One of the most promising control options is the devel-opment of resistant varieties of crop plants through plant breeding.. As new resistant varieties are found other methods of control
Trang 1using this method alone over a long period of time have not
been demonstrated The mechanisms that allow nitrogen
fer-tilization 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
con-tinues until the land is almost useless and must be abandoned
One of the most promising control options is the
devel-opment 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 do not have the means to buy
fer-tilizers, 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
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
Arceutho-bium, 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
spe-cific 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
south-western 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
eco-nomic 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
signifi-cant 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,
Englerina, Loranthus, Macrosolen, Oryctanthus, Phragmanthrea, Scurrula , and Struthanthus.
Some resistant varieties of poplars, pines, and oaks do exist, but very little variability for resistance to mistletoe in-fections 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, herbi-cides can be injected into infected limbs below the point of infection, accumulate in mistletoes, and eventually kill them without severely affecting the host
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 Chem-ical 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, mo-lecular 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
Hemiparasitism 77