The logic theory machine as a theory of human problem-solving

In 1956 A. Newell and H.A. Simon (with the aid of J.C. Shaw) published the first paper on the Logic Theory Machine (L.T.). In effect, L.T. was a computer program that proved theorems in propositional logic

SURVIVAL MECHANISMS OF PHYTOPATHOGENIC BACTERIA

Natural habitats usually do not provide bacteria the continuity of agricultural crops. With continuous culture. perpetuation of pathogen is no problem. Although agricul­ tural practices provide some discontinuity between crops. it is less than that in nature. Uniformity of crop germ plasm also favors inoculum buildup and perhaps perpetuation of the pathogens

Morphological and molecular characterisation of Scutellonema species from yam (Dioscorea spp.) and a key to the species of the genus

The yam nematode, Scutellonema bradys, is a major threat to yam (Dioscorea spp.) production across yam-growing regions. In West Africa, this species cohabits with many morphologically similar congeners and, consequently, its accurate diagnosis is essential for control and for monitoring its movement. In the present study, 46 Scutellonema populations collected from yam rhizosphere and yam tubers in different agro-ecological zones in Ghana and Nigeria were characterised by their morphological features and by sequencing of the D2-D3 region of the 28S rDNA gene and the mitochondrial COI genes. Molecular phylogeny, molecular species delimitation and morphology revealed S. bradys, S. cavenessi, S. clathricaudatum and three undescribed species from yam rhizosphere. Only S. bradys was identified from yam tuber tissue, however. For barcoding and identifying Scutellonema spp., the most suitable marker used was the COI gene. Additionally, 99 new Scutellonema sequences were generated using populations obtained also from banana, carrot, maize and tomato, including the first for S. paralabiatum and S. clathricaudatum, enabling the development of a dichotomous key for identification of Scutellonema spp. The implications of these results are discussed

Fecal Bacteria in Agricultural Waters of the Bluegrass Region of Kentucky

Agricultural runoff influenced by nonpoint pollution frequently exceeds the USEPA standards for bacterial contamination of primary contact water (200 fecal coliforms/100 mL). Few studies have evaluated the effect of cattle (Bos taurus) grazing on fecal contamination of ground water in the karst topography of central Kentucky. Our objectives were to: (i) observe the extent and pattern of fecal bacteria in agricultural waters from two central Kentucky watersheds; (ii) determine if monthly sampling accurately assessed the extent and variability of fecal contamination; and (iii) assess the fecal coliform/fecal streptococci ratio (FC/FS) as an indicator of fecal bacteria source. Springs, streams, and wells in two agricultural watersheds typical of central Kentucky were monitored for fecal coliform and fecal streptococci from December 1991 to January 1993. Springs and wells exceeded primary contact water standards, between 28 and 74% of the time; streams exceeded water quality standards between 87 and 100% of the time. When fecal bacteria were present, rainfall rapidly moved them from the soil surface into spring and well water. At two springs in Fleming county, only 29% of samples exceeded primary contact standards before cattle were present; 80% exceeded standards after cattle began grazing the surrounding pasture. Monthly sampling adequately reflected the extent of fecal contamination in our study, which had relatively continuous cattle grazing. Although the FC/FS ratio identified domestic animal contamination sources, it did not distinguish between domestic animal and human sources of contamination

Soil Organic Matter Fractions and Aggregate Distribution in Response to Tall Fescue Stands

The study was conducted to evaluate the influences of tall fescue management on soil organic matter fractions and macro- and microaggregate distribution. Soil samples were collected from four paired adjacent fields consisting of five years of tall fescue mono and poly stands in Western Kentucky. Soil samples from 0 to 15 cm and 15 to 30 cm soil depths were analyzed for soil organic C and N, particulate organic matter C (POM-C) and N (POM-N), macro- and micro aggregate distribution and C-associated with macro- and micro- aggregates. Significant effects were observed between stands for all the properties, except total C, microaggregates and C-associated with microaggregates. Sampling depth significantly influenced total C and N in both stands. Particulate organic matter C and N and C-associated with macroaggregates and the amount of macroaggregates were strongly affected by tall fescue management. This confirmed the hypothesis that early changes in soil properties were reflected in labile C and N fractions and soil structure. Tall fescue mixture stands had 44% higher POM-C, 50% higher POM-N, 26% more macroaggregates and 33% more C-associated with macroaggregates compared to the tall fescue mono stands at the soil surface of 0 to 15 cm

Particle Size and Temperature Affect Fecal Bacteria Survival in Sediment

When cattle have direct access to streams, fecal bacteria concentrations in stream sediments increase. If these bacteria persist, and if the sediments are resuspended, fecal bacteria may also appear in surrounding water for extended periods. Why do fecal bacteria persist, since dry conditions, high acidity or alkalinity, sunlight, competition from native microbes, and extreme temperatures all diminish their populations in soil? The effects of these environmental factors are much reduced in sediment. Water protects fecal bacteria from desiccation and ultraviolet light. High temperatures can promote their regrowth in wet environments. Fecal bacteria also survive on fine-sized sediments in streams because the sediments have a high surface area. These factors may help explain our observations that streams flowing through pastures typically exceed Kentucky standards for primary contact water (200 fecal coliforms/100 ml) long after cattle depart. The fecal coliform/fecal streptococci ratio (FC/FS), is a tool in water quality assessment that diagnoses the source of fecal contamination, whether from people (FC/FS \u3e 4) or animals (FC/FS \u3c 0.1). The ratio is extremely variable and sensitive to the persistence of the indicator bacteria used in it. For example, we observed in central Kentucky streams that as the temperature increased during spring, the FC/FS ratio also increased. Fecal coliform growth shortly after manure deposition might explain some of the variability we have observed in our water monitoring studies. In this study we tried to account for the seasonal variability of FC/FS ratios in agricultural watersheds, and determine whether sediment particle size and water temperature interacted to influence fecal bacteria persistence and the FC/FS ratio

Soil Organic Matter Fractions and Aggregate Distribution in Response to Tall Fescue Stands

The study was conducted to evaluate the influences of tall fescue management on soil organic matter fractions and macro- and microaggregate distribution. Soil samples were collected from four paired adjacent fields consisting of five years of tall fescue mono and poly stands in Western Kentucky. Soil samples from 0 to 15 cm and 15 to 30 cm soil depths were analyzed for soil organic C and N, particulate organic matter C (POM-C) and N (POM-N), macro- and micro aggregate distribution and C-associated with macro- and micro- aggregates. Significant effects were observed between stands for all the properties, except total C, microaggregates and C-associated with microaggregates. Sampling depth significantly influenced total C and N in both stands. Particulate organic matter C and N and C-associated with macroaggregates and the amount of macroaggregates were strongly affected by tall fescue management. This confirmed the hypothesis that early changes in soil properties were reflected in labile C and N fractions and soil structure. Tall fescue mixture stands had 44% higher POM-C, 50% higher POM-N, 26% more macroaggregates and 33% more C-associated with macroaggregates compared to the tall fescue mono stands at the soil surface of 0 to 15 cm