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Life Cycle

One of the unique features in the life cycle is the pre-adult or fourth stage larva, sometimes known also as 'eelworm wool' or 'nematode wool’ stage which is extremely resistant to desiccation. It may retain viability under dry conditions for many years. Although, other larval stages are capable also of causing infection to crops, under favorable conditions, the pre-adult .fourth stage) stage is the most important stage in the life history of the nematode.

The fourth-stage larva can enter into an anabiotic phase under adverse conditions. The anabiotic fourth-stage larva loses more than. 99 per cent of its body water yet maintains considerably reduced reversible metabolism". Recent studies on ultra structural changes in anabiotic larvae of the nematode explain the mechanism which aid in survival of D. dipsaci™. The anabiotic larvae maintain their struc­tural integrity despite decrease in thickness of cuticle because of packing of layers and condensation of sacroplasm of muscle cell. Following rehydration, there is a lag phase of 2-3 hours during which the animal undergoes repair before recovery. The lipid droplets within intestinal cell coalesce and the cuticle increases in thickness. The sacroplasm expands with increase in spacing of thick filaments. The mitochondria swell, become more spherical and the cristae become less distinct. In spite of impaired mitochondrial function and low ATP levels, the initial phase of rehydration is accompanied by a burst of metabolic activity. It can be said that the success of the ana­biotic larvae to survive and resume normal activity is due to their ability to undergo repair which may involve restoration of membrane damage, re-establishment of the ionic gradients essential for normal' muscle and nerve function, prior to recovery.

Among the plant parasitic nematodes, D. dipsaci perhaps shows greatest extent of physiological variations. There are more than twenty biological races, differing in their host range. The races have been named after the preferred hosts¹¹. At least 11 distinct races are known to occur in Europe, identifiable on the basis of host prefe­rences¹³. Races considered to be of economic importance are : garlic, onion, tulip, oat, alfalfa, red clover and rye. Several of these races, are polyphagous, capable of attacking diverse plant species, while

Others have limited host range¹¹. For example, the alfalfa, red clover and white clover races seem to be very specific for the named hosts. In contrast, rye, oat and onion races are polyphagous. The different populations of the same race may exhibit differences in host range and varied pathogenicity. The suggestion made earlier that many races are distinguishable morphologically and could represent true species, has not been supported by data from crossing experi­ments. It is possible to have successful crossings between a number of these biological races and their progeny could exhibit different host preferences.

The evolution of these races still remains in the speculative phase. Monoculture of specific host may favor certain gene combi­nations and suppress others, resulting in a change in the pathogenic potential of a population¹⁷. The race genesis in the stem and bulb nematode is further explained on the basis of the work done with a garlic isolate of D. dipsari¹⁷. Under aspectic conditions, the isolate reproduces on a wide range of host tissues, many of which are the key hosts reported for the biological races of the nematode. It has been theorized, on the basis of these observations, that the nominal species, D. dipsaci gene pool can be considered as wild type and polyphagous, with race deviations as part of gene continuum.

Temperature, moisture conditions, soil texture and accompany­ing plant growth are important soil factors and determine the activity of the nematode under natural environment, ft is likely that all these factors are inter-related and their resultant interactions determine the survival, persistence and activity of the nematode under field condi­tions. The desiccated and anabiotic pre-adult nematodes are capable to resist temperature extremes- It has thus become rather essential to moist the anabiotic larvae, residing inside propagation bulbs, in order to achieve effective control of the nematode, since they exhibit grea­ter tolerance to heat than their host plants¹⁸"". In contrast, the nematode has been demonstrated to survive low temperature of —80°C^a3. About 16-24 per cent of anabiotic larvae can survive after exposures to —196°C for 24 hours or —150°C for 18 mouths²⁴.

A temperature range of 20-25°C has been recorded as the opti­mum for raising D. dipsaci and symptom expression in alfalfa seed-

lings under laboratory conditions. The optimum temperature for reproduction of the nematode on many plant species is reported to be I9°C^S7. The degree of mobility, symptom expression, extent of host tissue damage and resistance of host are considered to be the function of temperature at the time of infectivity²⁸-³². Fewer nematodes invade tolerant Lahonta alfalfa than susceptible cultivar Grimm at 15.5°C^a8. But the susceptibility of Lahonta increases with the elevated temperature of 25-30°C. In a similar study, it was recor­ded that Lahonta alfalfa was less resistant to D. dipsaci at '5.5 and 2I°Cthan at II°C, indicating break-down in resistance with the increase in temperature²⁸.

Soil texture is also one of the important factors that govern the persistence of the nematode under field conditions. Heavy soils which contain more than 30 per cent clay tend to stabilize the population to about 40 nematodes per 500 g soil, a number sufficiently high to damage entire field plant population³³. In contrast, sandy or light clay soils are unfavorable for multiplication of population.

The amount of moisture present in the atmosphere also plays an important role in determining the activity of the nematode. In fact) temperature is of major importance primarily in its effect on relative humidity. The fact, that with an advent of high temperature, the pathogenic symptoms on alfalfa are suppressed, reflects on the effect of thermal influence on humidity.

Other factors like light and host nutrition may influence also reproduction of D. dipsaci. Exclusion of light and applicator of calcium respectively may increase and decrease, the reproduction of the nematode.

The extent of life span of anabiotic larvae is important from dissemination point of view. Pre-adult larvae survive in dried teasel for 23 years³⁸. Marked differences in the survival ability of ihe nema­todes, recovered from different host sources have been observed also. Anabiotic larvae, from onion can survive in dry soil for two years'⁸ and a year and half from carrots, without hosts. The extreme case is' of survival for 7 years in moist soil without a host.

Freezing soils usually contain very few nematodes since there is a high mortality during severe winter.  Nevertheless,   D. dipsaci can over winter in soil as adult or fourth-stage in absence of host⁴¹. Symptoms

The symptom expressions, in infected hosts, vary with plant species involved and environmental factors. Under field conditions, the specific symptom expression of nematode infected plants of alfalfa is noticeable during re-growth, in early spring. The infected plants are stunted; frequently, have a 'halo' of dead stem around them. The developing shoots of infected plants are swollen with

shorter node. The leaves on the shoots are distorted and usually, but not always, remain folded (Fig. 3). The rapidly growing shoots may carry the nematode with them as a result of which swelling may be seen high up the stem. Nematode-infected plants, in established fields, may or may not recover with the advent of hot dry weather.

The effect of moisture, under experimental conditions, on symptom expression, is rather remarkable. In alfalfa, the nematode enters the emerging seeding at the base of cotyledonary petioles. Subsequently, cavities are formed in the cortical parenchyma within 12 hours of infection." Characteristic swellings are observation the stem within next 24 hours. Depending on the degree of humidity that prevails in the atmosphere, the infected plants appear to be short and distorted. Continued low humidity even in the presence of sufficient moisture and moderate infection level may lead to some plant mortality within a few weeks.

Conspecific population of D. dipsaci exhibit differences in the quality and quantity of some hydrolytic enzymes but no apparent correlation can be detected between enzyme activity and differing pathogenesis¹¹'. Pectinase activity has been reported from various populations of D. dipsaci, raised on different hosts. The survival, multiplication and successful feeding in host depend on its ability to macerate host tissue through dissolution of middle lamellae enzymatically, without the death of cells. Because pectic compounds are considered to be important structural components of middle lamellae, it is presumed that pectolytic enzymes are instrumental as well as essential for dissolution of middle lamellae, which, in the process, can function as a factor for pathogenesis. A variety of pectolytic enzymes from D. dipsaci has been reported from different laboratories. Although earlier work did not provide conclusive evidences for the involvement of the nematode to induce pathogenesis through pectolytic enzymes, convincing results⁸⁴ are now avail-

able which demonstrate the implication of pectolytic enzymes in the symptom expression of infected hosts. Presence of two pectolytic enzymes in aqueous extractions of 0 dipsaci has been demons­trated. The enzyme, identified as endopolygalacturonase (endo-PG) is responsible for the separation of the host cell and it is of nematode origin and not of host.

The pectolytic enzymes may differ qualitatively as well as quantitatively in different populations of D. dipsaci. These differences may be due to procedural techniques for assays or source of the host plant-nematode culture. Perhaps, the host specificity is related to the differences in the kind of the enzyme activity, and could induce fundamentally different mechanisms of pathogenesis.

In association with other micro-organisms, the nematode is suspected to enhance, in certain cases, severity of plant diseases. The increase in pathogenic expression of rhubarb crown rot, a bacterial disease caused by Bacterium rhaponticum, was suspected as early as 1936 because of association of D. dipsaci with the disease. It has been also reported to transmit Corynebacterium Insidiosum and pre-dispose wilt resistant alfalfa to infection. D, dipsaci and Pseudomonas fluoresces are associated also in 'Caf.au lait', a disease of garlic.

Antagonistic type of relation has been demonstrated between D. dipsaci and tobacco mosaic and tobacco rattle viruses. Such associations result in reduced reproduction of the nematode. Contrary to this, plants infected with arabis mosaic and black ring viruses influence the nematode reproduction favorably.