Bacterial wilt caused by Ralstonia solanacearum has affected the potato industry in South Africa since 1914. Control of bacterial wilt is difficult and depends greatly on integrated management strategies. Little information is available on the longevity of R. solanacearum in soil under South African conditions and how it is influenced by cultural practices. Information regarding local weeds which can serve as alternative hosts is limited. Soil survival of R. solanacearum biovar 2 (race 3) was investigated in an artificially infested field managed by either one of four cultural practices, namely maize and potato monoculture, bare and weed-fallow. After a five-year period wilting of indicator plants was observed in all treatments with potato the highest, followed by bare-fallow, maize monoculture and lastly weed-fallow. Results demonstrated a greater ability of biovar 2 to survive in soil than generally accepted. Subsequently, the susceptibility of 22 weed and three grass species was investigated in the greenhouse. Five species were susceptible to biovar 2 and 13 to biovar 3. Preliminary in vitro studies conducted to determine suppressiveness of some weeds/grasses, indicate that microbial activity associated with some weeds could be involved in suppression of the wilt organism. Further studies are however required. The effect of maize on Ralstonia solanacearum populations was evaluated in a pot trial as well as in hydroponic culture. Results indicated that microbial populations present in the maize plant, could play a role in the susceptibility of maize to bacterial wilt infection. Antagonistic bacteria associated with some maize plants or with the maize rhizosphere could be partly responsible for suppression of wilt. / Dissertation (MSc Agric (Agronomy))--University of Pretoria, 2006. / Plant Production and Soil Science / unrestricted

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Thesis (Ph. D.)--Oregon State University, 2007. / Printout. Includes bibliographical references (leaves 192-209). Also available on the World Wide Web.

Two field experiments were conducted in Arizona in 1998 at two locations (Maricopa and Marana). The Maricopa experiment has been conducted for nine consecutive seasons, the Marana site was initiated in 1994. The purposes of the experiments were to validate and refine nitrogen (N) fertilization recommendations for Upland cotton. The experiments each utilized N management tools such as pre-season soil tests for NO3 --N, in-season plant tissue testing (petioles) for N fertility status, and crop monitoring to ascertain crop fruiting patterns and crop N needs. At each location, treatments varied from a conservative to a more aggressive approach of N management. Results at each location revealed a strong relationship between the crop fruit retention levels and N needs for the crop. This pattern was further reflected in final yield analysis as a response to the N fertilization regimes used. The higher, more aggressive, N application regimes did not benefit yields at any location. In 1998, fruit retention levels were low and crop vigor was high. As a result, even slight increases in N fertilization and crop vigor translated into lower yield.

Field experiments were conducted during the 1996 and 1997 growing seasons in Mohave Valley to determine the effect of late season nitrogen (N) fertilizer applications on top crop yield potential of upland cotton. A long season production system utilizing late season nitrogen (N) applications through peak bloom (August) was compared to a short season production system in which N was applied through mid-bloom (June). Mid-season N applications were based on UA guidelines utilizing plant mapping and petiole nitrate data for the short season production system.

Two studies were conducted in 1996 and 1997 at the University of Arizona Maricopa Agricultural Center (MAC) to evaluate the added nitrogen interaction (ANI) or ‘priming effect’ on the determination of nitrogen recovery efficiencies (NRE). The method employed was to compare NRE’s as calculated by two different methods; the difference technique and the isotopic technique. The difference in NREs observed between the two methods indicates the extent of an ANI. Results demonstrated no statistical differences between NRE’s calculated by the two methods. Therefore, no ANI was observed in the field. These results indicate that the less expensive method of calculating NREs (difference technique) is sufficient under irrigated cotton production systems in the desert Southwest.

Three different nitrogen fertilizer regimes were practiced in this study along with an unfertilized check. The same amount of nitrogen fertilizer was sidedressed in the plots in one, two or three applications. No significant differences were seen, but the trends looked like the split applications might have had some advantage.

Various field experiments were conducted during the 1997 and 1998 cotton season involving macro and micronutrient fertilization. A total of six experiments were conducted at various locations in Arizona. Each of the field experiments studied the effects of different nutrients and nutrient combinations on both Upland and Pima varieties. The purpose of these experiments were to evaluate University of Arizona fertility guidelines with respect to soil test results and to possibly fine-tune or calibrate these guidelines for common Arizona soils and cotton growing regimes. Results from these experiments based on soil test information, quantitative plant measurements, and lint yield showed no significant difference due to treatments for all the studies except for a phosphorus study conducted in Graham County.

Micronutrient fertilizers including zinc, boron, copper, and manganese in their sulfate forms were broadcast applied and incorporated preplant to determine their effects on lint yield of upland cotton.

A single field study was conducted on a sodium-affected soil at the University of Arizona’s Maricopa Agricultural Center (MAC) in 1998. NuCotn 33B was dry planted and watered-up on 5 May 1998. Two treatments were evaluated; treatment 1 received no acid and treatment 2 received water-run acid applications. The acid used in this evaluation was sulfuric acid (H₂SO₄). The acid was applied at approximately 11 gallons acid/acre at each scheduled irrigation throughout the entire growing season. All other agronomic inputs and decisions were uniformly applied to both treatments in the same manner throughout the season. The experiment was arranged in a randomized complete block design with two treatments and six replications. Significant differences were found among the two treatments in terms of plant growth and soil water content (P<0.05). Lint yields were significantly different (P=0.0013) with the check having the highest yield.