Nematodes

• Nematodes


  • Bilaterally symmetrical, and vermiform
  • No segmentation (no internal separation of body plans)
  • Slender worms - 0.5 to 2.0 mm in length
  • No legs/appendages
  • Some females are swollen at maturity (pear-shaped or spheroid)
  • Body is transparent with colorless cuticle and striations
  • Longitudinal muscles allow for movement

  
• Plant parasitic nematodes


  • Feed by puncturing plant cells and extracting the predigested contents with a hollow spear or stylet
  • Disseminated by infested soil, nursery stock, running water, or vehicles and machineries
  • Reproduce through eggs; sexual or parthenogenetic

  
• Nematodes are triploblastic, containing three body layers (ectoderm, mesoderm and endoderm) in the embryo

  
• Nematodes have a body cavity that is not totally surrounded by mesoderm, so they are pseudocoelomic

  
• Life cycle of a typical plant parasitic nematode


  1. Egg stage

  2. Four larval or juvenile stages

  3. Adult stage

  4. Molt happens during each juvenile stage where the cuticle is shed, allowing the nematode to increase in size

  
• Giant cells


  Formed by root-knot nematode (Meloidogyne)
• Syncytium


  Formed by cyst nematode (Globodera and Heterodera)
• Classification of plant parasitic nematodes

  • Phylum: Nematoda
  • Class: Tylenchida
  • Suborder: Tylenchina, Superfamily: Tylenchoidea
  • Family: Anguinidae, Belonolaimidae, Pratylenchidae, Hoplolaimidae, Heteroderidae, Criconematidae, Paratylenchidae, Tylenchulidae, Aphelenchoididae, Longidoridae, Trichodoridae

  
• Groups of plant parasitic nematodes


  • Belonolaimus - sting nematode
  • Paratylenchus - pin nematode
  • Trichodorus - stubby-root nematode
  • Xiphinema - dagger nematode
  • Longidorus - needle nematode
  • Pratylenchus - lesion nematode
  • Radopholus similis - burrowing nematode
  • Hoplolaimus - lance nematode
  • Helicotylenchus - spiral nematode
  • Ditylenchus - stem and bulb nematode
  • Aphelencoides - foliar nematode
  • Meloidogyne - root-knot nematode
  • Heterodera and Globodera - cyst nematode
  • Rotylenchulus reniformis - reniform nematode
  • Tylenchulus semipenetrans - citrus decline nematode

  
• Syncytia


  Formed by cell fusions after cell wall dissolutions between the initial cell on which the nematode starts feeding and an increasing number of neighboring cells
• Giant cells


  Formed by repeated nuclear divisions of the initial feeding cell without complete cytokinesis
• Hypertrophy


  Abnormal increase in the size of cells in a tissue or organ, often resulting in the formation of galls or tumors
• Hyperplasia


  Abnormal increase in the number of cells, often resulting in the formation of galls or tumors
• Reproduction in nematodes


  • Amphimixis - males are needed
  • Parthenogenesis - only females are produced and offsprings are clone of the female
  • Sex reversal - observed in juvenile stage during unfavorable condition

  
• Nematode classification by feeding position

  • Ectoparasitic - nematode remains outside of the plant and uses its stylet to feed from the cells of the plant roots
  • Semi-endoparasitic - nematode partially penetrates the plant and forms a permanent feeding cell
  • Migratory endoparasitic - nematode spends much of their time migrating through root tissues destructively feeding on plant cells
  • Sedentary endoparasitic - most damaging nematodes, including cyst nematodes and root-knot nematodes

  
• Examples of nematode classification by feeding position


  • Migratory Ectoparasite: Longidorus, Xiphinema, Trichodorus, Paratrichodorus
  • Sedentary Ectoparasite: Criconemoides, Hemicyclophora
  • Semi-Ecto, Semi-Endo: Tylenchulus, Rotylenchulus
  • Sedentary Endoparasite: Meloidogyne, Globodera, Naccobus

  
• The most damaging nematodes in the world have a sedentary endoparasitic lifestyle
• Two main nematodes in the sedentary endoparasitic group


  • Cyst nematodes (Heterodera and Globodera)
  • Root-knot nematodes (Meloidogyne)
• For further information about the soybean cyst nematode see the "Biology and management of the soybean cyst nematode" edited by R.D. Riggs and J.A. Wrather. For further information about the root-knot nematode see "An advanced Treatise on Meloidogyne" edited by J.N. Sasser and C.C. Carter.

  
• Nematode Classification (Examples)


  • Migratory Ectoparasite
  • Sedentary Ectoparasite
  • Semi-Ecto, Semi-Endo
  • Sedentary Endoparasite
• Migratory Ectoparasite


  • Longidorus
  • Xiphinema
  • Trichodorus
  • Paratrichodorus
• Sedentary Ectoparasite


  • Criconemoides
  • Hemicyclophora
• Semi-Ecto, Semi-Endo


  • Tylenchulus
  • Rotylenchulus
• Sedentary Endoparasite


  • Meloidogyne
  • Globodera
  • Naccobus
  • Hirschmaniella
• Root-Knot Nematode (Meloidogyne sp.)


  • Damages plants by devitalizing root tips and causing the formation of swellings of the roots
  • Infected roots develop the typical root-knot galls that are two to several times as large in diameter as the healthy root
• Root-Knot Nematode (Meloidogyne sp.) control

  1. Steam sterilization of the soil or soil fumigation with nematicides in the greenhouse

  2. Fumigating the soil with approved chemical nematicides in the field
• Soybean Cyst Nematode (Heterodera glycines)


  • The infective second-stage juveniles penetrate young primary roots or apical meristems of secondary roots
  • The juveniles pierce their stylets into and feed off cells of the cortex, the endodermis, or the pericycle, causing the enlargement of these cells
  • The groups of enlarged cells are called syncytia and serve as feeder cells for the nematode
  • Syncytia in contact with developing third- or fourth-stage males begin to degenerate, indicating cessation of feeding
  • Syncytia in contact with females degenerate after egg deposition

  
• Soybean Cyst Nematode (Heterodera glycines) control

  1. Use of resistant varieties

  2. 1 to 2 year crop rotation with nonhost crops
• Lesion Nematode (Pratylenchus)


  • Attack the roots of all kinds of plants, such as cereals and other field crops, vegetables, fruit trees, and many ornamentals
  • Reduce or inhibit root development by forming local lesions on young roots
  • Affected plants grow poorly, produce low yields, and may finally die

  
• Lesion Nematode (Pratylenchus) control


  Overall or row treatment of the soil with nematicides before the crop is planted

  
• Burrowing Nematode (Radopholus similis)


  • The most important banana root pathogen
  • Infected banana plants grow poorly, with fewer and smaller leaves, premature defoliation, and have smaller fruits
  • Browning and presence of cavities in the cortex followed by deep cracks on the root surface
  • Spends its life and reproduces inside cavities in the root cortex, where it completes a life cycle in about 20 days
  • Most of the spread of the nematode from plant to plant is through root contact or near contact

  
• Burrowing Nematode (Radopholus similis) control


  1. Using nematode-free plantlets produced

  2. Hot water treatment in 55°C for 20 minutes

  3. Flooding the field for 5 to 6 months where possible

  4. Soil fumigation or postplanting treatment with appropriate nematicides
• Stem and Bulb Nematode (Ditylenchus)


  • Causes heavy losses by killing seedlings, dwarfing plants, destroying bulbs, or making them unfit for propagation or consumption
  • Causes the development of distorted, swollen, and twisted stems and foliage
  • Many leaves become flaccid and are so weakened that they cannot maintain their erect growth and fall to the ground
  • The stem, neck, and individual scales of the bulb become softened, loose, and pale gray in color
  • Affected scales appear as discolored rings in cross sections of infected bulbs and as discolored, and giving off a foul odor
  • Infected bulbs continue to decay in storage
• Stem and Bulb Nematode (Ditylenchus) control


  1. Long crop rotation

  2. Use of resistant cultivars
• Sampling of Nematodes


  • Aggregated distribution and seasonal behavior of nematode
  • Crop type and history
  • Soil moisture, compaction and type
  • Temperature and seasonal changes
• Sampling Pattern


  • Random sampling
  • Systematic sampling
• Sampling Patterns


  • Pattern for Perennial Crops not exceeding 2 ha
  • Pattern for Row annual crops
  • Sampling within the drip line of a tree or shrub
  • X travsect Sampling pattern for bulb crops and 8 ha+ field crops
• Time of Sampling


  • Predictive sampling - done early in the season, such as before planting or at the end of the previous cropping season
  • Diagnostic sampling - in the middle of the season and/or at the final harvest
• Do not collect samples when the soil is too dry or extremely wet because the nematode populations are usually low under these conditions
• Baermann Funnel vs. Sieving Method