Introduction

The urban trees of Mexico City offer a great amount of environmental, social, scientific and economic benefits that improve the quality of life of its inhabitants and other living beings (^{GDF, 2000}; ^{SMA, 2013}). However, the inadequate selection of species, lack of planning and maintenance along with other anthropogenic activities have generated numerous problems (^{Chacalo et al., 1994}) that favor the incidence of pests and diseases (^{Velasco et al., 2002}; ^{Alvarado et al., 2009 3}). One of the principal causes of impact in the urban trees of Mexico City is the infestation by mistletoe of the genera Arceuthobium, Cladocolea, Phoradendron, Psittacanthus and Strathanthus (^{Pérez et al., 2006}; ^{Marchal, 2009}; ^{Arriola et al., 2012}). The highest incidence of Struthanthus interruptus occurs in Populus deltoides and P. tremuloides (^{Arriola et al., 2012}), which represent the most abundant urban tree species (^{Cibrián et al., 2010 4}), thus a considerable number of trees is affected by the infection of this true mistletoe.

Some mistletoes contain functional chlorophyll, but depend on their hosts to satisfy their needs through the use of haustoria (^{Tainter and Baker, 1996}; ^{Press and Phoenix, 2005}), and cause witches’ broom, descending death, reduction of growth, survival and reproductive capacity, higher susceptibility to other diseases and in some cases, the death of the host (^{Hawksworth, 1983}; ^{Knutson, 1983}; ^{Manion, 1991}; ^{Tainter and Baker, 1996}; ^{Geils and Vázquez, 2002}).

The management of these parasitic plants is focused on pruning branches and felling trees, methods whose high cost is justified in recreational and urban areas and in trees of high value (^{Adams et al., 1993}; ^{Vázquez et al., 2006}). There are alternatives of control of which there is more knowledge and may provide additional methods for reducing their impact (^{Kuijt, 1969}; ^{Hawksworth, 1983}; ^{Geils and Vázquez, 2002}; ^{Vázquez et al., 2006}; ^{Mathiasen et al., 2008}).

Biological control with pathogenic fungi is an alternative studied and developed in dwarf mistletoe, achieving death, the interruption of its life cycle and the reduction of its propagation, intensification and damage (^{Shamoun and DeWald, 2002}; ^{Shamoun et al., 2003}; ^{Mathiasen et al., 2008}). However, pruning and biological control are insufficient against the growing impacts and dispersal of mistletoe (^{Wood and Reilly, 2004}); therefore, the most promising option in the short term could be chemical control (^{Michailides et al., 1987}; ^{Minko and Fagg, 1989}). For this control the selection of an adequate product is difficult, an herbicide needs to be found which is easy to apply and causes the death of the mistletoe without provoking phytotoxicity to the host; if this is not achieved, then hopefully the abscission of the shoots, flowers and fruits would be obtained in order to reduce and retard the dispersal and intensification of the infection (^{Shamoun and DeWald, 2002}).

Chemical control in mistletoe includes the application of herbicides by direct aspersion to the plant (^{Vázquez et al., 1986}; ^{Adams et al., 1993}; ^{Vázquez, 1994}; ^{Hoffman, 2004}; ^{Wood and Reilly, 2004}), through aspersion to the stumps as complementary treatment to pruning (^{Michailides et al., 1987}; ^{Lichter et al., 1991}) and by injection to the trunk of the host (^{Minko and Fagg, 1989}; ^{Cibrián et al., 2010[2]}). But in urban areas the best option is injection because it permits the efficient use of the product and eliminates environmental contamination (^{Sánchez and Fernández, 2000}).

The application of herbicides is influenced by the physiological stage of the mistletoe and the period of application. According to ^{Berry et al. (1992}) and ^{Mallams and Mathiasen (2010}), effectiveness is higher in winter because the host is in repose and the physiologically active mistletoe absorbs the chemical product and reduces damage to the tissues of the tree, but in spring effectiveness is not insured. In some cases, the application prior to (^{Perry and Elmore, 2006}) or at the start of the emission of young leaves of the mistletoe interferes with the normal growth processes (^{Vázquez, 1994}), although in other cases there is no effect (^{Berry et al., 1992}). Furthermore, it is recommended that the application be made prior to the seed dispersal of the mistletoe (^{Adams et al., 1993}).

The frequency of application of treatments is a criterion that depends on the level of infection of the mistletoe tolerable for the tree and for the arborist (^{Adams et al., 1993}). In Arceuthobium, Phoradendron, Psittacanthus and Cladocolea, a single application with an adequate dose achieves favorable results which include the chlorosis of leaves, necrosis, partial or total defoliation, abscission of shoots, and in some cases, death (^{Vázquez, 1986}; ^{Michailides et al., 1987}; ^{Vázquez, 1994}; ^{Wood and Reilly, 2004}; ^{Cibrián et al., 2010[2]}).

Ethephon (2-chloroethyl phosphonic acid) is a growth regulator that releases ethylene and improves the process of maturation, which favors the abscission of the mistletoe shoot (^{Adams et al., 1993}; ^{Shamoun and DeWald, 2002}; ^{Hoffman, 2004}). The relative success of this growth regulator is reported in mistletoes of the genera Arceuthobium and Phoradendron (Adams et al., 1993; ^{Shamoun and DeWald, 2002}) and Cladocolea (^{Cibrián et al., 2010[2]}); however, the results can not be extrapolated to other species or genera of mistletoe or host without conducting more tests (^{Minko and Fagg, 1989}; ^{Cibrián et al., 2010[2]}). The response of ethephon applied by aspersion occurs more rapidly than by injection to the trunk, which manifests the first symptoms of yellowing and leaf fall starting the first week and loses effect around the seventh week after its application (^{Wood and Reilly, 2004}; ^{Cibrián et al, 2010[2]}.

Mistletoes of the genus Struthanthus are arbustive plants, hemiparasites that affect a reduced number of conifers and a wide range of other woody plants. They present long epicortical roots, which run the surface of the host tissue and form an intermittent haustorial connection. Its leaves are simple, opposing or alternate, well developed, coriaceous and glabrous. The branches are cylindrical or compressed. The flowers are frequently arranged in fascicles of three, organized in indeterminate inflorescences; small flowers in spikes, caliculate and rarely pediculate, 6 greenish or yellowish free tepals, males with 6 reduced stamen and pistil, the females with reduced androecium. The fruit is a berry or drupe with a seed encased in a viscous layer; the seed with succulent endosperm, lacking testa and rarely contains more than one embryo (^{Geils and Vázquez, 2002}; ^{Cibrián and Alvarado, 2007}; ^{Calderón, 2010}). In Mexico City, the flower formation of S. interruptus occurs in October, fruit formation from November to January, fruit maturation from February to June and seed dispersal from March to September (Cárdenas, 2014⁵).

Populus deltoides is a species used as an ornamental and shade tree in parks and streets. It is monoecious deciduous, from 25 to 30 m height, of rapid growth and short life. The leaves are deciduous, simple, widely ovoid-deltoid of 8 to 18 cm in length and almost equal in width, the margin is toothed, the apex is abruptly acuminated, with truncated base, heart shaped or abruptly cuneate, light green spathe, glabrous, with notorious principal nerves, from yellow to red. The flowers appear as catkins in spring; the males reddish or purple and the females are brown, with cottony hairs when the capsule breaks. The fruits appear in clusters, each one in the shape of a dehiscent capsule, and when they break, numerous cottony hairs and very small seeds appear. The cortex is thin and smooth, yellow in young trunks and gray to almost black in old individuals, deeply furrowed, with wide and flattened borders, which derive in scales (^{Martínez, 2008}).

Therefore, the objective of this research was to evaluate the application of three doses of the growth regulator ethephon injected in the trunk of trees of P. deltoides as a chemical control measure of the true mistletoe S. interruptus. The hypothesis was that the injection of ethephon causes the early fall of the mistletoe leaves.

Materials and Methods

Evaluation of foliage density of the mistletoe

To determine the effect of the treatments applied to the trunk of the host, evaluations were made of the foliage density of the true mistletoe, for which a crown density scale was used proposed by ^{Schomaker et al. (2007)}, and was adjusted for Struthanthus interruptus (Figure 1), which includes ten categories where the lowest percentage of foliage density of the true mistletoe is 5 % and the highest is 95 %. For this purpose, photographs of different true mistletoe plants were taken with a Nikon^® D5200, and an adequate photograph was selected to represent each category of density, which was later validated in the field. Measurements were taken with the crown density scale proposed by ^{Schomaker et al. (2007)} and that of the authors, registering the same percentages with both scales. The data was registered from April 17 of 2015, date of the first application, and concluded on September 30 of the same year, due to the start of remodeling activities of the garden which included the removal of pavement, structures and trees, all of the above with heavy machinery, which could generate problems of stress in the trees, affect their growth, health and structure (^{Coder, 2007}), and indirectly generate a bias in the evaluation. The evaluations were made every 4 d during the first six weeks, then one every 15 d, with a total of 20 evaluations, with which the graph of average defoliation dynamic of the true mistletoe S. interruptus was generated as effect of the treatments.

Figure 1 Evaluation scale of the foliage density of the true mistletoe Struthanthus interruptus. 

The percentage of defoliation of the true mistletoe S. interruptus (D_i ) was calculated with the following formula proposed by the authors:

Di=100×1-dfidf1, i = 1,2,3,…,20

where df_i is the foliage density of true mistletoe measured in the i-th evaluation.

Results and Discussion

The application of ethephon caused significant defoliation (p = 0.0008) of the true mistletoe S. interruptus developed on P. deltoides, in comparison with the absolute control. The defoliation of the mistletoe included the loss of leaves and the detachment of flowers and fruits. The high, medium and low dose of ethephon provoked 56.29, 31.14 and 31.00 % defoliation, respectively, with no statistical difference between the latter two (Table 1). One of the trees with low dose treatment of ethephon (2 mL) died at 99 d after the start of the experiment.

Independently of the treatment, the dynamic of defoliation was different in each case, which signified that the effect was observed in different time and intensity. The defoliation of the mistletoe from the effect of the first injection of ethephon applied in April appeared 4 d later, which represented a shorter time compared with the application made in August, which showed effect at 17 d (Figure 2). It is probable that the circulation of the chemical product in the second application was influenced by environmental conditions such as precipitation. According to ^{Doccola and Wild (2012)}, the effect of the injections is greater when the tree is transpiring, but this is reduced in rainy days due to the increment in relative humidity which provokes a greater accumulation of water in the soil (^{Sánchez and Fernández, 2000}).

Figure 2 Dynamic of the average defoliation of the true mistletoe Struthanthus interruptus as an effect from the treatments with sterile distilled water (control) and ethephon (Source: ^{CONAGUA, 2015}). 

During the first application of treatments with ethephon, the maximum values of defoliation appeared at 33 (high dose) and 56 (low and medium dose) days after application, while the maximum values in the second application were observed at the final evaluation, without observing when the maximum occurred and how long the effect lasted due to the short time of evaluation. Although the application was made during the rainy season, an effect was observed in defoliation. The control remained constant throughout the evaluation period.

After the first application of treatments with ethephon, a recuperation of the mistletoe foliage was observed from day 33 (high dose) and 56 (medium and low dose), which coincided with the increment in precipitation. The above may have been due to the relationship between the vigor of the host and that of the parasite plant (^{Knutson and Tinnin, 1980}; ^{Olsen, 2003}), given that some species have a positive response in their growth due to better conditions of the host, such as availability of water and carbon (^{Glatzel and Geils, 2008}).

In the final evaluation, at 165 d, a reduction in foliage density of the true mistletoe was observed which oscillated from 9.86 to 56.29 %. The average defoliation in the controls was no more than 10 %, whereas defoliation in the treatments with ethephon oscillated from 31 and 56 %. The effectiveness in the treatments could have been affected because the application was made after the growth of the parasite plant (April), given that the young leaves and developing buds are less affected by the ethephon (^{Berry et al., 1992}), thus it would be more effective to make the application before the emission of young leaves (^{Perry and Elmore, 2006}).

^{Berry et al. (1992)} and ^{Mallams and Mathiasen (2010)} reported that the application in winter is effective when the host is in repose and the mistletoe physiologically active; thus, it absorbs the chemical product and reduces damage to the tissues of the host by the product. The above suggests that the success of the chemical control is influenced by the period of application, as well as the physiological stage of the parasite plant and of the host.

The foliage density of the mistletoe varied in the time of evaluation (Figure 3). The treatment with sterile distilled water caused a gradual defoliation, almost undetectable, and was obvious from the second month after the application of the treatment. The low and medium dose of ethephon showed a very similar defoliation dynamic. The loss of foliage as effect of first application of treatments was observed during the first three months, followed by a slight increment of foliage, which decreased just after the second application of treatments. The high dose of ethephon provoked the greatest defoliation with respect to the other doses; the effect was observed during the first month after the application, but the defoliation decreased during the three posterior months and increased as an effect of the second application.

Figure 3 Treatment with sterile distilled water (A and B) and treatment with low (C and D), medium (E and F) and high (G and H) dose of ethephon evaluated days after the application of treatments (dat). 

The treatments with ethephon caused fissures in the cortex at the injection points in the host (Figure 4); although this effect is not reported as a symptom of phytotoxicity caused by this product, because according to ^{Cibrián et al. (2010)7} and Martínez (2015)⁸, the injections with ethephon caused defoliation and death of the mistletoe, and in some cases the death of the host, therefore the dose should be adjusted. During our experiment, the fissures were only observed in the points made in the first injection and were evident from the second month after the application; these measured 1 to 8 cm in diameter.

Figure 4 Fissures in the cortex of Populus deltoides as effect of the application of ethephon at: A) low dose, B) medium dose and C) high dose, with respect to the D) treatment with sterile distilled water (control). 

In Mexico, the chemical control of true mistletoe has been scarcely explored. The success of the injection of ethephon in the trunk of host trees was reported in Cladocolea diversifolia (^{Cibrián et al., 20107}), achieving the loss of leaves, fruits and twigs of the parasite plant. However, in Struthanthus the injection of this growth regulator did not have the same effect, thus these results are an antecedent for the chemical control of the true mistletoe S. interruptus.

The injection of three doses of ethephon was not lethal for the true mistletoe S. interruptus, caused the defoliation, seed reduction as well as the decrease in new infections. The mistletoes represent an attractive and nutritive food source for the dispersing birds (even when there is scarcity of other food sources), which consume the fruits and deposit the seeds in new trees, causing their infection (^{Kuijt, 1969}; ^{López de Buen and Ornelas, 2001}; ^{Coder, 2008}; ^{Mathiasen et al., 2008}).

Therefore, it is important to carry out more tests of this type in the urban trees of Mexico City, with the purpose of determining the most adequate dose, period of application and phenological stage of the mistletoe, which achieve the selective death of the parasite plant without damaging the host.

Conclusions

Chemical control with ethephon is an alternative that permits the reduction and delay of dispersal of the true mistletoe Struthanthus interruptus. The percentage of defoliation of the parasite plant due to the injection of the growth regulator in trees of Populus deltoides was higher with respect to those treated with sterile distilled water, and the high dose caused greater defoliation. The effects in foliage density of the true mistletoe were observed in shorter time in the first application of treatments, compared with the second; therefore, it is better to carry out the application prior to the emission of young leaves of the mistletoe and to the rainy season.