title: Studies on the Senecio vulgaris-Erysiphe fischeri Blumer, plant pathosystem
Creators: name: Harry, Ibiso Basoene
Publisher: name: University of Glasgow
Year: 1980
Subjects: subject: Plant pathology, parasitology.

description: This thesis is in two parts. The first part reports a study of resistance factors in Senecio vulgaris (Groundsel) and of them matching virulence factors in its parasite Erysiphe fischeri. The second part reports a study of the effects of E. fischeri infection on the growth, development and functioning of the leaf tissue of S. vulgaris. PART I Eight resistance factors to the mildew were identified in a number of groundsel plant lines on the basis of differential reactions of the plant lines to eight different fungal isolates. Plant lines possessing no resistance factors to any of the isolates and plant lines possessing three or more resistance factors were more common in the host population than expected while plant lines possessing one or two resistance factors only were less common than expected. The selective pressures leading to this situation may have been imposed by the parasite although other environmental factors could have played a part. The resistance factors were found to be inherited in a simple Mendelian manner with resistance generally dominant over susceptibility. Resistance factors R3, R4 and R5 were found to be in the same linkage group, R3 and R5 being closely linked with a recorridnation value of about 1.5 +/- 1.5%. An analysis of the virulence factors possessed by each of the eight isolates of the mildew revealed that each possessed virulence factors matching all but one of the eight resistance factors in the host. The isolates were thus complex races. Thus, natural selection presumably as a result of selection pressures imposed by the host population has led to the development of a series of complex races, each race possessing the ability to overcome many of the resistance factors in the host. Over half of the plants susceptible to each race were very susceptible giving an infection type 4 to that race while only about 20% gave more resistant infection types 2 or 3. Thus selection appears to favour the most virulent condition in the parasite. At least six morphological forms of groundsel were identified on the basis of stem characters (red or green; hairy or glabrous) and inflorescence characters (radiate or non-radiate) and these were present in all populations studied. The population structure of groundsel in relation to the distribution of resistance factors and of morphological characters indicates that groundsel populations are genetically very diverse and is therefore a more complex form of a mixed variety cropping system. It is thus likely that long term usage of a mixed variety cropping system to control crop parasites may lead to the development of complex races which are able to attack most, if not all, the cultivars. PART II When groundsel plants have more than 30% of their leaf surface infected with mildew, both dry matter production and leaf expansion are reduced. Factors affecting this reduction in growth have been investigated. Neither the chlorophyll content per unit area of leaf nor the unit leaf rate were reduced even with 90% total mildew cover. Thus reduction in the rate of dry matter production was not due to a reduction in the efficiency of the leaf as a photosynthesizing organ, but to a reduction in the total green leaf area. Infection reduced the total green leaf area by enhancing leaf senescence, by reducing the number of expanding leaves on the lateral branches and by reducing the rate at which individual leaves expanded. The extent to which infection reduced the area of individual leaves depended upon the time of infection. Infection did not affect fully expanded leaves, but it drastically reduced the area of leaves which were infected while still expanding. Such leaves contained fewer and smaller lower epidermal cells than did un-infected leaves, indicating that infection inhibits both cell division and cell expansion. The succulence of the leaves was reduced by levels of infection over 60% cover. In such leaves, the amount of water lost per unit area of leaf appeared to be related to the amount of mildew present. It is thus possible that water stress in the leaves was partly responsible for the reduced expansion of the leaves. This investigation thus indicates that S. vulgaris has a high level of tolerance to E. fischeri.