Management

Detection

To detect the shrubs and haustoria by naked eye is easy due to specific morphology of the plant when it is approachable. Pernar et al. (2007) developed an efficient method of mistletoe detection by remote sensing. They achieved overall accuracy in the range of 62.50 to 70.56% using a combination of unsupervised and supervised classification of imagery classification data acquired from three visible bands combined with one near infra-red band. The use of high-resolution satellite or aerial imagery in detecting mistletoe in forests under European conditions has so far been tested only in France and Croatia (Iszkuło et al. 2020). In recent years, there have been research results on unmanned aerial platforms in remote sensing, such as studies on mistletoe attacking eucalyptus trees in Australia (Maes et al. 2018), or comparisons of data acquired with their help to satellite imagery (Dash et al. 2018). According to Ančić et al. (2014), detection of trees infection by the European mistletoe is possible and requires further research, it is also cost-effective and should be used on a wider scale especially in management and planning in forest ecosystems.

Preventive measures and suppression measures

To prevent mistletoe infestation in urban areas it is possible to plant resistant or tolerant cultivars of trees. Investigation of less susceptible individuals revealed anatomic and biochemical markers of higher levels of resistance. For example, thick cortex, high density of polyphenol-containing cells, thick first layer of fibers, thick collenchyma or prompt flavonoid synthesis are favoring the host tree (El Hariri et al. 1991).

Zuber (2004) concluded that there is no generally efficient control method of mistletoe apart of mechanical removal of the shrubs, invaded branches or whole trees. However, in smaller areas like urban parks he founded this method appropriate. 

In North America, there have been attempts to chemically control mistletoe (Livingston et al. 1985, Baker et al. 1989, Geils et al. 2002, Zuber 2004; Hoyt et al. 2017). Some systemic herbicides proved to be able to kill V. album with only a little damage of the host. Phenoxybutyric (2,4-MCPB), 2,4-dichlorophenoxyacetic acid (2,4-D), trichlorophenoxyacetic acid (2,4,5-T), di-chloro ethane and glyphosates were tested, but showed maximum effect only 4-6 months after treatment, furthermore, the endophytic system of the mistletoe was barely affected (Adams et al. 1993, Geils et al. 2002, Zuber 2004). Interesting results were received with application of the plant growth regulators such as ethephon ((2-chloroethyl)-phosphonic acid). Experiments of Adams et al. (1993) did not turn out promising. Ethephon caused fast abscission of mistletoe leaves and desintegration of mistletoe shrubs, but did not affect the endophytic system. Therefore, mistletoe regenerated during the following season. On the other hand, Rozsypálek et al. (2022) achieved considerable improvement of ethephon efficiency delimiting the conditions of its application. 60 – 70% of haustoria were killed after ethephon spraying during deep dormancy of the host trees, i.e. in a period starting 15 days after total leave fall and finishing at least 60 days before budding. Furthermore, ethephon must be sprayed on dry leaves, from -5 to +10 °C. Application is significantly less effective on heavily infected trees where the mistletoe predominates the foliage of the tree (Rozsypálek et al. 2022). The use of chemicals to control mistletoe is often questioned (Adams et al. 1993). Thus the key is to minimize the negative effects of the herbicide on trees and other environmental elements (Wood and Reilly 2004). Experiments with biocontrol of mistletoe did not bring considerable outcomes yet, because the range of pathogens and pests of mistletoe is not very wide due to its effective defense system. Kotan et al. (2013) tested 48 isolates of fungi and 193 bacterial strains isolated from declining mistletoe shrubs to reveal their potential as biological control agents. He pointed out fungal isolates of Alternaria alternata and Acremonium kiliense which caused dead of the artificially infected shrubs. Recently, a new fungus (Septoria krystynae Ruszk.-Mich.) was also discovered and described, causing a disease known as septoria in mistletoe (Pawłowicz et al. 2024).

Despite the existence of pathogens and insect pests that feed on mistletoe, however, none of them currently significantly affect the reduction of V. album, either in Europe (Zuber 2004) or in North America (Geils et al. 2002), and the only real threat to mistletoe is humans (Reid et al. 1995). The most effective and widely used way to reduce mistletoe on trees is to eradicate infested parts or entire specimens (Hawksworth 1983). However, in forestry, removal of individual branches is very difficult to carry out. Moreover, mistletoe has the ability to form dehiscent shoots from haustoria, i.e. roots that are hidden in the host tissue, making it difficult to combat the semiparasite (Zuber 2004). Removal of entire trees seems to be an effective solution. However, in the case of mistletoe, the feasibility of this type of treatment should be assessed each time, since usually more than one tree becomes infected by the semiparasite (Sanguesa-Barreda et al. 2012; Durand-Gillmann et al. 2014; Pilichowski et al. 2018), which can affect the difficulty of performing the treatment. In addition, decrease in tree cover improves conditions for the spread of mistletoe (Geils et al. 2002; Zuber 2004; Hoyt et al. 2017). The issue of effectively reducing the occurrence of mistletoe in forests is therefore still unresolved.

Respond & Control

To control the amount of mistletoe on trees, observe the rate and extent of the plant spread, different inventory methods and approaches, based on the use of traditional techniques, including crown observation and estimating the intensity of the occurrence of the pathogen are used in forest ecosystems. One of the more frequently worldwide used is the six-grade mistletoe rating scale (6-class dwarf mistletoe rating system (DMR)), proposed by Hawksworth (1977). In the method an observer visually dividing the tree's crown into three parts and separately assessing each of them for the degree of mistletoe infestation. This method allows to calculate the intensity of stand infection by Viscum spp. on the basis of the arithmetic mean DMR for the tested trees (Parker and Mathiasen 2004). An alternative to the 6-point Hawksworth scale is Broom volume rating (BVR) method, that in Tinnin (1998) opinion is simpler and faster to use compared to the previous one. Similarly to the Hawksworth method, the tree crown is divided into three parts (upper, middle and lower), where branches without infection and infected branches are counted in each of them. Mistletoe and co-occurring black broom are taken into account.

A four-class system proposed by (Barbu 2009), after assigning inventoried trees, on the basis of the intensity of mistletoe infestation and parts of the tree, to a particular class, includes measurements of the breast height, the height of the trees and their annual growth.

The degree of trees infestation by Viscum spp., using the total broom volume rating (TBV) method was proposed by Parker (2001). Unlike previous methods, it assumes the evaluation of the degree of mistletoe infestation not only the crown of the tree, but also its entire heigh (Parker 2001; Parker and Mathiasen 2004). Live crown volume percent-age (PCB) is an estimation method for assessing live branch infested by mistletoe, in which grades determine the percentage of mistletoe in a tree's crown with an accuracy of 10% (Parker and Mathiasen 2004). Because the TVB and PCB methods rely on percentage estimates of mistletoe infestation on trees, posing a risk of significant over- or underestimation of infestation, there are some limitations in practical application, as evidenced by the lack of many examples of the use of these methods.

The needle weighting method, aimed to determine the biomass of mistletoe and needles from mistletoe-infected branches for comparison with the biomass of branches uninfected (Bilgili et al. 2014) can be used to model the effects of mistletoe on the host, which can contribute to its control.