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Parasitic Higher Plants • > 2,500 Species of Higher Plants Live Parasitically on Other Plants • Produce Flowers & Seeds • May or May Not Produce Chlorophyll • May or May Not Possess True Roots Sonja Lallemand Picture Genera that Cause Economic Impact to Agricultural Plants • Cuscuta (Dodders) • Arceuthobium, Phoradendron, Viscum (Mistletoes) • Orobanche (Broomrapes of Tobacco) • Striga (Witchweeds of Monocots) Symptoms • Orange or Yellow Strands Entwine Around Aboveground Parts of Host Plants • Patches in Field Enlarge during Growing Season • In Perennial Host Crops, Patches Become Larger Every Year • Produces Clusters of White, Pink or Yellowish Flowers in Late Spring, Summer • Form Seeds Dodder • Affects Growth & Yield of Infected Plants • May Transmit Viruses • Slender, Twining Plant • Leafless (Tiny Scales in Place of Leaves) Disease Development • Seed Overwinters in Infested Fields or Is Harvested with Seed of Crop Plants • Seed Germinates during Growing Season • Produces Slender Shoot but No Roots • Shoot Rotates in Search of Host • If No Contact with Susceptible Plant, Stem Falls to Ground, Lies Dormant a Few Weeks, Dies 1 • If Contact with Susceptible Plant, Encircles It & Begins to Climb • Sends Haustoria Inside Leaf or Stem • Seeks Vascular Tissues where Absorbs Food & Water • Base of Dodder Shrivels & Dies • Loses All Connection with Ground • Becomes Entirely Dependent on Host for Nutrition http://www.science.siu.edu/parasitic-plants/Cuscutaceae/images/Cuscuta.JPEG Dissemination of Seed • • • • Animals Water Equipment Contaminated Crop Seed Control Measures • Twisting Tips Attack Adjacent Plants • Forms Patches of Infection • Suppresses Growth of Infected Plants http://www.science.siu.edu/parasitic-plants/Cuscutaceae/images/Cusc.haust.JPEG Effect on Host Plants • Weakened by Parasite • Vigor Declines • Yields Decrease • May Be Smothered or Killed by Dodder Mistletoe • Dodder-Free Seed • Clean Equipment Before Going to Next Field • Limit Movement of Domestic Animals from Infested to Clean Fields • Spray Scattered Patches Early in Season with Contact Herbicides • Cut or Burn Patches • Frequent Tillage or Flaming to Control Widespread Infection in Field 2 Dwarf Mistletoe • On Conifers • Monoecious – Produces Flowers at Age 4-6 Years – Male Shoots Die After Flowering • Stunts, Deforms or Kills Hosts including Height Reduction by 50-80% – Female Shoots Die After Seeds Discharged • Fruits • Reduces Timber Quality – Mature 5 to 16 Months After Pollination • Shoots Grow in Tufts along Twigs of Hosts – Turgid with Considerable Internal Pressure • Sends Wedge-Shaped Haustoria Inside • Infected Twigs & Branches Develop Swellings & Cankers, Sometimes Witches’Brooms • When Disturbed, Fruit Expels Seed Upward or Outward at Distances up to 15 Meters – Seed Covered with Sticky Substance, Adheres to Surface It Contacts – Occasionally Birds Transport Seed on Bodies Disease Development • Seed Attaches to Bark or Twig of Susceptible Host • Germinates & Produces Germ Tube/Radicle • Grows along Bark Surface Until It Meets Bud or Leaf Base • Produces Rootlike Haustorium to Penetrate Bark, Reach Phloem & Cambium http://www.science.siu.edu/parasitic-plants/Viscaceae/images/Phor.tom.seed.JPEG – Absorb Nutrients from Host – Sinkers that Reach Cambium Embed in Wood as It Is Laid Down Each Year but Always Retain Connections with Strands in Phloem • When Well Established, Buds Are Produced & Develop Shoots in Subsequent Years • Center of Infection usually Deteriorates & Becomes Attacked by Secondary Fungi • If Witches’-Brooms Are Produced, Haustoria Invade All Branches & Produce Mistletoe Shoots Along Host’s Proliferating Branches • Develops System of Longitudinal Strands & Radial Sinkers Effect on Host Plants • Obtains Water, Nutrients from Host • Starves & Kills Terminal Portion of Branch • Saps Vitality • Upsets Hormonal Balance of Host – Causes Excessive Cell Enlargement & Division – Results in Swellings & Deformities on Branches – Stimulates Excessive Shoot Formation • Predisposes Trees to Wood-Decay & Root Pathogens, Beetles, Wind Breakage 3 True or Leafy Mistletoe • Infects Hardwood & Fruit Trees • Symptoms Similar to Dwarf Mistletoe • Parasitic Evergreens – Well-Developed Leaves & Stems • Usually Small, Dioecious Flowers • Haustorial Sinkers Grow in Branches & Stems • Spread by Birds that Eat Berries – Excrete Sticky Seeds in Tops of Taller Trees Broomrapes • Occur in Warm & Dry Regions Worldwide • Attack 100s of Species of Herbaceous Dicots • Pathogen Whitish to Yellowish Annual – Fleshy Stem, Scalelike Leaves – Numerous Pretty Snapdragonlike Flowers Arising Singly Along Stem • Seed Pods about 5 mm Long – Each Contains 100s of Tiny Seeds • Develops a Stem that Looks Like Asparagus • Secondary Roots Spread to Other Host Roots • End with Clusters of Broomrape Plants Arising from Soil around Infected Host Plants • Seeds Germinate – Produce Radicle, Grow Toward Root of Host, Attach, Produce Appressorium that Surrounds Root Excellent Information On Parasitic Plants & on Other Mistletoes: http://www.science.siu.edu/biologicalsciences/biol200/biol200b/Nickrent/DLNlectures/200.Nutrition.html “…I don't want to leave you with the impression that all mistletoes are bad! The vast majority of mistletoe genera & species occur in the tropics. They are actually members of a different family than our Christmas mistletoe. These "showy" mistletoes (family Loranthaceae) are spectacular plants, often with large, brightly-colored flowers [Macrosolen]. These plants have fascinating coevolutionary relationships with birds (Dicaeum) that not only pollinate their flowers but also disperse their seeds…” • Overwinters as Seeds – May Survive in Soil 10 Years or Longer • Germinate when Roots of Certain Plants Grow Near Them • Not All Plants Susceptible • Some Cells Differentiate into Parasite Xylem Vessel Elements – Connect Host Xylem with Main Vascular System of Parasite • Other Undifferentiated Cells Attach to Phloem Cells to Obtain Nutrients from Them Witchweed • Native to Africa, Asia, Australia • Discovered in Carolinas in 1956 • Quarantines Confined Parasite to that Area – Mass of Undifferentiated Cells Arise & Penetrate Host – Absorb Nutrients & Water from Xylem http://www.science.siu.edu/parasitic-plants/Scrophulariaceae/images/Striga.asia2.JPEG 4