Nematodes
• " If all the matter in the universe except the
nematodes were swept away, our world
would still be dimly recognizable..."
-N.A. Cobb, 1914
Morphology
Overview of Nematodes
• Animal Kingdom
• Wormlike But Not True Worms
• Several Hundred Species Attack Living
Plants
• Obtain Food with Spears or Stylets
• ~ $80 Billion in Crop Losses Annually
• Worse on Sandy Soils
Anatomy
• 300 to 1000 µm to 4 mm Long
• Invisible to Naked Eye, Visible Under
Microscope
• Eel-Shaped, Smooth, Unsegmented Bodies
• No Legs or Other Appendages
• Females of Some Species Pear-Shaped at
Maturity
• Mostly Transparent Body, Colorless Cuticle
• Body Cavity Contains Fluid through which
Circulation & Respiration Take Place
• Digestive System Is Hollow Tube from Mouth
to Anus
• Lips (USUALLY SIX IN NUMBER!!!) Surround
The Mouth
• Usually Hollow Stylet or Spear to Puncture
Holes in Plant Cells & to Withdraw Nutrients
http://nematode.unl.edu/baffi12.jpg
Direct Attack
Life Cycle
• Well-Developed Reproductive System
• Eggs Hatch into Juveniles that Look Similar
to Adults
• Juveniles Molt 4 Times, Then Differentiate
into Males & Females
• Female Can Produce Fertile Eggs after
Mating with Male or in Absence of Males
• Apply Mouth Suction to
Adhere to Plant
• Needlelike Stylet Thrusts
Back & Forth
• Rear Part of Body Sways
or Rotates
• Pierces Well & Stylet or
Entire Nematode Enters
Plant
– Called ‘Parthenogenesis’
http://nematode.unl.edu/agla2.jpg
1
• Life Cycle 2-4 Weeks in Optimum Conditions
• Slower in Cooler Temperatures
• Some Species Can Dry Out & Remain
Dormant in Juvenile Stage for Months
• Some Species Need Susceptible Host Soon
After Hatching
• Eggs May/May Not Remain Dormant in Soil
for Years
Ecology & Spread
• Usually Spend Part of Lives in Soil
• Many Only Feed Superficially on Roots or
Underground Stems
• Occur Mostly in Top 15-30 cm of Soil
• Usually Irregular Distribution in
Cultivated Soil, but Greatest Near Roots
• Can Follow Roots of Susceptible Plants to
Depths of 30-150 cm or More
Eggs of Steinernema feltiae
Mating of Steinernema feltiae
http://www2.oardc.ohio-state.edu/nematodes/photo_gallery.htm
Ecology & Spread
• Reproduce More Rapidly when Food Supply
Available
• Attracted to Hosts by Substances Released
into Rhizosphere
• ‘Hatching Factor’ Substances Released by
Plant Roots into Soil that Markedly Stimulate
Hatching Eggs of Certain Species
• However, Most Eggs Hatch Freely in Water in
Absence of Any Special Stimulus
• A Few Species Attack Aboveground Plant
Parts & Can Be Splashed onto Plants by
Falling Rain or Overhead Watering
• Some Ascend Wet Plant Stem or Leaf
Surfaces on Own Power
– May Spread to Adjacent Plants that Touch
Infected Plant http://hyg.ipm.illinois.edu/article.php?id=36
• 2 Genera Seldom if Ever Enter Soil
– Bud & Leaf Nematodes
– Pine Wilt (Vectored by Pinesawyer
Beetle) & Red-Ring Nematodes
– Survive in Tissues They Infect
– Pine Wilt Nematodes May Survive in Vectors
• Spread through Soil Slowly Under Own Power
• May Travel Few Meters Per Season
• Move Faster When Soil Pores Lined with Thin
Film of Water than When Waterlogged
• Can Also Easily Spread by Any Movement that
Carries Particles of Soil
–
–
–
–
–
Farm Equipment
Irrigation or Flood/Drainage Water
Animal Feet
Dust Storms
Farm Produce or Nursery Plants (Long Distances)
Symptoms on Plants
• On Roots
– Knots, Galls, Legions
– Excessive Branching, Injured Tips
– Root Rots if Secondary Bacteria or Fungi
• Aboveground
–
–
–
–
–
Reduced Growth, Nutrient Deficiencies
Excessive Wilting in Hot or Dry Weather
Reduced Yields, Poor Quality of Products
Galls, Necrotic Lesions & Rots
Distortion of Leaves, Stems, Flowers
2
What Nematodes Do to Plants
• Direct Mechanical Injury Only Slight Damage
• Secrete & Inject Saliva into Plants
– Puncture Cell Wall, Inject Saliva Into Cell
– Withdraw Part of Cell Contents
– Move to Another Cell
• Some Rapid Feeders
– Move to New Cell within Few Seconds
• Some Slow Feeders
– Remaine at Same Picture for Hours or Days
• Plant-Nematode Biochemical Interactions
Impair Overall Physiology of Plants
• Nematodes’ Mechanical Injury Allows Entry of
Other Pathogens
– Becomes Important Problem when Nematode
Population Explodes
• Their Enzymes Dissolve Some Infected Tissues
• Causes Dead Root Tips & Buds, Lesions,
Tissue Breakdown, Swellings & Galls,
Crinkled & Distorted Stems & Foliage
• May Inject Toxic Metabolites
–
–
–
–
Disintegrate Plant Tissue
Cause Abnormal Cell Enlargement
Suppress Cell Division
Stimulate Cell Division (Gall Formation or
Proliferation of Lateral Roots)
• Root-Feeding Species May Inhibit Plant Ability
to Take Up Water & Nutrients
Nematode-Fungus Disease
Complexes
• Fusarium Wilt, Verticillium Wilt, Pythium
Damping-Off, Rhizoctonia, Phytophthora
Root Rots Increase when Plants Also Infected
by Nematodes
• Varieties Ordinarily Resistant to Fungi
Apparently Become Infected After Previous
Infection by Nematodes
• Nematodes Somehow Lower Natural
Resistance to Fungus (Not Just Wound Entry
Site)
Nematode-Bacteria Disease
Complexes
• Only a Few Known:
– Pseudomonas solanacearum (Bacterial Wilt of
Tobacco)
– Clavibacter michiganense subsp. insidiosum
(Bacterial Wilt of Alfalfa)
– Pseudomonas marginata (Bacterial Scab of
Gladiolus)
Nematode-Virus Disease
Complexes
• Nematodes Vector Some Viruses
–
–
–
–
–
Grapevine Fanleaf
Tomato Ringspot
Raspberry Ringspot
Tobacco Rattle
Pea Early Browning
3
Control
• Usually Combination of Measures
– Physical Agents
• Heat, Flooding
– Chemical Controls: Nematicides
– Cultural Practices
• Crop Rotation, Fallowing, Cover Crops
– Biological Control
• Resistant Varieties
• Antagonistic Bacteria & Fungi
4