Parasitic Weeds of Jordan: Species, Hosts, Distribution and Management

Parasitic weeds or parasitic flowering plants are destructive species that negatively impact both agro- and ecosystems. They attack cultivated and wild grown annual, biennial and perennial plant species of different growth habits and habitats. They parasitize most vegetables, forage plants, shrubs, and fruit and forest trees. They reflect a high physiological/chemical and environmental tolerance and/or these parasites adapt to separate plant cells, dissolve tissues and food materials, and facilitate penetration secure food and water for their own use. Parasitic weeds are unusual and peculiar species in growth habits and habitats, hence not in direct contact (except certain hemiparasites) with soil and soil factors necessary for growth, development, and existence and for almost the entire life cycle. Instead, they usually attach themselves to other plant species of different genera (except self-parasitism) and families through an absorptive organ called haustorium (pl. haustoria) develops upon attachment to host tissues. Essentially, all haustoria are modified roots (Nickrent, 2002), sucker-like structures, and varied in size, shape, and morphology for different parasitic species. The parasite can derive its food and /or water from the host plant. Parasitic weeds are of a widespread significance in different parts of the world and serious concern to farmers and researchers. Certain species are a real threat to food security since they parasitize many important food crops. However, parasitic weeds are different in requirements for germination, attachment to host plants and in the host parts they attack, degree of dependence upon hosts, and presence or absence of chlorophyll. Most recent management methods are presented including (i) biological using natural enemies such as fungi, bacteria and insects including Trichoderma, Phytomyza for Orobanche; Smicronyx sp. and fungal species for Cuscuta and Orobanche; (ii) Chemical by the use of herbicides such as glyphosate for Orobanche control in fababean, different sulfonylurea herbicides for other species (e.g. O. aegyptiaca and O. ramosa in tomato) and selective application of MCPA and 2,4-D for mistletoes control; (iii) physical including soil solarization and plastic mulch for Orobanche and Cuscuta species; (iv) organic methods using plant residue and natural products or allelopathy (e.g. for Orobanche), root extracts (sunflower), plant oils (ginger, groundnuts, palm, sunflower, safflower, castorbean, linseed, neem, coconut or tobacco seed oils, niger, and mustard), orobanchol and alectrol germination stimulants, different strains of Streptomyces, certain fungal metabolites including cotylenins and fusicoccins; (v) trap and catch species in crop rotation or intercropping systems ; (vi) genetically engineered crops (herbicide resistant); (vii) plastic mulch with fertilizers (chicken manure, urea, ammonium sulphate); (viii) resistant cultivars for different crop species and screening studies on tolerant/resistant crop lines; (ix) mowing and general contact herbicides such as paraquat and dinoseb for Osyris and Cuscuta (at early infestation in perennial field crops) species control; (x) pruning and shading for mistletoes control; and (xi) integrated control methods. This book reviews the parasitic genera that occur in Jordan and are of great importance to many Middle East countries. It includes recent research achievements in combating these parasitic weeds and presents a brief description of species phonology, ecology, and economic importance as parasites of cultivated land, forestry, and wild species in the country. Recent literature on species biology, physiology, and ecology and developments on their control and management were reviewed for each parasitic genus separately as deemed necessary. 

Parasitism is a phenomenon of widespread importance among angiosperms and there are at least five orders having parasitic species (Musselman, 1982) that evolved independently in 20 plant families (Musselman, 1987) and comprising 3000-5000 species (Musselman, 1987; Sauerborn, 2001). Almost 4200 species of haustorial parasitic plants are spread in the world, representing just over 1% of all flowering plants. Nickrent (2002) reported that the total number of parasite species is not distributed evenly among the 18 families or 274 genera. Aly (2007) reported that about 20 families (3,000–5,000 species) of higher plants are parasitic in the plant kingdom and may cause production losses of 30–80% in staple food and industrial crops in every continent. Westwood et al. (2010) mentioned parasitic weeds include approximately 3,900 species included in more than 20 plant families consist the important genera Striga, Orobanche and Cuscuta. FernándezAparicio et al. (2011) reported at least 4500 plant species within 270 genera in over 22 families predominantly angiosperms rely on a parasitic association with a host plant for their mineral nutrition, water and/or carbon supply. Heide-Jorgensen (2013) mentioned that the total number of world parasitic plants account for 4500 species present in 275 genera of 28 botanical families. However, in a recent molecular study, Nickrent (2020) reported the occurrence of 4700 species that belong to 290 genera of parasitic angiosperms. Approximately 30 genera of these parasites have been reported to negatively impact cultivated crops, while only about 11% of all genera have members that could be considered pathogenic. Parasitic species belong to different plant families and are either root or shoot parasites, attack plants of different botanical families and growth forms and are distributed in different geographical regions of the world. They cause great yield losses and a heavy infestation can negate yield and lead to complete crop loss. Some parasites are regarded as highly damaging agricultural pests that exert significant threats to regional and global commodity and food production (Cudney et al.1992; Aly 2007; Parker 2012). However, out of plant families that have parasitic species, six were considered of great economic importance, including Scrophulariaceae, Orobanchaceae, Santalaceae, Cuscutaceae, Viscaceae and Loranthaceae. Other researchers also added Lauraceae and Balanophoraceae (Subhashini et al., 2019). In general, Cuscuta, Arceuthobium, Orobanche, and Striga were regarded as causing the most damage to economic host crops. 

Weeds represent a major threat to agriculture in Jordan and cause great yield losses due to their negative interference with crop plants (Qasem, 2003). Weeds affect crop plants through competition, allelopathy, some through parasitism and may be in some cases through all at once. Parasitic weeds are one group of a deep concern to local farmers and researchers and exert a real danger to agriculture hence resources from water and nutrients are extremely limited considering that 90% of the country’s area is a desert with drought and poor soil resources while the rest is devoted to forestry and agriculture. These parasites reported from the country (Table 1) subsist on the host species' root system or aerial vegetative parts. They can lead to severe growth damage hence obtain food, water, and nutrients from host plants, resulting in yield failure and death of the host plants under heavy infestation. Parasitic weeds may or may not have chlorophyll pigments and thus are partially or completely dependent on host plants for food and/or water (Parker and Riches, 1993). In Jordan, three families, including three genera, were reported to attack 34 wild and cultivated species (Abu-Iramileh, 1979). In more recent studies, Qasem (2009, 2011) reported eight parasitic genera of six plant families attacking many host species, among which 85 woody plant species from 57 genera belong to 33 plant families. The distribution of parasites throughout the country is shown in Fig. 3. The danger these parasites exert is mainly due to their difficulty to control with available weed control methods, except by soil fumigation mainly using methyl bromide, which has been banded a few years ago and its use is not economically feasible in the open field or for application at a wide scale. In addition, the huge number of seeds these parasites produce is accompanied by long seed viability and adaptability to disseminate by wind (Orobanche, Cistanche, and Cynomorium), animals or birds (Loranthus, Viscum and Osyris), or through crop infested seeds (Cuscuta, Orobanche and Thesium) add more complications to control methods. The interrelationship between the parasites and their hosts and how parasites connect to host plants through the absorptive organs (haustoria) is another means through which they challenge control measures. Morphological and physiological similarities between the parasitic species and their hosts and the ability of certain species to develop additional shoots from epicortical roots (Loranthus spp.) or embedded haustoria (Cuscuta spp.), especially after damage to or removal of the primary shoot (Viscum and Loranthus) hinder pruning and herbicides use as common methods of control. 

Dodders (Cuscuta spp.) are holo-parasites, usually annuals, or perennials under certain conditions. They are members of the morning glory family (Convolvulaceae) in old references that include Cuscuta, Convolvulus and Ipomoea and of dodder family (Cuscutaceae) in the more recent publications. Cuscuta is a yellow or orange plant, twinged with purple or red color sometimes but generally white. The stems can be very thin and thread like or relatively stout. However, all Cuscuta species are herbaceous vines with twining, slender, pale stems, with reduced, scale-like leaves, and have no roots. Plants have no or merely low amounts of chlorophyll, and usually do not have a photosynthetic activity (Hibberd et al., 1998; Garcia et al., 2014) or may be placed as intermediate between hemi- and holoparastic conditions (Nickrent and Muselman, 2004) hence some species show residual photosynthesis (Dawson et al., 1994; Hibberd at al., 1998) and have thus been designated as cryptically photosynthetic (Funk et al., 2007; McNeal et al., 2007a, b). All Cuscuta species are fully dependent on host plants to complete their life cycle, and therefore regarded as obligate holoparasites. Cuscuta spp. are the most problematic parasitic weeds in agriculture in the semi-humid and semiarid areas of Africa and Asia (Sauberborn, 1994). They are nearly cosmopolitan, occurring in a wide range of habitats and hosts with a high number of species in the Americas (Yuncker, 1932; Hunziker, 1950; Mabberley, 1997; Stefanović et al., 2007). Their wide geographical distribution and host range make them among the most damaging parasitic weeds worldover (Ashton and Santana, 1976; Holm et al., 1997). Damage can ultimately lead to total destruction and death of the host plants. The genus Cuscuta comprises some 200 species (Costea, 2007), usually grouped in three subgenera; Monogyna, Cuscuta and Grammica. Molecular studies on subgenera Grammica and Cuscuta, delimit major clades within these groups. Differences in seed morphology between these subgenera were reported. Seed exostructure features supported the subgeneric classification based on exosomorpholigical (vague term) characters (Kim et al., 2000). However, the sequences used were unalienable among subgenera, preventing the phylogenetic comparison across the genus (García et al., 2014). A new phylogenetic classification proposed by Costea et al. (2015) placed 194 accepted species of Cuscuta into four subgenera and 18 sections. While only 10–15 of these species are considered economically important pests (Parker and Riches, 1993; Costea, 2007), their broad geographical distribution and host ranges contribute to impacts on a wide range of agricultural cropping systems (Dawson et al. 1994; Costea and Tardif, 2006). Cuscuta spp. are not host-specific, although they show a wide variation in their host ranges. Severe infestations can stunt, smother and kill host plants, cause severe yield and stand reductions. The worldwide plant species that are commonly parasitized by certain parasitic genera, including Cuscuta spp. were thoroughly reviewed and documented (Qasem, 2006). Cuscuta spp. parasitize many wild and cultivated plant species. They are destructive to certain high-value crops and particularly troublesome where alfalfa, clover, and onion are grown for seeds (Ristau, 2001; Swift, 2001). Some species can parasitize a wide range of host of herbaceous and woody plants, while others may be more host restricted (Qasem, 2006). Cuscuta’s successful parasitism depends critically on efficient host-location mechanisms, as the free-living seedlings of these vine-growing species (Lyshede, 1985; Koch et al.,2004; Furuhashi et al., 2011) and, being obligate parasites, must rapidly make an attachment to a susceptible host before exhausting their limited stored food (Lanini and Kogan, 2005; Costea and Tardif, 2006).Lyshede (1985) indicated that host plants less than about three weeks old seem not to be attacked by Cuscuta seedlings.

The common name for the stem parasites- Mistletoe has a misleading tendency in the taxonomy of Loranthus (Family Loranthaceae) and Viscum (Family Viscaceae) (Steuber, undated). Sometimes Viscum species are referred to as the dwarf mistletoes to distinguish them from the genus Loranthus. Dwarf mistletoes are smaller plants than broadleaf mistletoes, with mature stems less than 6 to 8 inches long. However, both genera are related not only in gross morphology but also in the category of host attacked, and in fact, they fall under the order Santalales (Table 2.1.1). The Santalales cover 2000 species in 11 families and mostly occur in the tropics. The three most primitive families contain tree or bush-like plants. The representatives of the remaining, however, are half-parasites or parasites. Half-parasites are those with green pigments, whose hosts provide only water and feed salts, vitamins, and phytohormones (Steuber, undated). Mistletoes are highly specialized perennial flowering plants adapted to parasitic life on aerial parts of their hosts (Glatzel and Geils, 2009). 

Mistletoes include two main groups of Viscum and Loranthus species. Both are stem hemiparasites attacking different species of fruit and forestry trees and certain woody ornamentals. These parasites are green plants that have chlorophyll and can do photosynthesis. Viscum belongs to the family Viscaceae in the sandalwood order (Santalales) distributed in the tropics, North Temperate Zone and in tropical, sub-tropical and Mediterranean climatic regions in Jordan. Mistletoe has confusion in the taxonomy of Loranthus (Family Loranthaceae) and Viscum (Family Viscaceae) (Steuber, undated). Sometimes Viscum spp. is referred to as the dwarf mistletoes to distinguish them from the genus Loranthus. However, both genera are perennial leafy mistletoes of 6-7 years life span, morphologically and taxonomically related in which sometimes included under Viscaceae and similar in parasitic habit and in the form of host plants they attack. They mostly occur in the tropics, and their families are most primitive contain tree or bush-like plants. The mistletoe (Viscum and allied genera), family Viscaceae, are referring to those shrubby, parasitic cousins of sandalwood (Santalum album).

Parasitic flowering plants are of wide spread significance in different parts of the world. Some important genera are common in the Middle East countries and exert a major threat to many economic crops and forestry species. These include a large number of root and stem parasitic plants, among which Orobanche, Cuscuta, Viscum and Loranthus are widely spread and represent a major threat to agriculture. These species exhibit wide variations in their growth habit and habitats;they are also different in the size of their host range, many of which can attack both cultivated and wild species. Reviewing their ecological, biological and physiological requirements and behaviour revealed great differences between species in their responses. Ecological and biological factors affecting the germination, growth and development of these parasites were reviewed. Possible control measures for each species were carefully considered and analysed. This review indicated some promising herbicides, synthetic and natural chemicals and emphasized the importance of some physical, mechanical, agricultural and biological control measures. However, different workers have strongly recommended integrated control approaches for parasitic weed control. In this part (Volume 2), the parasitic weeds of Jordan stem parasitic species were introduced and emphasized. These included species of Cuscutaceae, Loranthaceae and Viscasceae. All species belonging to these families are serious threats to agriculture and the ecosystem. While 11 species Cuscuta species have been reported to occur in Jordan, six species appeared of much concern since being found attacking 120 plant species belonging to 37 families, among which 41 are cultivated as crops. Cuscuta species were found parasitizing herbaceous field crops and vegetables as well as fruit trees of economic value in addition to some forestry species. Loranthus and Viscum each was represented by a single parasitic species but are serious threats to woody plant species, including fruit and forest trees. Loranthus acaciae was found parasitizing 26 plant species of 12 plant families, while Viscum cruciatum attacked 14 plant species belonging to 8 plant families. Both mistletoes parasites, however, attack woody shrubs, fruit and forest trees and appear destructive to these plants in certain parts of the country.