J Nematod
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Stem and Bulb Nematode 535 Control
Root-lesion nematodes can b e st b e controlled by overall or row treatment of the soil with nematocides before the crop is planted.
D-D, E D B, and Brozone give good control of these nematodes, but they usually fail to eradicate them completely.
In hot and dry climates fairly good control of lesion nematodes can be achieved by summer fallow, which reduces nematode populations by exposing them to heat and drying and by eliminating host plants.
Control through crop rotation is at present rather unsuccessful d ue to the w i de host ranges of the lesion nematodes and to the lack of infor-mation on their precise host preferences.
Selected References
C h a p m a n, R. A. 1958. T h e effect of root-lesion n e m a t o d es on the growth of r e d clover a nd alfalfa u n d er g r e e n h o u se conditions. Phytopathology 48: 5 2 5 - 5 3 0 .
D i c k e r s o n, O. J., Ç . M. Darling, a nd G. D. Griffin. 1964. Pathogenicity a nd population trends of Pratylenchus penetrans on potato a nd corn. Phytopathology 54: 3 1 7 - 3 2 2 . G o o d, J. M., L. W. B o y l e, a nd R. O. H a m m o n s. 1958. S t u d i es of Pratylenchus
brachy-urus on p e a n u t s. Phytopathology 4 8 : 5 3 0 - 5 3 5 .
Mountain, W. B., a nd Z. A. Patrick. 1959. T h e p e a ch replant p r o b l e m. V I I. T h e p a t h o g e-nicity of Pratylenchus penetrans. Can. J. Botany 37: 4 5 9 - 4 7 0 .
Odihirin, R. Á., a nd W. R. J e n k i n s. 1965. Host-parasite relationship of Impatiens bal-samina a nd certain n e m a t o d e s. Phytopathology 55 : 7 6 3 - 7 6 6 .
Parker, K. G., a nd W. F. Mai. 1956. D a m a ge to tree fruits by root lesion n e m a t o d e s.
Plant Disease Reptr. 40: 6 9 4 - 6 9 9 .
Pitcher, R. Á., Æ. A. Patrick, a nd W. B. Mountain. 1960. S t u d i es on the host-parasite rela-tions of Pratylenchus penetrans ( C o b b) to a p p le s e e d l i n g s. I. Pathogenicity u n d er sterile conditions. Nematologica 5: 3 0 9 - 3 1 4 .
R o h d e, R. A. 1963. L e s i on n e m a t o de injury to carrot. Phytopathology 53: 8 8 6 - 8 8 7 (abstr.).
Stem and B u lb N e m a t o de Occurrence and Importance
T h e stem and b u lb nematode is worldwide in distribution but is particularly prevalent and destructive in areas with temperate cli-mate. It is one of the most destructive plant-parasitic nematodes. It attacks a large n u m b er of host plants including onions, narcissus, hy-acinth, tulip, oats, rye, alfalfa, red clover, strawberry, and phlox.
Al-though all these hosts are attacked by nematodes belonging to one species, different populations or biotypes of the stem and bulb nema-tode have certain host preference s and, when transferred to them, will attack one or a few of these hosts but not the others. On most crops it causes heavy losses by killing seedlings, dwarfing plants, destroying bulbs or making them unfit for propagation or consumption, by caus-ing the development of distorted, swollen and twisted stems and foli-age, and, generally, reducing yields greatly.
Symptoms
In fields infested with stem and bulb nematodes, e m e r g e n c e of seedlings such as onion is retarded and stands are r e d u c ed considera-bly. Half or more of the emerging seedlings may b e diseased, appear-ing pale, twisted, arched, and with enlarged areas along the cotyle-don. Cotyledons are usually puffy with the epidermis cracked in a lacelike fashion. Most infected seedlings die within 3 weeks from planting and the remainder usually die later.
Whe n bulbs are planted in infested soil, symptoms begin to appear on the developing plants within about 3 weeks and include stunting, light yellow spots, swellings ("spikkles"), and open lesions on the fo-liage. Young plants and sprouts d e v e l op swellings on the stem and foreshortening and curling of the leaves. Many outer leaves may show a flaccid condition instead of or along with " s p i k k l e s ," their leaf tips may show necrosis or dieback, and the older leaves may b e so weak-e n weak-e d that thweak-ey cannot maintain thweak-eir weak-erweak-ec t growth and thweak-ey fall to thweak-e ground (Fig. 126A). T h e stem and the neck of the bulb b e c o me soft-ened , and the softening gradually proceeds downward into individual scales, which b e c o me soft, loose, and pale gray in color. Affected scales appear as discolored rings in cross sections of infected bulbs (Fig. 126B) and as discolored, unequal lines in longitudinal sections.
In more advanced cases large areas or the whole bulb may b e affected.
Infected bulbs may also split and b e c o me malformed or may produce sprouts and double bulbs. T h e outer scales may be loosened and de-tached by applying a little oblique pressure with the thumb on the upper half of the bulb and reveal the soft mealy, frosty-looking tissue beneath. In dry weather the bulbs b e c o me desiccated, odorless, and very light in weight. In wet seasons a soft rot d ue to secondary invad-ers sets in, destroying the b u lb and giving off a foul odor. Infected bulbs are sometimes superficially healthy, but they continue to decay in storage, where the outer scale often sloughs off exposing the lower puffy, soft scales with the characteristic frosty mealiness.
Stem and Bulb Nematode 537
F i g. 126. (A) O n i on plants infected with the s t em a nd b u lb n e m a t o de (Ditylenchus dipsaci) a nd s h o w i ng the prostrate condition, thin stand, stunting, a nd m a ny d e ad outer leaves. (B) Narcissus b u l bs s h o w i ng c o m p l e te or i n c o m p l e te brown rings c a u s ed by infection by the stem a nd b u lb n e m a t o d e. (Photos by courtesy of the D e p a r t m e nt of Plant Pathology, Cornell University.)
The Pathogen: Ditylenchus dipsaci
T h e nematode is 1.0-1.3 m m long and about 30 μ in diameter (Fig.
127). Its cylindroid body is marked by faint transverse lines, about 1 μ apart. T h e tail of both males and females is acute. E a ch female lays 2 0 0 - 5 00 eggs. T h e first molt occurs in the egg. Second-stage larvae e m e r ge from the e g g and quickly undergo the second and third molt
Fig. 127. Disease cycle of the stem and bulb nematode Ditylenchus dipsaci.