Population size of Clavibacter michiganensis subsp. sepedonicus in potato cv Russet Burbank and plant response as affected by drought were assessed in a greenhouse experiment. Water deficit stress and no stress treatments, and inoculum densities of 0 or 2 X 10��� cfu C. m. sepedonicus/seed piece were arranged factorially. Stem populations of C. m. sepedonicus were significantly lower in the water deficit stress treatment compared to the non-stressed treatment at every sampling date in both experiments. In seven of the eight harvests the number of C. m. sepedonicus cells/g of stem tissue for the water deficit stress treated, infected plants was a factor of 10 lower than the non-stressed treatment. Foliar symptoms of bacterial ring rot were not observed, but symptoms developed in tubers. Compared to the noninoculated control inoculum reduced aerial biomass from 12 to 21% and tuber yield from 15 to 38% in samples taken four times after the drought was terminated. Reduction of these same variables due to water deficit stress ranged from 17 to 21% and 15 to 41%, respectively, compared to the non-stressed control. Therefore, both water deficit stress and C. m. sepedonicus had similar effects on growth and tuber yield of potato. / Graduation date: 1995

Graduation date: 1994

The potential of hot water dipping, vapour heating or ultraviolet irradiation to eliminate the use of chemicals for control of sprouting and post-harvest diseases of the potato (Solanum tuberosum L.) was examined. The microorganisms on which these treatments were tested were the fungal dry rot (Fusarium solani) and the bacterial soft rot (Erwinia carotovora pv. carotovora), two major post-harvest pathogens of potatoes. The study focused on short-term storage (three months) at 8$ sp circ$C or 18$ sp circ$C, which are representative of storage temperatures used by producers in northern temperate and semi-arid tropical regions, respectively. / Response surface methodology was used in experimentation to facilitate analysis of data and identification of optimal operating conditions for the treatments. The following parameters were used to assess tuber quality after the treatments and 3-month storage: firmness, color and structure. / It was found possible to obtain 100% control of sprouting and diseases for the three-month storage without resorting to the use of chemicals. This was achieved without significantly altering the quality attributes of the tubers under certain conditions of hot water, ultraviolet radiation or combinations of these two with storage at 8$ sp circ$C for three months. Although 100% control was not possible for the storage at 18$ sp circ$C, treated tubers performed much better than the controls. Vapour heat (50-70$ sp circ$C) was much less effective at controlling sprouting and was therefore not tested on the pathogens. / A numerical model of the heat transfer phenomenon in the tuber was also developed. It was used to predict the transient temperature distribution in the tuber. The model was solved using the line-by-line technique and model simulations were validated against experimental data.

Rhizoctonia solani (AG-3), the causal agent of Rhizoctonia disease of potato, overwinters as sclerotia on potato tubers. To develop a biocontrol strategy based on the prevention of the sclerotial germination, an isolation of microorganisms colonizing sclerotia of infected potato tubers (cultivars Norland, Atlantic and Souris), was conducted. In vitro screening was used to select effective antagonistic fungi against Rhizoctonia solani. Fifty fungal isolates were selected in order to cover all identified genera and potato variety and examined for their ability to inhibit germination of sclerotia which were incubated with the test fungus for 14 days. Twenty-four (24) fungal isolates were retained based on their ability to reduce sclerotial viability by more than 50% as compared with 100% viability of untreated sclerotia. These 24 isolates were further examined for their ability to protect Table beet seedlings against the pathogen in greenhouse soils. Based on their ability to protect Table beet seedlings from Rhizoctonia infections and to increase the number of secondary roots and root length isolates, F2, F11, F132, F158, and F258 were screened and test their efficacy to increase beet seed germination in field soils. (Abstract shortened by UMI.)

Field experiments were conducted during the 1989 dry season (July to December) at the Manggoapi Farm of the Faculty of Agriculture, Cenderawasih University in Manokwari, Irian Jaya, Indonesia. The objectives of the experiments were to determine the effects of four sweet potato cropping systems on the population density of sweet potato weevils (SPW) and on the diversity of other insects within these agroecosystems. / Fewer SPW were found in intercropped sweet potato + corn (2 weevils per kg infected tubers), sweet potato + soybean (21 weevils), sweet potato + corn + soybean (8 weevils) than in monoculture sweet potato (37 weevils); percentage of damaged tubers followed the same trend, ranging from 2.6% to 14.0% in intercropped sweet potato, to 21.9% in the sweet potato monoculture. However, the higher number of SPW and damaged tubers in the monoculture did not reduce yield below that in the intercropped plots. / Insect and spider populations were more diverse in the intercropped sweet potato systems than in monoculture. Number of arthropods increased throughout the growing season. Intercropping may reduce the population density of other insect pests associated with sweet potato and may increase the population density of natural enemies.

Potato leafroll virus (PLRV) causes yield and quality losses in potato. PLRV is identified by plant symptoms and serological tests such as an enzyme-linked immunosorbent assay (ELISA). A similar serological test, direct tissue blotting assay (DTBA), was used to detect and monitor PLRV movement in field-inoculated Russet Burbank plants and plant tissues from Russet Burbank and Russet Norkotah seed tubers submitted by growers for winter certification tests. DTBA was as accurate as ELISA and easier to use for detecting tuber-perpetuated PLRV in stems and petioles of plants grown from grower-submitted seed tubers. ELISA detected twice as many PLRV positives as DTBA in leaflet tests. DTBA detected PLRV in tuber tissue but results matched ELISA in only 74% or less of the samples. Results of DTBA tuber tests were sometimes difficult to interpret while stem and petiole results were distinct and unambiguous. As inoculations were delayed later in the season and as plants matured, PLRV infection levels decreased sharply, most often within a two week period in early July. In same-age plants inoculated 43 days after planting but 18 days apart, early inoculation produced higher PLRV levels. Conversely, when same-age plants were inoculated 62 days after planting but 19 days apart, late inoculation produced higher PLRV levels. This discrepancy is not fully understood, but larger tuber size at the later inoculation probably produced a stronger sink for source-to-sink translocation of nutrients and phloem-limited viruses. Results of DTBA winter grow-out tests of summer-infected tubers approximated those of ELISA and visual inspections. Indirect DTBA testing of tubers utilizing stem and petiole tissues from winter growout plants detected more PLRV than directly testing tuber tissue 21 days post inoculation in summer. DTBA detected current season (primary) PLRV less reliably than secondary (tuber-borne) PLRV, similar to reported ELISA results. PLRV infection increased tuber numbers but decreased size. Size reduction was most evident in plants infected early in the season. Average tuber size in healthy plots was always larger than the average tuber size in infected plots. Within an infected plant, small tubers tended to be infected less often than large tubers. / Graduation date: 1993

Three pages of addenda pasted inside back cover. / Bibliography: leaves 166-189. / xii, 189, [12] leaves, [24] leaves of plates : ill. (some col.), map ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Rhizoctonia solani is a complex species comprising morphologically basidiomycetous imperfect fungi. This study aimed to determine genetic diversity within R. solani AG-3 causing rhizoctonia disease of potato in South Australia. For this purpose, pectic zymogram, PCR, DNA fingerprinting and RFLP techniques were used in conjunction with traditional plant pathology procedures. / Thesis (Ph.D.)--University of Adelaide, Dept. of Crop Protection, 1996?

Bibliography: leaves 119-127 / viii, 138, 10 leaves, 8, [11] leaves of plates : ill. (some col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Plant Pathology, 1985

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