pathogens/parasitic nematodes


Meloidogyne incognita affected soybean roots
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Plant parasitic nematodes



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Nematodes are roundworms that occur worldwide in nearly all environments. Many nematode genera are beneficial to agriculture because they contribute to degradation of crop residue and are important members in the food chain. However, about 15 percent of the species are parasitic to plants and cause crop losses valued at $8 billion annually in the United States and $78 billion worldwide. Most plant-parasite nematodes are tiny (less than 1 mm or 0.04 inch long) and live in soil.

All plant-parasitic nematodes have have a hollow, protrusible mouth spear called a stylet that is used to inject secretions into host tissues and pierce plant cell walls to withdraw nutrients. Some phytonematode species feed upon and quickly destroy plant cells as migratory parasites, whereas other nematode species become sedentary in later life stages and must modify plant cells to provide a sustained local source of nutrition (cysts or root knots). The majority of phytonematode species are parasites of plant roots

Tylenchida is an order of nematodes containing many species of sedentary (cyst and root-knot) plant parasitic nematodes. Female worms lay eggs that hatch either in soil or in the host plant.

cellular organisms - Eukaryota - Opisthokonta - Metazoa - Eumetazoa - Bilateria - Pseudocoelomata - Nematoda - Chromadorea - Tylenchida - Tylenchina -

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Organism Disease Transmission Management
Tylenchoidea - Heteroderidae - Heteroderinae - Heterodera - Heterodera avenae
Common name: oat cyst nematode; cereal cyst nematode (CCN)
Distributed worldwide in temperate cereal-producing regions in all soil type except very heavy poorly structured soils. Females are initially white, then, turning cream or brown- colored, almost black when they die and become cysts. Low soil temperatures (5-15 C°) and high moisture stimulate hatching. Symptoms are usually shown as patches of pale and stunted plants. The effects on the aerial parts mimic severe nitrogen and other mineral deficiencies. Affected plants tolerate drought poorly because of damage to root system. Infected roots are characterized by matted appearance and proliferation of secondry roots. Easily spread with soil carried on equipment, animals, shoes, or root, tuber, and ornamental crops. It also is dispersed by wind (in dust) and water. Main goal is maintaining nematode populations below economic thresholds. Chemicals are not environmentally sustainable or economic and the major emphasis on control has been with host genetic resistance applied with other integrated pest managent options. Due to the number of pathotype variation genetic control of CCN is complex.
Tylenchoidea - Heteroderidae - Heteroderinae - Heterodera - Heterodera goettingiana
Common name: pea cyst nematode
Distinguished from other Heterodera spp. by lemon-shaped cysts. Cysts persist in soil for many years. Second-stage juveniles (J2) emerge from the cysts, penetrate host roots and establish a specialized feeding site (syncytium). They develop into swollen females that rupture root cortex and protrude outside. At the end of reproductive phase, females die and become lemon shaped brown cysts. Infects broad range of leguminous plants. Broad bean (Vicia faba) and garden pea (Pisum sativum) are the most seriously damaged field crops. Infected plants are stunted, with pale leaves which later turn yellow. These plants produce a few small pods with a few seeds. Infected roots are poorely developed, with suppressed Rhizobium nodulation. Subsequent root invasion by fungi (Fusarium oxysporum) can cause complete loss of the crop. This cyst nematode is dispersed with soil debris, plant material and machinery contaminated with the cysts. Survive in soil for many years in the absence of the host, however, population decline of about 50% was reported during the first three years. Crop rotation with non-hosts is recommended. Nematicides are used in some countries. However, these treatments are expensive and pose environmental risks. Moderately resistant pea hybrds have been obtained by crossing susceptible and edible Pisum sativum with inedible moderately resistant P. abyssinicum.
Tylenchoidea - Heteroderidae - Heteroderinae - Heterodera - Heterodera glycines
Common name: soybean cyst nematode, SCN
SCN's 3-stage life cycle is completed in 24 to 30 days. The juvenile emerging from the egg in spring will die from starvation if it doesn't find host plant roots within several days to a week. After a juvenile has penetrated the soybean root, it moves through the root to the vascular tissue, where it establishes a feeding site. Eventually swollen female bursts through the root tissue. Male nematodes migrate out of the root, fertilize the swollen females and die. The fertilized females produce eggs that fill their entire body cavity and become cysts. Moderate levels of SCN may not produce any visible, above ground symptoms other than reduced yield. The first visual sign of an infestation is usually a slight variation in the height of otherwise healthy-looking, dark-green plants. Higher SCN levels can cause stunting, chlorosis, and even plant death. Areas of SCN injury are often elongated in the direction of tillage practices, since cysts are spread by tillage equipment. SCN can be spread passively by farm machinery, vehicles and tools, wind, water, animals, and farm workers. Seed-sized particles of soil, called soil peds, often contaminate harvested seed. Consequently, SCN can be spread when seed from infested fields is planted in uninfested fields. There is even evidence that SCN can be spread by birds. Once present in the soil, SCN can never be eliminated. However, the nematode can be managed to minimize its reproduction and maximize crop yields. Management practices for SCN fall into five categories: plant health maintenance, sanitation practices, planting resistant cultivars, growing nonhost crops. No nematicide will kill all SCN in the soil completely.
Tylenchoidea - Hoplolaimidae - Rotylenchulinae - Rotylenchulus - Rotylenchulus reniformis
Reniform nematodes are semiendoparasitic (partially inside roots) species in which the females penetrate the root cortex, establish a permanent-feeding site in the stele region of the root and become sedentary or immobile. The anterior portion (head region) of the body remains embedded in the root whereas the posterior portion (tail region) protrudes from the root surface and swells during maturation. The term 'reniform' refers to the kidney-shaped body of the mature female. Primary economical nematode pest of cotton (Gossypium hirsutum) in the southern states of Alabama, Louisiana, and Mississippi. The amount and type of damage often depends on the host species and/or cultivar as well as the nematode population. General symptoms include reduced root systems, leaf chlorosis, overall stunting of host plants, and reduced yields and plant longevity. Female nematodes and their eggs are often visible when plant roots are viewed under a dissecting microscope. SCN can move through the soil only a few inches per year on its own power. However, nematodes can be easily transported by farm equipment, irrigation, flood or drainage water, animals and humans, and dust storms spread nematodes in local areas, while over long distances nematodes are spread primarily with farm produce and nursery plants. The primary crop recommended to be rotated with cotton for managing R. reniformis in the southeast region is corn. One growing season in corn can reduce R. reniformis populations by 90%. Other measures include sanitation practises, planting resistant cultivars, weed control, biocontrol and chemocal control.
Tylenchoidea - Tylenchulidae - Tylenchulinae - Tylenchulus - Tylenchulus semipenetrans
Common name: citrus nematode
Sexually dimorphic. Immature female migratory, wormlike, small, under 0.5 mm long. Males and male juveniles do not feed; female juveniles are ecto- and endoparasitic on roots. Second-stage juveniles hatch from eggs. Third and 4th stage juveniles feed upon root cells of the hypodermis. The young adult females penetrate more deeply leaving the posterior one-half of the body outside the root. The exposed part becomes saccate and produces a gelatinous matrix to where eggs are deposited. Both male and female J2s are hatched from unfertilized eggs. The J2 female is a persistent stage, and has been recovered from stored soil after 2.5 years and from field soil 4 years after pulling lemon trees. Narrow host range. Citrus, olive, grape, lilac, persimmon. Mature trees can tolerate large numbers of these nematodes before exhibiting lack of vigor or decline symptoms; however, young trees grow poorly if replanted into nematode-infested soils. Aboveground symptoms include stunting, slow growth, yellowing, reduced foliage, reduced fruit size, and yield caused by damage to the root system. As for belowground symptoms, feeder roots heavily infected by the citrus nematode are slightly thicker than healthy ones and have a dirty appearance because of the adhesion of soil particles to the gelatinous matrix deposited by the female nematode on the root surface. TThe worldwide distribution of the nematode suggests that it has been spread by infested planting material. Application of preplant and postplant nematicide on citrus and grapes; using certified material, soaking bare root in hot water; phytosanitation; breeding for resistance.
Tylenchoidea - Meloidogynidae - Meloidogyninae - Meloidogyne - Meloidogyne incognita group - Meloidogyne incognita
Common name: Southern root-knot nematode; cotton root-knot nematode
When M. incognita attacks the roots of plants, it sets up a feeding location where it deforms the normal root cells and establishes giant cells. The roots become gnarled or nodulated, forming galls, hence the term "root-knot" nematode. M. incognita is sexually dimorphic. The female is 0.4-1.3 mm. long and usually embedded in root tissues which are often swollen or galled. Its body is soft, pearl white in colour and does not form a cyst. In addition to the adult and egg, there are four juvenile stages and four moults in the life cycle of M. incognita. M. incognita has a very wide host range including weeds. Above-ground symptoms exhibited by sweet potato plants include poor shoot growth, leaf chlorosis and stunting. Galling of rootlets and severe cracking of storage roots on some varieties or formation of small bumps or blisters on other varieties are important below-ground symptoms in sweet potato. There may also be brown to black spots in the outer layers of flesh which are not evident unless the storage root is peeled. Presence can be diagnosed by the pearl-like swollen female nematodes in flesh of storage roots or in fibrous roots, within the galls or dark spots. Juveniles hatched disperse horizontally from plant to plant only slowly. Long-distance dispersal is largely passive and by chance including movement of soil on equipment and plant parts, crop transplants, water, animals and contaminated containers such as burlap bags. Nematicides have been very important in control of this nematode. They are, however, less effective when nematodes are embedded in plant tissue. Other management methods are planting resistant cultivars and crop rotation, addition of organic fertilizers, use of trap crops and antagonistic crops.
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Organism Disease Transmission Management
Aphelenchoidoidea - Aphelenchoididae - Aphelenchoides - Aphelenchoides besseyi
A. besseyi has been found in upland or irrigated rice in many rice growing countries in Asia, Tropical America, USSR and Africa. May reproduce parthenogenetically. Has a slender body 0.44-0.84 mm long and 14-22 μm wide. At optimum temperature 21-25°C, the life cycle takes about 10 days to complete and there are several generations in a season. Up to 14 nematodes, mostly pre-adults, have been found on a single seed, where they remain coiled up inside the palea. The grain itself is never invaded. The nematode can retain viability for 2-3 years on dry grain, but dies in 4 months on grain left in the field; the nematode is not thought to survive long periods in the soil between crops. The main host plants are strawberries and rice. Causes white-tip disease of rice. Feeding at leaf tips in rice results in whitening of the top 3-5 cm of the leaf, leading to necrosis. There is also distortion of the flag leaf that encloses the panicle. Symptoms may be confused with calcium and magnesium deficiency. In the seed-bed, emergence of severely infected seedlings is delayed and germination is low. Yields of rice may be reduced 50%. The pest is liable to be carried on rice seed and accompanying chaff, and also on strawberries and other host plants. Although there is some risk of injury, the most effective means of controlling A. besseyi is through seed treatments by hot water (at 55-60 °C for 15 minutes). Chemical treatments of seed have also been reported as being effective with timing of field applications of chemicals is critical for the success. Preventative measures include using nematode-free seeds and planting in nematode-free fields. Planting of resistant cultivars have strongly reduced yield losses, however, resistant varieties may not be compatible with production practices or market requirements.

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