Tylenchulus semipenetrans

Citrus nematode
Scientific classification
Kingdom:	Animalia
Phylum:	Nematoda
Class:	Secernentea
Subclass:	Diplogasteria
Order:	Tylenchida
Superfamily:	Criconematoidea
Family:	Tylenchulidae
Subfamily:	Tylenchulinae
Genus:	Tylenchulus
Species:	T. semipenetrans
Binomial name
Tylenchulus semipenetrans

Tylenchulus semipenetrans (Citrus nematode, Citrus root nematode) is a plant pathogenic nematode and the causal agent of slow decline of citrus. T. semipenetrans is found in most citrus production areas and diverse soil textures worldwide. Their feeding strategy is semi-endoparasitic and has a very narrow host range among commonly grown crops. These nematodes are considered as major plant-parasitic nematode because they can cause 10-30% losses reported on citrus trees. They also parasitize other hosts such as olive, grape, persimmon and lilac. The citrus nematode was first discovered in California in 1913 by J.R. Hodges and was later described and named by Nathan Cobb that year.

T. semipenetrans is the only species of Tylenchulidae that are economically important to agriculture. Citrus nematode range from 0.25-0.35 mm long. They have an amalgamated procorpus and metacorpus, distinct isthmus, and a bulb-shaped postcorpus. They are distinct in juveniles. Both the juvenile stages and the adult male stage are vermiform in shape. The male has significantly reduced esophagus and stylet. The posterior end of the female citrus nematode becomes swollen upon feeding. She contains a single ovary and the vulva is subterminal. The female will lay up to 100 eggs deposited in a gelatinous matrix secreted from the nearby excretory pore. The pore is surrounded by small, irregularly shaped lobes; and the excretory duct is directed forward. The rectum and anus are atrophied or absent; non-functional.

The life cycle of the female citrus nematode is 6–8 weeks long, whereas the male citrus nematode only lives for about 7–10 days. These nematodes reproduce by amphiximis and parthenogenesis. The first-stage juvenile (J1) undergo one molt while still in the egg. The J1 has no stylet. The second-stage juveniles (J2) hatch from the eggs and the sex can be distinguished at this stage. The J2 male is short and fat. Juveniles will undergo two more molts into the J3 and J4 before becoming young adults. The citrus male nematodes are required for reproduction with females when their posterior end is exposed on the root surface. The J2 male has a stylet while the J3 and J4 have a weaker stylet. The J2 female is longer and thinner than males and they do not molt until feeding site is established. The female juveniles begin feeding ectoparasitically on epidermal root cells. It is not until the female citrus nematode becomes a young adult that she becomes the infective stage. The anterior end of the young female penetrates into the cortex of the root and begins feeding on 3-6 nurse cells. This intense feeding by the adult female will cause the posterior end to enlarge outside the root and start producing eggs. After fertilization, the female lays its eggs outside of the root in a gelatinous matrix extruded from excretory pore located near the vulva.

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Wikipedia