1. INTRODUCTION TO PLANT PARASITIC NEMATODES Lecturer: Mr. Esava Tabua

2. The History of Nematodes
Members of the phylum Nematoda (round worms) have been in existence for one billion years, making them one of the most ancient and diverse types of animals on earth (Wang et al. 1999).
Thought to have evolved from simple animals some 400 million years before the "Cambrian explosion" of invertebrates able to be fossilized (Poinar 1983).
The two nematode classes, the Chromadorea and Enoplea, have diverged so long ago, over 550 million years, that it is difficult to accurately know the age of the two lineages of the phylum.

3. Cont’
Nematodes are multicellular animals in the group Ecdysozoa, or animals that can shed their cuticle.
Also included in this group with nematodes are insects, arachnids and crustaceans. In contrast to some of their relative invertebrates, nematodes are soft-bodied.

4. Structure of a Nematode

5. Cont’
Nematodes were noted early in human history because some serious human diseases are caused by relatively large vertebrate-parasitic nematodes.
Today plant parasitic nematodes are recognized as major agricultural pathogens and are known to attack plants and cause crop losses throughout the world.
Some estimates suggest they cause 77 billion dollars of damage worldwide each year (Sasser and Freckman 1987).
As the full extent of damage caused by plant-parasitic nematodes is recognized by agricultural scientists, the study of the biology of plant-parasitic nematodes will become increasingly important.

6. Nematode Biology
The study of nematode biology has led to a dramatic increase in understanding of how all animals function.
The reason nematodes are so useful for biological research is due to their simple anatomy and transparent bodies.
Nematodes are simple animals, often only containing 1000 cells or less.
Nematodes in all or part of their life cycle are worm-shaped (vermiform), although some species become swollen and rounded in later life stages.
The basic body plan of a nematode is a tube within a tube 

7. Cont’
Nematodes have outer skin or cuticle that is secreted from an inner hypodermis.
The muscles are attached longitudinally to the nematode’s hypodermis, allowing them to move only in the dorsal ventral direction (snake-like movement).
Inside the nematode there is an inner tube, the alimentary canal, which runs inside the nematode from head to tail.
Between the alimentary canal and the body wall is fluid that provides pressure against the wall to maintain body shape and allow movement.

8. Cont’
At the head of a plant-parasitic nematode is a hollow mouth spear (like a hypodermic needle) called a stylet.
The nematode uses this stylet to puncture plant cells, to withdraw food and also to secrete protein and metabolites that aid the nematode in parasitizing the plant.
The stylet is connected to the pharynx that, in turn, is connected to the intestine.
The intestine ends at the rectum in the female nematode and the cloaca in the male.
Attached to the pharynx are three - five salivary glands which produce secretions that may be emitted from the stylet and that assist the nematode in plant invasion and parasitism.

9. Cont’
The nematode pharynx is muscular and specialized areas can contract and expand the esophageal lining.
The expansion and contraction of the pharynx muscles allow the nematode to pump food into its intestine through its stylet or eject secretions from its salivary glands into and around plant cells.
In enoplean nematodes, the esophageal muscles are more spread out and do not form a compact pumping organ.

10. DIAGRAM OF NEMATODE

11. Nematode Habitats
Nematodes are very diverse and of the world's population:
50% are marine inhabitants
25% are free-living soil inhabitants
15% are animal parasites
10% are plant parasites
The focus of this topic is the 10% of nematodes which are plant parasitic.
They have a significant impact on agriculture, causing 10-15% annual losses on crops worldwide.

12. Plant Parasitic Nematodes
Plant parasitic nematodes are classified according to their feeding habit as:
Ectoparasitic: feed outside the root; or
Endoparasitic: feed inside the root and
Sedentary: establish specialised feeding sites and stay there until they die; or
Migratory: move through plant tissue, feeding as they go

13. Cont’
The most severe damage to plants is done by endoparasitc sedentary species of nematodes e.g. root knot nematode (RKN).
Location of feeding
Feeding habit
Example nematodes
endoparasitic
sedentary
root knot, cyst*, citrus
migratory
lesion, stem, bulb, burrowing, leaf, lance*, spiral*
ectoparasitic
ring
dagger, stubby root

14. Cont’
Plant parasitic nematodes are attracted by the host-plant exudates. They probe the host tissue, and if the plant is perceived as a suitable host, the stylet penetrates the host tissue by battering its way through.
The nematode sucks out contents of host cells through the stylet and cell-by-cell. An exception to this is some species of Meloidogyne (root knot nematode) and Heterodera which alter the genetic make-up of their host plant to set up specialized feeding sites.
Non-plant parasitic nematodes use other methods for feeding e.g. bacterial feeders have a type of sucking mouthpart; some animal parasites channel the host blood continuously through their body.

15. Nematode mobility, morphology and function
Nematodes are aquatic animals that require free water in the soil to be able to move. Because of this requirement, nematodes thrive best in sandy soils where the soil particles are larger, since the associated water films are thicker and more continuous, aiding their movement and dispersal (c.f. clay soils with very fine particle size).
Nematodes also require oxygen for survival, and their life cycle responds to t emperature. They have a dormant state termed 'quiescent state' through which they can survive unfavourable conditions.
The nematode has four major body regions/systems:

16. Cont’ 2. Oesophagus and intestine 3. Reproductive system
The nematode has four major body regions/systems:
1. Stoma
2. Oesophagus and intestine
3. Reproductive system
4. Nervous system

17. Nematode Life Cycle
The generalised life cycle of plant parasitic nematodes is:
egg
4 juvenile stages
adult
eggs produced
The adult nematodes can either be both female and males, or solely females. If the nematode species only produces female adults, these adults either possess the reproductive ability of both a male and female nematode (hermaphroditic) or reproduce by asexual means (parthenogenic).

18. Damaged caused by nematodes
The damage caused by nematodes involves:
direct damage caused by feeding,
indirect damage such as:
- being vectors for plant disease or enabling entry to the host(e.g. plant viruses, the fungus Fusarium oxysporum), and
- providing wound sites for secondary pathogens to invade via (e.g. bacterial rots)

19. Cont’
The symptoms of nematode damage can be divided into above ground and below ground symptoms.
Note the above ground symptoms may not always be evenly distributed throughout a crop due to differences in microenvironment, soil type, previous cropping history etc .
Nematodes are typically named according to the type of damage that they cause to the below ground plant parts i.e. roots. Types of root damage at the infection site include galls, cysts, lesions, browning, stubby root etc.

20. Root functions affected by Nematode Damage
Uptake of water and nutrients
Synthesis of growth regulators
Metabolism
Redistribution of photosynthesis products
Nitrogen fixation
Mycorrhizal function and survival
Anchorage

21. Above ground symptoms as result of Nematode damage
Stunting
Chlorosis
Wilting
Premature senescence
Premature or delayed flowering

22. Factors that influence nematode population level
Host plant
Environment
Form of parasitism Life cycle Reproduction rate Adaptation to environment
Resistance/susceptibility tolerance Length of cropping season Age of plant Growth stage of plant
Temperature Water availability Soil type Presence of other host plants eg weed species

23. Nematode Distribution
The distribution of nematodes in any given soil will vary in terms of time and space:
Spatial distribution: affected by cropping rows, soil fertility, soil type, root depth, environment etc.
Seasonal distribution: reproduction of nematodes generally increases in spring and autumn as this is when the environment is most favourable. They tend to migrate downwards into the soil profile as the soil dries

24. Quantification of nematode within a crop
Threshold level: the minimum density of nematodes which produces symptoms or yield loss in the crop (not necessarily representing an economic loss)
Tolerance level: density at planting which will eventually reach the threshold level
Economic threshold level: nematode density which causes a yield loss equal to the cost of nematode control.
The economic threshold level is dependent on both the market (for the produce) and the environment, both of which are very difficult to predict! It is also difficult to accurately quantify the number of nematodes at the beginning of a season since their numbers are very low and their distribution is uneven.

25. Nematode Control
There are many approaches available to control nematodes that encompass:
biological control
cultural control
plant breeding
chemical control
Not all nematodes are bad! Some nematodes are being investigated for their ability to control other plant pathogens, namely fungi.

26. Biological Control: Nematode-trapping fungi
Methods used by fungi to trap nematodes include:
Constricting rings
Adhesive knobs
Adhesive nets

27. Cultural Control Crop rotation Organic amendments to soil
Steam sterilization
Hot water treatment
Farm sanitation/hygiene, quarantine
Planting healthy propagating material
Changing time of planting
Disinfestations of irrigation water

28. Plant Breeding for Nematode Control
Two general approaches can be taken for breeding:
Breed crop to be resistant i.e. able to reduce reproduction of nematode
Breed crop to be tolerant i.e. able to withstand attack by nematode

29. Cont’ Resistance can occur by a plant… Failing to attract nematodes
Inhibiting hatching of nematode eggs
Being resistance to nematode penetration
Triggering a hypersensitive response
Failure of nematode to induce compatible response in plants

30. Advantages of resistance
Disadvantages of resistance
· No extra management costs
· Does not rely on environment
· May cause population decline
· A resistant host may still be damaged if it is intolerant takes a long time to develop
· Expensive to develop
· Resistance may be overcome

31. Advantages of tolerance Disadvantage of tolerance
Tolerance can occur by a plant:
growing in excess of requirements
growth compensation for damage
lack of response e.g. does not gall
Advantages of tolerance
Disadvantage of tolerance
· No selection pressure to cause resistance in pathogen population
· Cheaper to develop
· Nematodes can reach damaging levels

32. Any Questions?? ?