: 25.98 ± 0.42 (23.2–27.85) μm; and c: 7.14 ± 0.18 (6.99–7.91) μm. No males were found for the morphological and morphometric analyses. According to Rammah and Hirschmann (1990), the measurements obtained from the second-stage (J2) juveniles correspond to those of the species M. morocciensis. The esterase isoenzyme phenotype was characterized as A3 by PAGE (Fig. 1F) in the esterase identification method (Carneiro et al., 2008).

Meloidogyne morocciensis is a species less frequently found worldwide when compared to traditional species such as M. javanica and M. incognita. In Brazil, this nematode has been found in beetroots (Machaca-Calsin et al., 2021), peach trees (Silva et al., 2020), tomato plants (Barros et al., 2018), and soybean (Dalla Nora et al., 2020). Based on the results of the species identification analyses, this is the first report of M. morocciensis on hop roots. We observed that infected hop plants showed developmental damage, such as reduced growth, chlorosis on leaves, and nodules on roots. The disease could reduce hop production and affect other crops reported to be hosts of economic importance in Brazil, such as soybeans and tomatoes. Thus, we recommend hop growers in Brazil, especially in the northern region of Espírito Santo, to take care and avoid the spread of this nematode to other agricultural crops and to other regions of the country.