This thesis is concerned with an analysis of swimming of oomycete zoospores, particularly in relation to the hypothesis that electrotaxis mediated host root colonisation. A correlation was found between the electrotactic behaviour of zoospores of <I>Phytophthora palmivora </I>and <I>Pythium aphanidermatum</I> and their localisation relative to anodic or cathodic regions of the majority of non-host roots. Cathodotropic <I>P. aphanidermatum </I>zoospores were found to be attracted to the cathode generated at the wound site on monocotyledonous and dicotyledonous plant roots studied. Zoospores of <I>P. aphanidermatum</I> were also found to become gradually less attracted to these wound sites as they dissipated over time. Anodotropic <I>P. palmivora </I>zoospores were found to be repelled by the cathodic wound sites on roots of all plants investigated, with the exception of <I>Petunia hybrida. </I>In addition to this zoospores of <I>P. aphanidermatum</I> were found not exhibit chemotaxis or encystment in gradients of the wound-specific metabolite acetosyringone. This suggests that electrotaxis and chemotaxis both operate in directing zoospore accumulation around roots. This reduction of the conductivity of the bathing medium by the addition of different concentrations of sodium chloride salts did not appear to affect the accumulation of both zoospore species around roots of rye grass. The results of this study supports the hypothesis that zoospores use electrotaxis as one means to locate new plant hosts in the rhizosphere. However, chemotaxis may still augment the regulation of zoospore colonisation and encystment.

No description available.

New Jersey, Georgia, and the Eastern Shore of Virginia (ESV) are important snap bean (Phaseolus vulgaris L.) growing regions, but profitability is threatened by Pythium blight. Causal agents of Pythium blight on snap bean were identified using morphological characterization and sequence analysis of the rDNA-internal transcribed spacer (ITS) regions of 100 isolates. Most isolates were Pythium aphanidermatum (Edson) Fitzp. (53%), and also included Pythium deliense Meurs (31%; all from Georgia), Pythium ultimum Trow (12%), Pythium myriotylum Drechsler (2%), Pythium catenulatum Matthews (1%), and unknown Pythium sp. (1%). To our knowledge, this is the first report of P. deliense in Georgia and on common bean and squash (Cucurbita pepo L.); as well as the first report of P. catenulatum on lima bean (Phaseolus lunatus L.) and in New Jersey. Fungicide labeling and cultivar selection for Pythium blight management is hindered by difficulties associated with conducting successful trials, because the disease occurs sporadically and clustered in the field. Three P. aphanidermatum-infested inoculum substrates were evaluated at three concentrations. The vermiculite/V8 juice (5:3 weight to volume) inoculum (10,000 ppg/0.3 m) consistently caused at least 50% disease in 3 field trials. Sensitivity of the Pythium blight pathogens was determined in vitro against five fungicides. Twenty-two Pythium isolates representing P. aphanidermatum, P. deliense, P. ultimum, and P. myriotylum were inoculated to media amended with each active ingredient at 0, 100μg/ml, the concentration equivalent to the field labeled rate if applied on succulent beans at 187 L/ha, and the equivalent if applied at 374 L/ha. All isolates were completely sensitive (100% growth reduction, or GR) to all active ingredients at the labeled rates, except azoxystrobin. At 100μg/ml azoxystrobin, one P. deliense isolate had 8.9% GR. All isolates had 100% GR to copper hydroxide at 100μg/ml, and the lowest GR on mefenoxam-amended medium was 91.9%. At 100μg/ml cyazofamid, all P. deliense isolates were completely sensitive and variation was observed in P. aphanidermatum isolates. At 100μg/ml potassium phosphite, significant GR similarities were recorded within isolates of the same species, and less than 50% GR was observed in all P. deliense isolates. / Ph. D.

Call number: LD2668 .T4 1979 T73 / Master of Science

No description available.

Root cap cells of two cotton species elicited a specific chemotactic response in zoospores of Pythium dissotocum. When roots of cotton seedlings were placed into a suspension of P. dissotocum zoospores, there was immediate attraction, accumulation and encystment exclusively in the root cap cell region. Furthermore, root cap cells remained attractive when isolated from the root: attraction, accumulation, and encystment on individual root cap cells occurred within seconds after contact. Zoospores penetrated and killed isolated root cap cells within 15-30 minutes, and seedlings died within 24 hours. In contrast, zoospores of P. catenulatum, which exhibited a chemotactic response to roots of Bentgrass, were not attracted to and did not infect seedlings or isolated root cap cells of cotton. Preliminary studies indicate that both Pythium species are capable of infecting cotton seedlings in sand culture, though it is not known if either are pathogens on cotton grown in the field.

Air-filled porosity (AFP) affected the suppressiveness of organically based plant growth media against Pythium ultimum infection. AFP also interacted with microbial activity, moisture levels altered by covering/uncovering media and water severity of pythium damping-off. It has not been possible to determine a narrow range of AFP required to achieve consistently suppressive media, as AFP has been found to interact with many factors such as microbial activity, temperature shock of media and plants and moisture levels modified by covering and uncovering media, or using different pot heights to modify WHC. The use of different batches of compost and ageing of compost in the seven experiments reported here may also account for some of the different effects of AFP on the severity of Pythium disease. It is not recommended that AFP be used for controlling Pythium damping-off, unless certain conditions, such as microbial activity, can be controlled to a range of specifications also. Variability of compost is one of the principal factors limiting its widespread use, so therefore, improving the quality control during the composting process would be the principal factor which should be investigated in future. / Master of Science (Hons)

Cotton seedling disease complex is caused by a number of pathogens inducing similar symptoms and can lead to thin, uneven stands that grow slowly and yield poorly. Preliminary work indicated that a set of high tannin cotton lines developed and released in 1989 by Texas A&M AgriLife Research, (Smith et al., 1990a, Smith et al., 1990b, Schuster et al., 1990) may possess increased resistance to disease. This evidence, along with additional studies that show a clear role of tannin in disease resistance, suggest that these high tannin cotton lines may prove useful in breeding for increased resistance to cotton seedling disease complex. High tannin cotton lines were screened for their resistance to Rhizoctonia solani and Pythium aphanidermatum. While no high tannin germplasm line was more resistant to R. solani than the resistant control, Tamcot SP 21, the potential for significant gains from selection was demonstrated. Fifteen high tannin lines expressed resistance to P. aphanidermatum equal to the resistant control, Tamcot Sphinx (El-Zik and Thaxton, 1996). This resistance was not shown to be correlated with tannin content, though it is still unclear whether or not this lack of correlation is real or due to limitations in measuring tannin in infected seedlings.

Xanthomonas axonopodis pv. malvacearum(Smith), Pythium aphanidermatum(Edson) and Rhizoctonia solani(Kuhn) have all been shown to cause significant yield losses in cotton. Previous work has demonstrated that a set of high tannin cotton germplasm lines developed and released in 1989 by Texas A&M AgriLife Research may possess resistance to these three diseases. In this research, the usefulness of these high tannin lines in breeding for resistance to these pathogens as well as the role of tannin in conferring this resistance were examined. The high tannin lines were screened for their resistance to Xanthomonas axonopodis pv. malvacearum and five highly resistant lines were subjected to generation means and diallel analysis in order to determine the relative importance of different types of gene action in conferring resistance as well as which lines would be most useful in breeding for resistance. The effect of selection for R. solani resistance was measured and selected lines were subjected to diallel analysis. Lines showing elevated resistant to P. aphanidermatum were subjected to a single cycle of selection and the effect of selection was measured. Lines possessing high degrees of P. aphanidermatum resistance were subjected to generation means and diallel analysis. The importance of tannin content in conferring resistance to X. axonopodis and R. solani was also measured. Four high tannin lines were found to be not different from Tamcot Sphinx in their resistance to Xam. Generation means analysis for Xam resistance showed that in the five high tannin lines tested, additive gene effects were of the greatest importance. No correlation between tannin content and Xam resistance was detected. Under the conditions of this study tannin content was also shown to be unimportant in R. solani resistance. Two rounds of recurrent phenotypic selection were effective in increasing the resistance of selected HT lines to R. solani. One cycle of phenotypic selection for P. aphanidermatum resistance was found to produce significant improvements in seven of the most resistant HT lines. Generation means analysis for P. aphanidermatum resistance showed that in the HT lines tested, additive genetic effects were by far the most important.

No description available.