Tylenchulus semipenetrans

Taxonomy, Common Name, Disease

Historical

First reported by J. R. Hodges in 1912 - Horticultural inspector for L.A. County. Described by N.A. Cobb in 1914. Thomas at the Citrus Expt. Station reported association with poor-growing trees. Baines at UCR worked on nematode for 30 years. Recently retired Dean of the College of Agricuture at UC Riverside, Seymour Van Gundy, was hired in 1954 to work on this nematode.

Hosts

Narrow host range. Citrus, olive, grape, lilac, persimmon. Currently, there are 4 known biotypes on various hosts; these can be distinguished by their ability to parasitize citrus rootstocks (host range test).

Distribution

Worldwide on citrus. Grapes - Australia, California and Chile. Olive - California. Occurs in approximately 95% of citrus in California. Common on grapes in Coachella and San Joaquin Valleys.

Life Cycle

Sedentary endoparasite. Reproduction occurs by parthenogenesis, males are present but are not required; both male and female J2s are produced by unfertilized females. Second-stage juveniles hatch from eggs. These and the 3rd and 4th stage juveniles feed upon root cells of the hypodermis. The young adult females which are vermiform penetrate more deeply into the cortical parenchyma, leaving the posterior one-half of the body outside the root. The exposed part of the body becomes saccate and a gelatinous matrix is produced. Egg masses contain about 100 eggs deposited in the matrix, these hatch and the second-stage juveniles attack roots. The feeding site consists of 8-10 nurse cells which have thick walls, a large nucleus and nucleolus. The area may become invaded by other micro-organisms. Males do not penetrate roots and they are about the same size as the second-stage juveniles. The male passes through 3 molts without feeding, and the stylet becomes progressively less distinct; males reach maturity in one week. 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.

Symptoms-Pathogenicity

Trees heavily infected with citrus nematode exhibit symptoms of "slow decline", dieback of small branches and yellow foliage. Size of fruit is reduced. There are reduced numbers of feeder roots. Feeding causes the formation of necrotic areas and cells are killed. Heavy infections cause disintegration of the cortical parenchyma cells and the feeder roots eventually are killed. Root destruction causes plant decline over 3-5 years. Nematodes occur in very high numbers. Female body outside root swells, and eggs are produced in a gelatinous matrix (the soil adheres to matrix, causing a "dirty root" symptom).

Management

Preplant nematicides. Postplant nematicides on citrus and grapes. Avoidance - clean planting stock, use of certified material, bare root dips in hot water (45 C for 25 minutes) effective. Prevent spread - give attention to machinery, planting stock, and irrigation water. Resistant rootstocks - Poncirus trifoliata - trifoliate rootstock, Troyer citrange.

Characteristics

Tylenchulus - In all stages the excretory pore is anterior, 68-85% of the body length with a prominent duct directed anteriorly. Sexually dimorphic. Immature female migratory, vermiform, small, under 0.5 mm long. Lip region continuous with body contour; head skeleton weak, stylet about 13 um long moderately developed with small basal knobs. Esophagus with prominent, valved, median bulb, a narrow isthmus and small basal bulb. Vulva very posterior, about a body width behind the excretory pore; gonad single outstretched anteriorly. Tail end conical, anus and rectum not visible. Mature female a sedentary endoparasite; the anterior third of the body embedded in root tissue; posterior body swollen, irregular in shape, vulva and excretory pore with prominent lips, excretory cell well developed producing a gelatinous matrix. Male body short and slender, stylet and esophageal region much weaker than female; spicules narrow, ventrally curved; tail elongate, conical without a bursa. Males and male juveniles do not feed; female juveniles are ecto- and endoparasitic on roots.

References

  1. Evans, K., D.L. Trudgill, and J.M. Webster. 1993. Chapter 1. Extraction, Identification and Control of Plant Parasitic Nematodes. in Plant Parasitic Nematodes in Temperate Agriculture. CAB International, UK. 648 pages.
  2. Siddiqi, M.R. 1974. Tylenchulus semipenetrans. C.I.H. Descriptions of Plant-parasitic Nematodes. Set 3, No. 34lus semipenetrans. C.I.H. Descriptions of Plant-parasitic Nematodes. Set 3, No. 34. Commonwealth Institute of Parasitology. C.A.B. International. 4 pages.