Parasitic Diseases Plants killing plants
Parasitic Plants According to the American Heritage Dictionary a parasite is- An organism that grows, feeds, and is sheltered on or in a different organism while contributing nothing to the survival of its host More than 2,500 species of plants are known to be parasitic on other plants
Characteristics of Parasitic Plants Most parasitic plants have developed specialized organs that penetrate the host plant and establish connections to that plant s vascular tissue Parasitic plants have various degrees of dependence on the host plant
Characteristics of Parasitic Plants (2) Mistletoe has chlorophyll but no roots, it is dependent on the host only for support, water and basic nutrients Other plants like dodder have little or no chlorophyll and no true roots and are completely dependent on the host plant
Significant Parasitic Plants Though there are more than 2500 identified parasitic higher plants only a very few are significant, the following occur in the U.S. and cause notable damage Dodder Dwarf mistletoes True mistletoes Witchweeds
Dodder Widely distributed in the Americas and attacks several ornamentals Affects the growth and yield of infected plants Losses range from slight to complete destruction May also serve as a bridge for transmission of viruses from virus infected plants to virus-free plants if both plants are infected by the same dodder plant
Dodder Symptoms Orange or yellow vine strands grow and wrap around stems and above-ground plant parts Growing tips reach out and attack nearby plants, causing infestations up to 10 feet in diameter Dodder continues to enlarge during the growing season and, if on perennial plants, from year to year
Dodder Symptoms (2) During late spring dodder produces clusters of white, pink or yellowish flowers which form seed Infected plants become weakened and vigor declines Plants may be smothered and may be killed
Dodder Symptoms (3)
Dodder growing tips rotate counterclockwise (viewed from top) around host plant and reach out seeking nearby plants Dodder Symptoms (4)
Dodder Symptoms (5) Dodder on impatiens Note the swellings on the side of the dodder that contacts the host. Dodder secretes enzymes that break down host plant surface tissue and allow structures called haustoria to enter the host and attach it its vascular system
Dodder Symptoms (6) Dodder on tomato This dodder has just begun attacking the tomato stem. Note the growing tip is starting to stand away from the stem. As the dodder gets larger the tip will be farther away from the host stem and more likely to contact adjacent plants
Dodder Symptoms (7) Dodder flowers Dodder has masses of flowers in late spring which quickly produce seed that drop into the soil. Dodder seeds may germinate immediately or remain dormant until next season Seeds germinate producing a stem but no root, the stem immediately begins rotating, apparently searching for a nearby host plant
Dodder Symptoms (8) Dodder flowers Note the difference in color from the previous flower. Flower color can range from white to pink to yellowish
Dodder Disease Cycle Dodder overwinters either as seed or in some cases on perennial plants Seeds germinate in spring and produce a shoot but no root The shoot begins rotating, apparently searching for a nearby host plant, if no host is found the shoot drops to the ground and lays dormant for a few weeks then dies
Dodder Disease Cycle (2) If a host plant is found the dodder emits enzymes which break down surface tissue of the host and sends haustoria into the host Dodder then climbs the host, wrapping in a counter-clockwise clockwise direction (viewed from above) as it grows The growing tip stands away from the host apparently searching for adjacent host plants
Dodder Disease Cycle (3) The haustoria make a connection to the host s vascular tissue from which it takes water and nutrients (dodder has no chlorophyll and cannot make its own food) The base of the dodder shrivels and dies and the dodder loses all contact with the soil and is completely dependent on the host plant for all water and food
Dodder Disease Cycle (4) The host plant s s growth is suppressed as the dodder takes its water and nutrients The host may eventually die either from loss of food, loss of light from being covered by dodder or both If a host plant is infected with a virus, the virus may travel through the dodder to adjacent host plants
Dodder Disease Cycle (5) The dodder plant develops flowers and seeds The seeds drop to the ground and may be spread by water, animals and equipment Seeds can be spread over very long distances by contaminated crop seed
Dodder Cultural Control The best control is to prevent dodder from invading growing areas by Using dodder-free seed or plants Cleaning equipment before moving it from infested fields to clean fields
Dodder Chemical Control Dodder can be controlled using contact herbicides such as paraquat or diquat early in the season Herbicides kill the host plants as well so prevention by cultural control is preferred Note that translocated herbicides such as Roundup won t t kill dodder since it has no roots to destroy
Witchweed Witchweed is a serious parasitic plant in Africa, Asia and Australia It was discovered in the U.S. in 1956 in the Carolinas Effective state and federal quarantines have so far contained the parasite to the original infection sites Witchweed is a small (6 to 12 ) ) plant that is attractive with pink, red or orange flowers
Witchweed Symptoms Affected plants are stunted, wilt and turn yellowish. They may die if heavily parasitized One to several witchweed plants may be growing above ground next to the infected plants Roots of host plants contain many haustoria from witchweed roots where the connection to the host is made
Witchweed Symptoms (2) Witchweed plant at base of corn plant Witchweed has small, elongated leaves and red or orange flowers Attachment to the host plant is made by roots, not the above-ground parts
Witchweed Symptoms (3) Witchweed in corn field
Witchweed Symptoms (4) Witchweed flower and bud
Witchweed Symptoms (5) Witchweed attachment The larger white structure is witchweed which can grow underground and make its attachment to the host plant Some witchweed plants never grow through the surface of the soil, those plants will eventually die
Witchweed Disease Cycle Witchweed overwinters as seed in the soil Most seeds require a dormant period of 15 to 18 months before germination though some can germinate immediately without any dormant period Seeds close to host roots germinate and grow toward those roots attracted by exudates of the host roots
Witchweed Disease Cycle (2) As soon as the witchweed root contacts the host root its tip develops a bulb-shaped haustorium The haustorium dissolves and penetrates the host roots within 8 to 24 hours Leading cells of the haustorium force their way into the vascular tissue of the host roots
Witchweed Disease Cycle (3) Witchweed develops xylem which connects to the host xylem It does not develop typical phloem cells but connects to the host phloem as well The chlorophyll of witchweed is functional but it still takes manufactured food from the host Often several hundred witchweed plants will infect the roots of a host
Witchweed Disease Cycle (4) Most of the witchweed plants do not survive to reach the surface because the host plant cannot support so many The elapsed time from seed germination to producing new seeds can be 60 to 120 days
Witchweed Cultural Control Witchweed is difficult to control so infestation should be avoided at all costs Catch crops of susceptible host plants force the germination of witchweed which can then be plowed under or killed by herbicides Trap crops of non-susceptible host plants cause witchweed to germinate but since the hosts are non-susceptible the witchweed starves and dies
Witchweed Chemical Control Witchweed can be killed by contact herbicides or by translocated herbicides both of which also kill the host plants Seeds may remain in the soil so treatment may be necessary for several years
Dwarf Mistletoes Dwarf mistletoes infect conifers and occur wherever conifer trees grow In the U.S. they are more severe in the western half of the country Damage in conifer forests is extensive Trees become stunted, deformed or killed Size may be reduced 50% to 80% Infected wood has large knots and spongy abnormal grain
Dwarf Mistletoe Symptoms Dwarf mistletoe occurs as tufts along the twigs, branches and trunks Infected twigs and branches develop swellings and cankers on the infected areas The swellings contain haustoria of the parasite which grow into bark, cambium and xylem Infected branches often produce witch s brooms
Dwarf Mistletoe Symptoms (2) Young dwarf mistletoe plant on pine
Dwarf Mistletoe Symptoms (3) Dwarf mistletoe on pine Note the swellings on branches at infection sites
Dwarf Mistletoe Symptoms (4) Dwarf mistletoe on hemlock Note swelling on stem
Dwarf Mistletoe Symptoms (5) Damage from dwarf mistletoe on trunk The damage resembles a burl but the wood quality is very poor The damaged area reduces the transfer of water and minerals up to the needles and of manufactured food down to the area below the damage
Dwarf Mistletoe Symptoms (6) Dwarf mistletoe fruit Dwarf mistletoe has separate male and female plants. The plants can flower after 4 to 6 years. After flowering the male shoots die. The female shoots die after discharging seeds from fruit. When disturbed, ripe fruit explodes expelling seeds forcefully up to 15 meters The sticky seed adheres to whatever it comes in contact with
Dwarf Mistletoe Disease Cycle When disturbed a ripe fruit forcefully expels seeds up to 15 meters The seeds are covered with a sticky substance and adhere to whatever they come in contact with If on a susceptible host it germinates and produces a radicle which grows along the bark surface until it meets a bud or leafbase
Dwarf Mistletoe Disease Cycle (2) At a bud or leafbase the radicle produces a haustorium that penetrates the bark directly and reaches the phloem and the cambium From the haustorium the plant develops strands of radial sinkers which absorb nutrients Once established inside the host the parasite produces buds that develop shoots the following year or several years later
Dwarf Mistletoe Disease Cycle (3) The shoots emerge first from the original point of infection Later shoots emerge in increasing concentric zones The center of infection usually decays and is attacked by decay-producing fungi If witches brooms are produced on the affected area haustoria pervade all the branches
Dwarf Mistletoe Disease Cycle (4) The parasite takes water, minerals and photosynthates (manufactured food) from the host which starves and kills the branch beyond the infection It affects the vitality of the rest of the branch It also upsets the hormonal balance in the affected area which causes excess cell enlargement and division resulting in swellings
Dwarf Mistletoe Cultural Control The only method of control is by physical removal of the parasite This can be done by pruning infected branches or by removing entire trees Uninfected trees can be protected by maintaining a protective zone around them
True Mistletoes Evergreen parasitic plants Found throughout the world, particularly in warmer climates Attack mainly hardwood forest and shade trees but also common fruit trees such as apple and even some gymnosperms such as juniper and cypress Economic losses are generally much lower than those caused by dwarf mistletoe
True Mistletoe Symptoms The symptoms are very similar to those caused by the dwarf mistletoes Infected areas become swollen Plants produce witches brooms Sometimes so numerous that they account for more than 50% of green foliage in infected trees In winter they can make deciduous trees appear evergreen
True Mistletoe Symptoms (2) Infected trees may live for years but growth is affected and trees can be stunted and distorted Portions beyond the infected site can be starved and killed Mistletoes have well developed stems and leaves and can be up to a meter high
True Mistletoe Symptoms (3)
True Mistletoe Symptoms (4) Mistletoe in winter Typical mistletoe ball is several years old Mistletoe is easier to see in winter as it is evergreen while most of its hosts are deciduous
True Mistletoe Symptoms (5) Mistletoe detail The stems and leaves are green and are capable of photosynthesis Mistletoe manufactures much of its own food taking less manufactured food from the host plant than many other parasites but still causes significant damage to the host Each of the white berries contains a single sticky seed. The berries are consumed by birds which deposit the seeds on other branches or other trees
Mistletoe Control Control of true mistletoe is virtually the same as for dwarf mistletoe, removal of infected branches or entire plants A major difference is the method of spreading seed, since true mistletoe seeds are spread by birds establishing a protective zone around uninfected plants is generally not effective