Field resistance

Resistances in and of electrolytes are exclusively measured with low or audio frequency ac so as not to falsify the results with polarization effects. Measurement is mainly by four electrodes, which eliminates voltages due to the grounding field resistances of the measuring electrodes. 

Amplifier gain K2 = 3.5 Servomotor constant = 15 Nm/A Field resistance = 20H Gear ratio = 5... 

Ebbesen[4] was the first to estimate a conductivity of the order of lO fim for the black core bulk material existing in two thirds of tubes and one third of nanoparticles. From this observation, it may naturally be inferred that the carbon arc deposit must contain material that is electrically conducting. An analysis of the temperature dependence of the zero-field resistivity of similar bulk materials[14,15] indicated that the absolute values of the conductivity were very sample dependent. 

Above 2 K, the temperature dependence of the zero-field resistivity of the microbundle measured by Langer et al. [9] was found to be governed by the temperature dependence of the carrier densities and well described by the simple two-band (STB) model derived by Klein [23] for electrons, , and hole, p, densities in semimetallic graphite ... 

Mooi, J.C. (1965). Experiments on testing field resistance to Phytophthora infestans by inoculating cut leaves of potato varieties , European Potato Journal, 8, 182-183. 

However these compounds either singly or in combination do not account for the total lack of feeding activity observed either in the laboratory or in the field. Resistance of S. nemoralis to Insect attack seems to Involve, as is expected, not a single class of compounds but a multichemical response. Other compounds isolated from nemoralis showing antifeedant activity to Trirhabda canadensis are at present under investigation. 

Different lines, each with Insect resistance, may possess different ratios of antibiotic compounds. Thus, It may be possible to Increase resistance by crossing lines where each contributes genes for biosynthesis of different antibiotic compounds. The tobacco budworm was selected for study In preference to the cotton bollworm because It Is easier to rear and use In the laboratory, Is more resistant to Insecticides In the field, and It Is approximately as susceptible to cotton constituents Incorporated In laboratory diets (14). This present study was carried out to Identify and analyze for cotton constituents that were toxic In laboratory feeding tests, and to determine whether there were positive correlations of their content In leaves and/or other tissue with field resistance. From this Information, the generation of lines with multiple factors for resistance could be Initiated. 

At about the same time, the first information was received about product failure after continuous and exclusive use. The investigation of the pathogen populations of such fields yielded highly resistant strains (28, 29, 40), whereas so far no field resistance has been reported for cymoxanil or fosetyl. Thus, in contrast to the favorable results of a broad range of model studies, resistance had appeared very fast under field conditions. The lesson to be learned from this experience is that results of model studies have to be used with caution. Model studies must include the use of chemical mutagens and highly active, systemic fungicides should be used as if a risk of resistance exists until their mode of action is known. 

Laboratory tests with M. fructicola (26) and field experience with the dicarboximides for Botrytis control in strawberry fields (14) suggest fungicide resistance may be a problem if the dicarboximide fungicides are used extensively and continuously season after season. Although many workers consider dicarboximide resistant strains less fit than wild-type strains ( 1 6, 17 ), the research suggests that field resistance will develop if steps are not taken to... 

Obviously, one can not precisely quantify the elements of Figure 8. It is nevertheless highly indicative. Resistance of a practical nature is rarely, if ever, encountered in normal field practice by the sulfenimides (or for example, the dithio-carbamates, Figure 9) - insoluble, multisite protectives. The water soluble single site and variably translocatable antibiotics, benzimidazoles, oxathiins, etc., etc., all exhibit degrees of field resistance. As far as this author knows, this analysis can be extended over the whole class of plant protection fungicides. 

Figure 8.8. The zero-field resistivity of powder samples of Lao.ssCao.asMnOs are highly dependent on the grain size. (After Fontcuberta et al. (1999), Nano-Crystalline and Thin Film Magnetic Oxides. Nedkov and Ausloos (eds.) Kluwer Academic Publishers. Reproduced with permission.)...

The elucidation of the primary site and biochemical mode of action of inhibitors is often difficult and not necessarily associated with the mechanism of resistance in field isolates. Nevertheless, information on the mechanism of resistance can provide evidence to determine the site of action. Therefore, the classification of fungicides is based on crossresistance reactions rather than chemical similarities of structures or proposed modes of action (Table 1). Based on available information in the literature, three categories of inhibitor classes can be made Classes with known mode of action and known mechanism of resistance, classes with proposed mode of action and unknown mode of resistance but wide-spread field resistance, and classes in which resistance is claimed to occur in the field but both mode of action and resistance are not known. 

In H. virescens, loss of expression of a cadherin-like protein was found to be associated with CiylA toxin resistance and consequently this protein plays a crucial role in B. thuringiensis toxicity in vivo [149]. In the laboratory-selected resistant H. virescens strain YHD2, a retrotransposon insertion in the cadherin gene was linked to high levels of CtylAc resistance. More recently, disruption of a cadherin gene may also be linked to the development of field resistance to CiylA toxins in the pink boUworm, Pectinophora gossypiella [150]. 

In fact, the clearing or field is itself a powerful selector for resistance. Even if the cultivator were to randomly choose the seed stock for the next season or, for that matter, leave the crop standing in the field to reseed itself, the resistance of next year s crop will increase, in a phenomenon called field resistance. Whichever landraces (including random crosses and mutants) do best over time against pests, adverse weather, and so on will contribute, willy-nilly, more of their seed to the subsequent season s crop. See Harlan, Crops and Man, pp. 117-33. 

Peanut phytoalexins appear to be involved in resistance to drought-induced preharvest aflatoxin contamination of immature peanuts. Mature peanuts are considerably more resistant to environmentally-induced preharvest aflatoxin contamination of peanuts than are immature peanuts. The mechanism of this latter resistance is unknown. The identification of this resistance mechanism and other resistance may provide one approach to subsequent use of biotechnology to incorporate field resistance traits into commercially acceptable varieties. Biotechnology may also be a valuable approach to exploiting genetic resistance to preharvest aflatoxin found in wild species that have evolved in an arid environment. 

The use of biotechnology to incorporate phytoalexin-based genetic resistance from wild species that have evolved in an arid environment into commercially available varieties is a potentially viable approach to developing field resistance against A. flavus invasion and subsequent aflatoxin production in peanuts. 

Since the 1950 s, field resistance in Heliothis spp. has been reported for nearly every chemical used to protect cotton (1-9). 

In January of 1984, following the documentation of field resistance in fl. armioera in Australia, seven prominent entomologists (F. L. Carter, D. F. Clower, J. R. Phillips, F. w. Plapp, H. T. Reynolds, R. T. Roush, T. C. Sparks) from major cotton producing states issued a warning about the potential for pyrethroid resistance in Heliothis spp.. It was a timely statement. 

In the dicarboximldes the situation is again different. In vitro and in vivo experiments easily yield resistant strains (6), but in practice some populations of the major pathogen, Botrvtis cinerea, showed extreme fluctuations in sensitivity from year to year. Only recently has field resistance reached critical levels in some regions of intensive use (2) ... 

The occurrence of the first field resistance phenomena of mildew towards azoles provoked the question of the molecular origin of this resistance. Again we had to trust in the reliability of our model systems, biochemical studies at the molecular level with resistant mildew strains being practically impossible. 

Field resistance Significant shifts in sensitivity not yet leading to loss of control ... 

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