This study is concerned with the resistance of tobacco seedlings to a second attack of downy mildew caused by Peronospora tabacina Adam. Observations were made on the development and duration of resistance following recovery of tobacco seedlings from downy mildew. The duration of the period of resistance is variable and the observations indicate that plants in the greenhouse recovering during the summer remain resistant longer than do plants recovering from an initial attack during the winter. Samples of leaf tissue from recovered (resistant), infected and previously uninfected (susceptible) plants grown in soil in the greenhouse were analyzed for ammonia-nitrogen, nitrate-nitrogen, reducing sugars and total sugars. Similar analyses were made of samples of leaf tissue from tobacco plants grown in crocks of quartz sand and irrigated with nutrient solutions and from plants grown in crocks of sand-peat mixture to which various mixed fertilizers were added. There was no correlation between the ammonia-nitrogen content of tobacco plants, as determined in these experiments, and the response of the tobacco plant and its susceptibility to attack by P. tabacina, but there was a strong indication that such a relationship exists. Recovered plants contained a higher ratio of total sugar to nitrate-nitrogen than did comparable plants which had never been infected. Furthermore the ratio of total sugar to nitrate-nitrogen was lower in plants infected for a second time than it was in plants from the same lot shortly after they had recovered from the initial attack. It seems probable however that the changes in the nitrate-nitrogen and sugar content in recovered tobacco leaves are an indication of recovery and are not responsible for the resistance possessed by such plants. A series of experiments were performed to test the immediate effect of several nutrient salts and of sugar upon the sporulation of the fungus on the leaves of diseased tobacco plants. There was a very definite response in sporulation to some of these treatments, especially to potassium nitrate, calcium nitrate, and sucrose, but it is not known whether the response of sporulation of the fungus to such treatment is related to the type of resistance possessed by recovered plants. Sodium chloride and calcium chloride were dissolved in water and applied to the soil in which young tobacco plants were growing in the greenhouse. Although increased chloride in one of the sand cultural experiments seemed to protect the plants somewhat against downy mildew, application of solutions of these salts to the soil at the rates used (100 lbs. And 138 lbs. Per acre respectively) produced no noticeable effect on the severity of the disease or subsequent recovery of the plants. The leaves of recovered (resistant) tobacco plants are usually reduced in number, the lower ones having been killed by the initial attack of the fungus. Removal of the leaves from healthy tobacco plants in imitation of this condition did not produce resistance to an attack by P. tabacina. The artificial production of necrotic areas on healthy tobacco leaves in simulation of those usually following sporulation of P. tabacina on diseased leaves had no effect either on the susceptibility of the plants to infection or upon sporulation of the fungus on leaves already infected at the time the necrotic areas were induced. Results of attempts to confer immunity from downy mildew upon an entire plant by infection of a single leaf of the plant were inconclusive, since under the conditions of the experiment it was impossible to be sure that the single leaf was infected. However, the indication is that infection of a single leaf will not render the entire plant immune from a second attack by the fungus. / Master of Science

The scope of this thesis was to identify, describe and quantify important life-history traits for the pseudoannual aquatic plant Ranunculus lingua in different ecological settings, by comparing populations from geographically marginal vs. central habitats. Results from a four-year field study showed that abiotic factors (water-level fluctuations and associated processes) tended to have a greater influence in marginal populations, whereas biotic factors (competition, insect grazing and fungal infections) dominated in central populations. This was reflected in different depth distribution of ramet numbers and ramet sizes between the areas, and In different dynamic patterns, with a higher flux of ramets in marginal populations. In a reciprocal transplant experiment, marginal ramets produced more but smaller rhizomes, whereas central ramets produced Individually larger but fewer rhizomes, irrespective of transplant site. A possible selection for genotypes producing large rhizomes in the central habitat was supported by the fact that initially smaller ramets were more likely to be diseased by the fungal pathogen Peronospora gigantea and damaged by insect grazing. In the marginal population, where density-independent mortality factors tend to dominate, a high reproductive output, expressed in production of high numbers of rhizomes, was suggested to be a favoured life-history trait. In a glasshouse experiment, ramets from marginal and central populations were grown in low and high densities and under three contrasting nutrient levels. The allocation to sexual structures was generally very low, and did not incur any costs in terms of reduced rhizome production. Rhizome production showed strong positive allometrical relationships to mother ramet size. Increasing mother ramet size resulted in a larger increase in rhizome numbers for the marginal than for the central population, whereas the increase in mean rhizome mass was more pronounced for the centred population. Both populations showed similar reductions in rhizome production in response to increased density and lowered nutrient levels, which could not be explained by size-dependent effects adone. The dispersal, dynamics and distribution of R. lingua were studied in a marginal river population in northern Sweden, where the only means of dispersal is by vegetative diaspores, i.e. floating rhizome fragments. Stranding occurred mainly in river curves and at obstacles, and the distribution of established stands was also highly correlated with these features. Relative changes in ramet numbers were correlated with water-level fluctuations during the present and previous growing seasons, with winter low-water, and with duration of spring-flood. The predictability of change was high within but low between stands. It was concluded that the patterns and mechanisms of dispersal are fundamental for local distribution patterns as well as variation in regional abundance in R. lingua / <p>Diss. (sammanfattning) Umeå : Umeå universitet, 1992, härtill 4 uppsatser</p> / digitalisering@umu

Le cucurbitacee possono essere attaccate da più di 200 malattie, una corretta gestione è dunque fondamentale per ridurre il rischio di perdite di produzione, sia in termini di quantità sia in termini di qualità. Tra le malattie più importanti che attaccano le cucurbitacee ci sono l’oidio e la peronospora delle cucurbitacee. Podosphaera xanthii e Golovinomyces cichoracearum sono gli agenti causali dell’oidio. È stato studiato l’effetto della temperature sulla germinazione dei conidi in ambiente controllato a 6 temperature costanti (da 10 a 35°C, ad intervalli di 5C°) da 3 a 72 ore. La temperature ottimale per la germinazone, l’infezione e la sporulazione è risultata essere rispettivamente 24.4, 25.7 e 21.3°C per P. xanthii e 17.9, 17.3 e 16.2°C per G. cichoracearum. Un modello meccanicistico è stato sviluppato per il rischio che i due agenti patogeni causassero un attacco di oidio. Il modello simula la germinazione su foglie infettate sulla base delle condizioni ambientali di temperatura e umidità relativa. Le equazioni che calcolano la germinazione delle dei conidi di entrambi i funghi sono state sviluppate sulla base di dati presenti in bibliografia. Un secondo modello meccanicistico è stato sviluppato per Pseudoperonospora cubensis, agente causale della peronospora delle cucurbitacee. Il modello calcola la data di comparsa dei sintomi e la probabilità del superamento di determinate soglie di gravità della malattia, sulla base di sotto-processi dell’infezione. Il passaggio da uno stato dell’infezione al successivo è dipendente dalle condizioni ambientali. Entrambi i modelli sono stati validati confrontando i dati ottenuti dai modelli con dati indipendenti rilevati in coltivazioni del nord Italia. / Cucurbits are potentially affected by more than 200 diseases of diverse etiologies, so a good disease management is crucial to reduce the risk of high yield losses in terms of quantity and quality. Among the more important diseases there are powdery and downy mildew. Podosphaera xanthii and Golovinomyces cichoracearum are the causal agents of cucurbit powdery mildew. The effect of temperature on conidial germination was studied in controlled condition at 6 constant temperature (from 10 to 35°C, step 5°C) for 3 to 72 hours. Optima temperature for conidial germination, infection and sporulation were 24.4, 25.7 and 21.3°C respectively for P. xanthii and 17.9, 17.3 and 16.2°C for G. cichoracearum. A mechanistic model was developed for the risk posed by P. xanthii and G. cichoracearum to cause cucurbit powdery mildew. The model simulates germination on infected leaves on the base of environmental conditions of temperature and relative humidity. Equation regulating spore germination of both fungi were developed using published data. Another mechanistic model was develop also for Pseudoperonospora cubensis, causal agent of cucurbit downy mildew. The model calculates the symptoms appearance and the probability of overtaking severity threshold based on sub-processes of infection. Changes from one state of the infection to the following one depend on environmental conditions. Both models were validated by comparing model outputs with independent data sets collected in fields located in the north of Italy.