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Mistletoes are a taxonomically diverse group of aerial stem hemiparasites belonging to the Loranthaceae, Viscaceae, Santalaceae, Amphorogynaceae, and Misodendraceae families (order Santalales). They attach to the susceptible host trees by a specific anatomical bridge called a haustorium (endophyte) and are largely dependent on the host for water and nutrient supply, but are still able to photosynthesize via the green aerial portion called exophyte1,2. Although mistletoes may be considered hotspots of diversity providing food and shelter for various organisms, some mistletoe species are indeed biological pirates, affecting host health, fitness, and lifespan by the reducing their annual apical and lateral growth, flowering and fruiting as well as facilitating attacks of biological invaders such as fungi and bacteria3,4,5. At the same time, mistletoes are intensely used in both traditional and folk medicine, are components of compost, mulch, forage, and act as seasonal ecotourism attractions6,7,8.
Some of the notorious parasitic mistletoes are the dwarf mistletoes (Arceuthobium spp.; Santalaceae) reducing the growth of coniferous trees in North America9 and Mexico10; Tapinanthus (Blume) Rchb. mistletoes (Loranthaceae) attacking cacao (Theobroma cacao L.) and Citrus L. plantations in Africa11,12; hairy mistletoes (Taxillus tomentosus (Heyne ex Roth) van Tiegh.; Loranthaceae) affecting the Indian Euphorbiaceae trees, Phyllanthus emblica L. and Phyllanthus indofischeri Bennet, both known locally as Amla13; and the European mistletoe (Viscum album subsp. album L.; Viscaceae) having a broad host spectrum (more than 450 tree species)14,15 and rapidly spreading in Europe with its range shifted northwards16.
Walnut (Juglans regia L.) is an important horticultural cash crop in Kashmir valley constituting approximately 85% of India's annual walnut production17. Besides other biotic stresses such as leaf blotch, anthracnose, crown galls, stem cankers, walnut blight, or lace bug (Paracopium cingalense (Walker, 1873); Heteroptera: Tingidae) that attack the walnuts in Kashmir Valley, the V. album, locally called banada, kaw-khoor or nalkachul, also heavily infests these trees and can also rarely parasitize poplars (Populus L.; poplars in Kashmir valley are secondary hosts)18. Such a localized infestation might be explained by the possible concomitant introduction of V. album at the early developmental stages of 2-3 years, when its exophytes are almost invisible, with the planting material of walnuts from some plant nurseries. In turn, local poplar trees, the widespread susceptible primary host of this mistletoe in Europe, might be attended by birds vectoring sticky viscin-covered mistletoe fruits to the neighboring J. regia orchards.
Besides walnuts, leafy mistletoes parasitize other nut-producing trees, for example, almonds (Prunus dulcis (Mill.) D.A. Webb) are attacked by V. album in the Mediterranean Europe and in Crimea (Ukraine) as well as by the representatives of Phoradendron Nutt. genus in North America14,15,19; common hazel (Corylus avellane L.) by V. album in Europe14,15; pecan (Carya illinoinensis (Wangenh.) K.Koch) by oak mistletoe (Phoradendron flavescens (Pursh) Nutt.) in the southeastern United States20,21, and V. album in Europe14; macadamia (Macadamia F. Muell.) by shiny-leaved mistletoe (Benthamina alyxifolia (F.Muell. ex Benth.) Tiegh.), concomitant coast mistletoe (Mullerina celastroides (Sieb. ex. Schult & Schult. f.) Tiegh.), erect mistletoe (Amyema congener (Sieber ex Schult. & Schult. f.) Tiegh.), smooth mistletoe (Dendrophthoe glabrescens (Blakeley) Barlow), and long-flowered mistletoe (Dendrophthoe vitelline (Sieb. ex. Schult & Schult.f.) Tiegh.) in Australia22 (Watson, 2018); kolanut (Cola nitida (Vent.) Schott & Endl.) by Phragmanthera incana (Schumach.) Balle and Tapinanthus bangwensis (Engl. & K.Krause) Danser in Nigeria23; and cashew (Anacardium occidentale L.) by Psittacanthus plagiophyllus Eichler in Amazonian savanna24.
Currently, the strategy of mistletoe control develops in frames of the integrated pest management paradigm combining mechanical, chemical, and biological means to reduce infestations4,7,25,26,27,28. An environmentally friendly mistletoe biocontrol method involves the use of the natural enemies of mistletoe such as bacteria, necrotrophic fungi, herbivores, or even epiparasitic mistletoes3,4,7,29,30 but have limited applications due to the non-specificity and small-scale of action of the biotic agents. Although cutting and pruning are recognized as the most traditional and efficient means of mistletoe control26,30,31,32, exophytes frequently resprout. For spraying of mistletoe exophytes with chemicals, mistletoe stubs or host stems are injected with systemic herbicides such as glyphosate, 2,4-dichlorophenoxyacetic acid (2,4-D), 4-(2,4-dichlorophenoxy) butyric acid (2,4-DB), or 2,4,5-trichlorophenoxyacetic acid (2,4,5-T)3,33 or ethylene-releasing defoliants and ripening enhancers such as ethephon or (2-chloroethyl)-phosphonic acid)34,35. The combination of exophyte pruning to 2-3 cm stubs along with ethephon application to cut end of mistletoe stubs with a fine brush resulted in reduced V. album resprouting. This inspired the development of Mistletoe Eradicator, which can perform mechanical and chemical control of mistletoe growth simultaneously.
Here, we introduce an easy-to-handle and broadly available tool called Mistletoe Eradicator, which combines the benefits of mechanical and chemical means of mistletoe control and prevents its resprouting. The tool allows the certified arborists, plant pathologists, foresters, horticulturists, silviculturists, plant ecologists, experienced gardeners and other instructed users to deal with mistletoe challenge from the ground, from aerial platforms or from the trees. The tool performs a targeted pruning of mistletoe exophyte with the simultaneous dropping of a small amount of growth regulator directly on the pruned mistletoe stem. Mistletoe Eradicator was patented in India36 and used for the successful control of an increasing V. album subsp. album infestation on walnuts in Kashmir Valley. An updated version of this tool was designed in Ukraine and introduced to local arborists to practice in field tests with just a food color additive to track the herbicide dropping. It is noteworthy that Mistletoe Eradicator can be easily assembled and used for the control of other host-aggressive mistletoe species at the sites of their infestation in accordance with the national environmental law and after the approval of the local human communities.