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The work contained in this thesis investigates aspects of the physical, chemical and microbial components of the phylloplane of broad beans and snapdragons in relation to rust diseases of these plants caused by Uromyces fabae and Puccinia antirrhirii respectively. Although experiments were performed with both rusts initially, most work was done with uredospores of U. fabae. The microflora of uredospores and leaves of field-grown plants was sampled at frequent intervals during two summers using a variety of techniques. These studies showed that the dominant microorganisms in the phylloplane of both plants were bacteria and yeasts of the genus Cryptococcus. However composition of the phylloplane microflora varied with the host species, the age and severity of rust infection on sampled leaves, and the time of year. Isolations from uredospores collected from field-grown plants usually included a higher proportion of bacteria than occurred from isolations made from the phylloplane. Experiments using 14C-labelled exogenous nutrients indicated that utilisation by Cryptococcus cells was much greater than by germinating uredospores. As solutions of various exogenous nutrients of similar type and concentration to those found in the phylloplane failed to increase the germination and growth of germ tubes of mature uredospores, and as these uredospores can germinate in deionized water, it was deduced that competition for nutrients by microorganisms in the phylloplane would not inhibit germination of nutrient-independent uredospores. However it was found that solutions of yeast extract or various vitamins and growth substances could stimulate germination and growth of germ tubes of older uredospores. The effect of addition of various microbial epiphytes isolated from leaves and uredospores to germinating uredospores was investigated. Various reactions were seen including stimulation, inhibition or no effect on uredospore germination and growth of germ tubes. No isolate was shown to consistently inhibit uredospore germination and hence a microbial antagonist capable of controlling rust infection was not found. Addition of isolates of phylloplane microorganisms to leaves prior to inoculation of uredospores or addition of nutrients did not increase antagonism. However, interaction studies indicated that uredospore germination, growth of germ tubes and infection could be increased by the presence of cells of Cryptococcus, Sporobolomyces and Aureobasidium pullulans. The greatest stimulation of uredospore germination was achieved with cells of Cryptococcus and hence this interaction was studied in more detail. Addition of Cryptococcus cells stimulated uredospore germination and pustule formation on both attached and detached broad bean leaves when inoculated simultaneously with or prior to uredospores. Leachate from Cryptococcus cells also stimulated uredospore germination, growth of germ tubes and infection on detached leaves but to a lesser extent than the presence of the yeast cells. Uredospores utilised only a small proportion of substances leached from 14C-labelled Cryptococcus cells, whereas they could utilise a greater proportion of the membrane-sterilised leachate from similar yeast cells. However, the presence of uredospores appeared to affect the partitioning of the endogenous reserves of 14C-labelled yeast cells, increasing amounts of 14C in compounds leaked into the external solution and as CO2 but reducing the proportion remaining in the yeast cells. In interactions in which the 14C-label originated in uredospores, addition of Cryptococcus cells decreased the amount of 14C in the external solution without significantly affecting the proportions of label in the cells or as CO2. However, addition of membrane-sterilised leachate from Cryptococcus cells did not alter the partitioning of the 14C-label from uredospores to the external solution or as CO2.

In recent years, in Great Britain and countries situated along the coastal fringe of Northwestern Europe, "top-dying" has become the most destructive disease of Norway spruce (Picea abies (L.) Karst.). The disease affects healthy trees in the dormant period during years characterized by mild, windy winters (March included). The first effect to appear on trees in the interior of thinned plantations is reduction of height growth which may be followed by foliage loss. It is thought that this pattern is also followed by edge trees. When dry flay to August periods follow such winters the growth reduction is steep, anddeterioration of the health status of affected trees is speeded up. Occurrence of severe outbreaks of the disease may result when weather such as the above takes place for more than two consecutive years. The present outbreak in N.E. Scotland is regarded as commencing in 1971t thereafter continuously developing due to unusually mild windy winters accompanied by dry summers (May included) during the period 1971-75. Data for wind duration, air temperature and rainfall for the years 1961-75 were used as variables and annual height increments from three repeat stands were used as the dependent variable in a stepwise multiple regression analysis. Significant correlations with height growth, which precedes needle-browning, were always found for edge trees of stands and occasionally for trees inside stands. Trees, once affected by the disease, go through four distinguishable stages of deteriorating health until they die. These stages can be recognized by the significantly different amounts of needle loss. During the "top-dying" annual cycles-1971-75 and 1975-76 needle-browning in all tree-health categories started building up in mid, late winter-spring and culminated just before and in the early flushing period. Sudden, sharp increase of the rate of needle-browning was found to be closely associated with short spells characterized by clear sunny, warm weather accompanied by high velocity wind and low or even freezing temperature at night, occurring in the dormant period. Needle-browning was successfully prevented by bagging shoots from the early flushing period until late August. Tip needles of current and second year were found to retain a significantly lower water content than base needles (expressed on needle dry weight) as did needles from bare 1 shoots compared withbaggedones during the period January-May. Water uptake by roots of Norway spruce plants, four years of age was significantly lower when cold water was supplied. Uptake at l u was found to be only 49.51% of that at 20G. Close observations and a number of experiments failed to show that R. kalkhoffii Bubak or any other needle fungus has a primary involvement in "top-dying". Similarly, no fungus likely to be strongly pathogenic to Norway spruce was isolated from various tissues of current and second year shoots. Strong evidence suggested that the fungus R. kalkhoffii isolated from Norway spruce and dealt with in this work is a different strain from those isolated in the U.S.A. and Japan from other host species. Its optimum temperature for diameter growth on malt agar was 18C. Malt agar pH's ranging from 3.5 to 9 (before autoclaving) did not have any significant effect on growth. It was shown to be a very successful colonizer of dying needles during its infection period which in 1976-77 started building up in August, culminated in October and stopped in late February. The hypothesis was developed that "top-dying" is caused by severe water stress created by adverse climatic conditions, chiefly mild, windy weather during the dormant period enforced by drought in the first part of the growing season. When conditions such as these occur for more than two consecutive years severe outbreaks of the disease may occur. In a test of this hypothesis, one young Norway spruce plant out of two tested, exhibited in vitro symptoms very similar to those recorded in the field after a 75 hr, exposure period under conditions consisting of air temperature 14C, relative humidity 70% and photoperiod of 12 hrs.

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Root parasitic plants such as Striga, Phelipanche and Orobanche species cause severe loss of crop production worldwide. In some areas of East Africa, intercropping with Desmodium species has given good control of Striga hermonlhica. The root exudates of Desmodium contain compounds that affect germination, early development and attachment of S. hermonthica to the host plants. Laboratory and glasshouse experiments were carried out at the Seed Science and the Plant Environment Laboratories at the University of Reading, UK to investigate whether these known effects of Desmodium on Striga also apply to Phelipanche ramosa and Orobanche crenata. The results of this study indicated that Desmodium uncinatum and D. intortum both provided a high level of control of Ph. ramosa and O. crenata while confirming the effect on S. hermonthica. Significant reductions in infestations by these parasites were obtained when hosts (tomato, pea, maize / millet, respectively) were planted in soil with high densities of parasitic seeds and were irrigated by the water draining from the pots where D. uncinatum and D. intortum plants were being grown. Exposing to Desmodium root exudates for twelve rather than four weeks was more effective in controlling the parasites. Likewise, applications of Desmodium root exudates at different concentrations were also effective. Experiments were also conducted to investigate the effects of environmental factors on the activity of Desmodium root exudates as germination stimulants and attachment inhibitors. Desmodium root exudates from plants grown at different temperatures had no effect on S. hermonthica and Ph. ramosa Dl germination, but they did affect the germination of 0. crenata. In contrast, drought significantly decreased the germination Ph. ramosa D6 compared to non-drought treatments. D. uncinatum proved more effective in controlling attachments of Ph. ramosa D6 compared to D. in/orlum. Root exudates were collected at several different times and all samples were effective at reducing attachment of Ph. ramosa Dl, Ph. ramosa D6 and O. crenata compared to control treatments. However, no significant effects were found for different temperatures on the production of attachment inhibitors.

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