|
|
Class |
Secernentea |
|
Order |
Tylenchida |
|
Superfamily |
Tylenchoidea |
|
Family |
Pratylenchidae |
|
Genus |
Pratylenchus |
Asia, the Pacific,
America, and other tropical and temperate areas where sweetpotato is grown.
Pratylenchus is considered second only to Meloidogyne
in terms of plant species parasitised and extent of crop damage and loss. The
thresholds vary somewhat with species, climate, soil type and host crop but a
density of 1-2 nematodes g-1 soil at planting is a reasonable
guide; thresholds range from 0.5-1.8 g-1 soil.
In Japan the principal species is P. coffeae,
which caused significant losses, while in the U.S., P. brachyurus, is the
most common.
The nematode causes small, necrotic root lesions. Fibrous
root necrosis may lead to some stunting of vines and a significant reduction in
the quality of fleshy storage roots. Small, brown to black, necrotic lesions are
also produced on storage roots, which make the roots unmarketable.
Lesion nematodes
are small nematodes with adults, being
usually less than 1 mm long. The head region is low and flattened, with 2- 4
head annules. It has distinct head skeleton, continuous with the
body contour. The stylet is 20 µm or less, and moderately developed with
distinct basal knobs. The esophagus has a well developed median bulb, and the
posterior gland lobes overlap the intestine ventrally. The female has posterior
vulva (V= 70-80%) with a single anterior gonad and a short post vulval sac; It
is tall, cylindrical to conoid, and with two to three anal body widths long
(?). The male tail is conical with a distinct bursa that reaches the tail
tip.
This genus has the typical nematode life cycle with four
juvenile stages, and the adults in some species are parthenogenic
and in others amphimictic. The
juveniles of P. coffeae mature and differentiate within the root, and
adult females deposit eggs singly or in small groups within the tissues of the
root. Commonly, the duration of the Pratylenchus life cycle is given as
about 35-42 days, but this, considerably varies with species, e.g. P.
penetrans can range from 30-86 days (Corbett, 1973) while P. coffeae
in sweetpotato is from 30-40 days at 25-30oC to 50-60 days at 20oC.
The variability is due partly to the length of the life cycle being temperature
dependent (Corbett, 1973; Townshend and Anderson, 1976).
Lesion nematodes are migratory root endoparasites. They are
also extremely diverse in their adaptive capacity. Individual species show
special preferences for temperature. Some are less able than others to tolerate
extremes, especially cold, or the extreme drought areas, where vegetable
production is less common except under irrigation. The influence of moisture as
rain is not as significant. However, P. penetrans moves best when soil
water has drained so that 8-12% of the soil volume is air-filled and
survives a shorter time in wet than in dry soil . Preferred soil types include
coarse-textured sandy loam, silt loam or occasionally, organic soil (muck). More
P. penetrans penetrate roots in a sandy loam than in silt loam or loam.
They are less common in clays and similar fine-textured soils; adults and
juveniles of P. penetrans move farther in coarse than in fine-textured
soil. Pratylenchus may be disseminated by transportation of plant root
parts or soil, and by surface or irrigation water.
Cultural control
Crop rotation. Peanut crops resulted in significantly lower population of P.
coffeae. Paddy rice or increase fertilization with potassium reduced
severity of infection.
Addition of organic amendments. Chicken manure is very effective in reducing
the nematode.
Biological control
Paecilomyces lilacinus, a fungal egg parasite was evaluated and found
effective against Pratylenchus sp. in corn.
Chemical control
Use of soil fumigants.
Clark, C.A. and J. Moyer. 1988. Compendium of Sweet Potato Diseases. APS
Press. 74 p.
Corbett, D.C.M. 1973. Pratylenchus penetrans. CIH Descriptions of
plant parasitic nematodes. Set 2, No. 25. Commonwealth Institute of
Helminthology, St. Albans, UK.
Evans, K, D. L. Trudgill and J. M. Webster. 1993. Plant Parasitic Nematodes
in Temperate Agriculture. University Press, Cambridge. 648 p.
Filipjev, I.N. and Schuurmans Stekhoven, J.H. 1941. A manual of
agricultural helminthology. Leiden : Brill. 878 p.
Gapasin R..M. 1996. Evaluation of Paecilomyces lilacinus
(Thom.) Samson for the control of Pratylenchus sp. in corn. Biocontrol 1:
41-49.
Goodey, T. 1933. Plant parasite nematodes and the diseases they cause.
London. 306 p.
Yoshida, T. 1985.Correlation between successive yield tests for agronomic characters in
sweetpotato. Jpn. J. Breed. 35: 204-208.
Contributed
by: Ruben Gapasin |
