Phoradendron nervosum, colloquially referred to as “Matapalo,” “Muérdago,” or “Injerto” in South and Central America [1], is a hemiparasitic shrub that infects stems, a typical characteristic of the Phoradendron genus [2]. Despite the extensive native distribution of P. nervosum, which extends from Mexico to Bolivia, research about its natural history and ecology is limited. Particularly, studies related to host specificity have been performed mainly in Mexico and Costa Rica, with recent observations from Ecuador (Table 1) [1-12].

The objective of this study is to expand the list of reported host species of P. nervosum within urban environments in Quito, the capital of Ecuador. Existing literature and preliminary field observations demonstrate that P. nervosum is parasitizing a wide range of plants including native, introduced, and cultivated species within Quito [11,12]. Due to its wide distribution, we assume that P. nervosum infects native, introduced, and cultivated species in similar frequencies. This information holds the potential to enhance our understanding of the ecological dynamics and natural history of this widely distributed species within urban settings across the Americas.

The city of Quito is located in the central-northern part of the Inter Andean region of Ecuador, at an average of 2850 m.a.s.l. Its valleys sit on the hillside of numerous volcanos, creating a variety of climatic floors and plant formations, including piedmont rainforest, high Andean scrub forest, dry forest, and others [13]. This study was performed across urban and densely populated areas in north and southern Quito, including the adjacent valleys of Los Chillos, Tumbaco, and Pomasqui, which are now part of the urban area of Quito [14]. The sampling limit to the north was San Antonio de Pichincha, and to the south was Chillogallo neighborhood. The eastern limits were Tumbaco and Los Chillos valleys, and the western limits were the Guagua Pichincha and Atacazo volcanoes. For the most part, these urbanized localities harbor small green areas that consist of gardens, parks, and abandoned land, which have some native vegetation, but most of the coverage consists of ornamental species and invasive vegetation.

Sampling was performed by three researchers between May and December of 2019, traveling by car in equidistant transects along and around eight principal roads inside our study area. We sampled a total distance of 120 km. The sampled roads were Antonio José de Sucre Avenue, Simon Bolivar Highway, Rumiñahui Highway, Manuel Cordova Galarza Avenue, Antonio José de Sucre Avenue, Ruta Viva Highway, 6 de Diciembre Avenue, and 10 de Agosto Avenue.

P. nervosum is easy to recognize due to its oblong or lanceolate leaves that contrast with those of the host tree, often presenting a conspicuous yellowish color in the stems. P. parietarioides also occurs in our study area and is similar in morphology when compared to P. nervosum but easily distinguished by a greyish coloration [11,12]. Each time we recognized an infested tree, we approached it to identify the host species and took a photograph. We registered the frequency of infected individuals of each species, and to avoid pseudo replication we marked each host individual. Additionally, removing parasitic plants may disrupt the co-evolutionary dynamics between the parasites and their host species, also removing the transmission of adaptive information for successful infection in their seeds [15]. In this context, no collections were made during this study; all records and species identification were photographic [16]. For some species, photographs were not possible to take because the infected trees were growing on private land and access was not granted.

We identified each host species using samples from the “Herbario de Botánica Económica de la Universidad San Francisco de Quito (QUSF)” and classified them into four categories: native, introduced, introduced and cultivated, and native and cultivated. These categories are based on the growing status of this species in Ecuador, which is compiled in the Catalog of the Vascular Plants of Ecuador [17]. To test if one of these categories was more prone to infection by P. nervosum, we performed a chi-square goodness-of-fit test including the standardized residuals as post-hoc analysis using R statistical analysis software version 4.1.1 [18]. The relative frequency of each species was calculated.

FIGURE 1-a
Photos of P. nervosum host species. A. Phoradendron nervosum. B. Acacia baileyana. C. Acacia macracantha. D. Acacia mearnsii. E. Annona cherimola. F. Callistemon subulatus. G. Croton wagneri. H. Ficus carica. I. Hibiscus rosa-sinensis. J. Nerium oleander. K. Nicotiana glauca. L. Prunus persica.
Photos by Martín Carrera, Lía Altamirano, and Karla Barragán.

P. nervosum appears to be a generalist hemiparasite plant, infecting 21 species from 14 families from the Eudicotyledonae class. The family with more frequently infected species is Fabaceae (4 species), followed by Malvaceae (3 species) and Solanaceae and Rosaceae (each with 2 species). In families like Apocynaceae, Asteraceae, Bignoniaceae, Lythraceae, Myrtaceae, Salicaceae, and Verbenaceae, we only found one infected species (Fig 1 a-b). From the 140 infected individuals reported, the four species with highest relative frequency are Acacia mearnsii with 22.30 %, followed by Callistemon subulatus with 14.29 %, Salix humboldtiana with 13.57 %, and Nicotiana glauca with 10.00 %. We registered 3 infected species with only one individual (Tecoma stans, Acacia baileyana, and Annona cherimola, each with 0.71 %) (Table 2).

FIGURE 1-b
Photos of P. nervosum host species. A. Prunus serotina. B. Punica granatum. C. Salix humboldtiana. D. Solanum brevifolium.
Photos by Martín Carrera, Lía Altamirano, and Karla Barragán.

Table 2

Host species of Phoradendron nervosum Status is derived from Tropicos introduced and cultivated native native and cultivated and introduced Common names are from Ecuador Common use from Tropicos and geographic reference is given from one host species locality for each species Bolded relative frequencies show a species with a high relative frequency of infection

The chi-square goodness-of-fit test suggests that not all the categories have the same probability of infection (X2 = 9.2857, df = 3, . = <0.05). The category with higher standardized residuals is introduced and cultivated (2.0730), suggesting that this category is more prone to infection by P. nervosum. From the four categories in Figure 2, 53.06% of the identified infected individuals correspond to the introduced and cultivated category (10 species), 22.15% to native species (7 species), 14.99% to native and cultivated species (3 species), and 10% to introduced species (1 species) (Fig. 2).

Figure 2
Relative frequency of the individuals infected by Phoradendron nervosum represented by their respective status according to Tropicos Introduced and Cultivated Native Native and Cultivated and Introduced

Our results increase the host species list of P. nervosum in Ecuador from 11 species reported in the literature to 27 species, contributing to the knowledge of the natural history and ecology of this species [11-12]. Despite being a relatively abundant species in Quito, studies of P. nervosum in Ecuador are scarce. Further sampling in areas within its natural distribution could give us a better understanding on the abundance and preferences of parasitizing of these species.

This research suggests that most of the infected individuals are introduced and cultivated species, mainly ornamental plants from gardens. The most logical explanation is that introduced and cultivated individuals are more prone to infection just because they are more abundant in urban areas such as Quito. Complementary to this explanation, ornamental plants often receive abundant water and nutrient provisioning, from which parasitic plants could also benefit and spread onto surrounding individuals. Another possible explanation is that introduced species have not co-evolved with P. nervosus, and therefore, they lack defense mechanisms against this parasitic species’ infection strategies [15]. These hypotheses open new doors for research on the ecology and infection mechanism of P. nervosum in urban areas.

Personal observations suggest that P. nervosum prefers dryer areas (e.g., dry forest) within Quito, especially around the north of Quito and Tumbaco Valley [19]. In the south of Quito and Los Chillos Valley, which are more humid areas, individuals were harder to find. These observations create new research questions about P. nervosum environmental preferences and biogeography.

Acknowledgments

We would like to thank Vlastimil Zak and Pablo Riera (QUSF) for useful comments about the host species of P. nervosum and for allowing us to check QUSF collections. We are grateful to the several landowners who allowed us to take photos of P. nervosum on their land. This research was funded by the authors exclusively.

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Author notes