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Nematodes in soil and root extracts

Many of the nematodes observed in soil and root extracts are not plant parasitic in nature. In ecological studies of soil, root and shoot extracts, nematodes can be placed into a number of feeding groups. These are :-

1. Bacterial feeding
2. Fungal (hyphae) feeding
3. Predactious
4. Plant pathogen
5. Plant associated
6. Omnivores

They are placed into these feeding groups mainly on the appearance of the head and oesophageal region. Bacterial feeders tend to belong to the Rahbditids which have a plain mouth region of varying length. The oesophageal region is divided into at least two parts with the area just before the junction with the intestine having a muscular bulb to grind the bacteria upon which they feed. The fungal feeders generally have a very short stylet or piercing structure in the mouth region and an oesophagus which varies in shape and size. The predatious nematodes are generally large with a piercing tooth and an oesophagus that is one long muscular tube. The plant pathogens have a piercing structure in the mouth region which can vary in size from very small (under 10 um to very long (over 200 um); the shape of this structure also varies from curved (Paratrichodorus) to straight (most of the others). The oesophageal region tends to be muscular and has varying glandular structures located before the start of the intestine which provide chemicals that allow these nematodes to feed on plant cells. The plant associated nematodes tend to be smaller than the plant pathogens with a less robust piercing structure and oesophagus. The omnivores have a piercing structure which is plain in shape and an oesophageal region which is very muscular and separated into two parts without valves.

When doing ecological work it is important to determine the proportion of each group within the soil extract. This is done using a dissecting microscope under high power and by experience researchers. The compound microscope can be also used with a special counting disc that holds 1ml of extract. Work in being done to determine if specific groups of nematodes can be used to determine "soil health" as nematodes, being secondary consumers of algae, bacteria, fungi, plants and other nematodes, are closely involved in the regulating of the cycling of nutrients within the soil.

Cereal Cyst Nematode (Heterodera avenae)

Cereal cyst nematode (CCN) is a damaging pathogen of broad acre cereal crops in South Australia and Victoria. It affects wheat, barley, oat and triticale and can cause yield loss of up to 80%. Only one race of CCN has been recorded in Australia.

CCN is a sedentary plant parasitic nematode i.e. after invasion of the host it sets up a fixed feeding site within the plant root. The life cycle of CCN begins with a hard, brown cyst containing eggs. This cyst is formed from the swollen body of the female nematode from the previous season. Each cyst is approximately 1mm in diameter and is covered with a hard, brown outer casing which enables the eggs to survive in soil over the dry Australian summer.

After opening rains in April - June, and soil temperatures are approximately 15C, juvenile female nematodes hatch from the cyst and invade host plants. Each nematode sets up a feeding site (syncytia) and begins to produce eggs. The body of the female nematode gradually swells as eggs grow. The cyst can be seen on the outside of the roots as a small white nodule. At the end of each season, the cyst hardens and becomes brown. There is only one generation of nematodes each season.

The damage caused by the feeding nematode results in a proliferation of roots at the feeding site and forms a knot in the root which gives the plant characteristic symptoms.

View the Lifecycle of Cereal Cyst Nematode.

The best method for control of CCN is the use of resistant and/or tolerant varieties, a list of which is found in the Cereal Variety Disease Guide.
 

Root lesion nematode (Pratylenchus spp.)

The root lesion nematodes Pratylenchus neglectus and P. thornei are common in broad acre field crops of southern Australia and can cause damage of up to 40% in intolerant plant varieties. While other Pratylenchus species (P. penetrans, P. brachyurus and P. teres) have been found in Western Australia, South Australia and Victoria, damage caused by these nematodes has not been determined.

P. neglectus and P. thornei have a wide host range and both species can affect wheat, barley, oat and chickpea.

Root lesion nematodes are migratory plant parasitic nematodes i.e. they migrate freely between roots and soil providing the soil is moist. The life cycle of P. neglectus or P. thornei begins after opening rains in April - June. Juvenile and adult nematodes (the over-summering survival stage) invade plant roots and feed and multiply as they move through the root. Individual eggs are laid within the root from which juvenile nematodes hatch, grow to adults and lay eggs. There may be 3-5 cycles (egg to egg) within the plant host each season. As plants and soil dry out in late spring - summer in southern Australia, root lesion nematodes enter a state of anhydrobiosis and can survive high soil temperatures and desiccation.

As nematodes feed and multiply, lesions and/or sections of brown discolouration are formed on the plant root. Other symptoms include the reduction in the number and size of lateral roots and root hairs.

The best method for control of P. neglectus and P. thornei is the use of resistant and/or tolerant varieties, a list of which is found in the Cereal Variety Disease Guide.

View Life Cycle of P. neglectus or P. thornei

Stem nematode - Ditylenchus dipsaci (Oat race)

Two races of stem nematode have been recorded in South Australia - the oat and the lucerne race. The oat race is found in parts of Yorke Peninsula and the mid North of SA and the main hosts are susceptible oat and faba bean varieties. Symptoms in oat include stunted plants and the bases of each tiller become swollen. Other crops, such as field pea, chickpea, canola and lentil, are damaged extensively by stem nematode when they are seedlings i.e. there is seedling intolerance. Symptoms include stunted and distorted leaves and stems. As these plants from these crops mature, they become both resistant and tolerant.

The lucerne race of stem nematode has been recorded in lucerne growing areas of the South East of SA and along the eastern coast of Australia.

Stem nematode are obligate endoparasites. The life cycle of stem nematode begins after opening rains in April - June. The 4th stage juveniles (the over-summering survival stage), invade seedlings and damage the growing and emerging shoot tips of leaves and stems. This causes characteristic symptoms in oat, faba bean, canola, lentil and chickpea. Although the seedlings of a number of crops may be damaged, the oat race of stem nematode only multiplies in oat (including wild oat) and faba bean varieties. After juveniles have entered a host plant, they mature to become adults and feed and lay eggs. There may be 3-6 cycles within a susceptible host each season. As plants and soil dry out in late spring - summer in southern Australia, 4th stage juvenile stem nematode enter a state of anhydrobiosis in either plants or soil and can survive high soil temperatures and desiccation.

The best method for control of stem nematode is the use of non host crops such as wheat or barley, and the use of resistant and/or tolerant oat varieties, a list of which is found in the Cereal Variety Disease Guide.

View the Life Cycle of Stem Nematode

Anguina

Anguina agrostis is a plant parasitic nematode that infects rye grass. This nematode species (in association with a Corynetoxin bacteria and a bacteriophage) can cause death of cattle, sheep and horses. This disease complex is known as Annual Ryegrass Toxicity (ARGT) and a test is available through the Diagnostics Group for assessing the presence of the toxin responsible for stock death in hay.