Hemiparasitism using this method alone over a long period of time have not been demonstrated The mechanisms that allow nitrogen fertilization to decrease the impact of the parasitic plants are not fully understood It does seem clear, however, that on poor soils, the witchweed infestations increase in intensity as the quality of the soil decreases; the result is a feedback that continues until the land is almost useless and must be abandoned One of the most promising control options is the development of resistant varieties of crop plants through plant breeding As new resistant varieties are found other methods of control become unnecessary Resistant varieties of many crops have not been discovered, however, therefore this is a very active area of research Parasitic plants may pose the largest problem to rural, subsistence farmers As pressures on land use increase, the ability to use crop rotation methods decreases Furthermore, subsistence farmers often not have the means to buy fertilizers, herbicides, or expensive resistant seeds For these farmers, the feedbacks between poor soils and parasitic plant infestation may eventually cause the farmers to abandon plots of land and move elsewhere 77 Englerina, Loranthus, Macrosolen, Oryctanthus, Phragmanthrea, Scurrula, and Struthanthus Some resistant varieties of poplars, pines, and oaks exist, but very little variability for resistance to mistletoe infections exists and, in general, breeding for resistance has not been a viable option Biological control by insects, pathogens, or other mistletoes remains a possibility, but none of these methods have been successful Instead, control of the leafy mistletoes involves pruning off infected limbs, or chemical control Pruning infected limbs removes the mistletoe, but the limb must be cut below the point of the infection to remove the entire endophyte This practice is extremely labor intensive and impractical on a large scale With Arceuthobium, the endophyte is often extensive, and the limb must be severed 20 to 30 cm below the lowest parasite shoots In extreme cases, it may be more cost-effective to burn the forest In less extreme cases, early harvest followed by controlled burns may reduce the mistletoes Some successful chemical control methods have been found The leaves of mistletoes may be sprayed with ethephon once the host drops its leaves Alternatively, herbicides can be injected into infected limbs below the point of infection, accumulate in mistletoes, and eventually kill them without severely affecting the host Mistletoes Several species of mistletoes are economically important pests of fruit trees, ornamental trees, or timber Most mistletoes have a similar ecology, so the impact and control measures are easy to generalize The exceptions are dwarf mistletoes (genus Arceuthobium, Viscaceae) that have a much more highly developed endophytic component and inconspicuous, scaly leaves Because of the highly developed endophyte, physical removal of dwarf mistletoes is more difficult than other mistletoes Most species of dwarf mistletoes are extremely host specific For example, A douglasii is a parasite of Douglas fir, and A tsugense is a parasite of the western hemlock The dwarf mistletoes are parasites of pine trees in North America, the Himalayas of India, Pakistan, and Bhutan, and in southwestern China Dwarf mistletoes are common throughout the Rocky Mountains where large regions of forests are infested and the timber volume and quality are affected Two other genera in Viscaceae are notable for their economic impact Phoradendron is a genus of about 190 species distributed throughout North and South America Of these, the most important are P serotinum, found on an extremely wide variety of hosts (but never conifers), and P piperoides, found on cocoa in Costa Rica Viscum is a genus of about 60 species that include V album that has an extremely wide host range including fruit trees, pines, and poplars in Europe and persimmon in China Another agricultural pest is V cruciatum that parasitizes olives in the Middle East Many species of Loranthaceae cause economically significant loss Dendrophthoe falcata is a pest of fruit trees in India and teak in Kerala Dendropthoe pentandra parasitizes rubber and kapok in Indonesia Helixanthera mannii is a pest of citrus and coffee in West Africa, and H parasitica is a pest of citrus in the Himalayas Tapinanthus bangwensis is an important pest of cocoa and cola in Ghana, while T globiferus grows on coffee, citrus, and other fruit trees in Ethiopia Some other genera that have economically important species are Amyema, Dodders The economic impact of Cuscuta species is greatest in forage crops such as alfalfa and clover; however, crops like citrus, coffee, peach, litchi, flax, linseed, and other crops as well as ornamental plants all suffer from dodder attack The most economically important dodder is Cuscuta campestris This species attacks alfalfa and has been shown to reduce forage yield by as much as 57% over a 2-year period It also affects Niger seed in India and many vegetable and flower crops Of the 20 species of Cassytha, the most damaging is C filiformis It is distributed throughout the tropics and parasitizes a wide range of hosts It is a problem on citrus in India and Tanzania and on Pinus massonia in China Mechanical control methods for Cuscuta include hand pulling, crop rotation, burning, delaying planting until after Cuscuta have germinated, or deep ploughing to reduce the seed bank Few resistant varieties of crops are known Chemical control methods include fumigants to eliminate the seed bank, herbicides applied to the soil to prevent seedling growth, and herbicide application after seeds have germinated to prevent establishment Some methods of biological control have been established with insects and pathogens, but the scope of these has been limited Conclusions Parasitic plants have been understudied, but recent studies have improved our understanding of their evolution, ecology, molecular biology, and physiology Haustorial parasitism has evolved independently at least 12 times within angiosperms and, surprisingly, complete loss of photosynthesis has occurred multiple times within some lineages Most parasitic plants exhibit near-normal levels of photosynthesis like their