No description available.

No description available.

No description available.

Late blight of potato incited by Phytophthora infestans (Mont.) de Bary, is one of the plant diseases particularly well suited for fundamental studies on the physiology of parasitism. For this reason, and also because of its economic importance, this disease has been the object of numerous and diverse investigations. However, it is only recently that studies on the biochemistry of the alterations resulting from infection, and of disease resistance and susceptibility, have been initiated. [...]

Disease symptoms often have diagnostic value. They are used also to determine the nature of the alterations in the physiology of the diseased plant, and to estimate the severity or extent of the damage caused. The search for diagnostic features is usually performed in white light. Descriptions of symptoms and conclusions as to the nature and extent of damage reflected by symptoms are usually based on the appearance of symptoms in that light. [...]

Very high infestation levels of Thrips tabaci Lindeman were observed on onions at Ste-Clotilde (Quebec) with seasonal means of 149 and 161 thrips/plant for 1988 and 1989 respectively. No significant differences in thrips densities were detected among six yellow onion cultivars (Capable, Flame, Norstar, Progress, Rocket, Taurus) through weekly comparisons in 1988 and biweekly comparisons in 1989. / A new variable, the cumulative number of thrips-days/plant, was used to model the impact of T. tabaci on onion yields. This variable allowed for better-fitted models, especially in 1989 where greater variability of the onion yields was observed. A modified Gompertz equation gave the best fit for 1988. No yield reduction due to T. tabaci was observed up to the 1000 cumulative thrips-days/plant point, beyond which the thrips impact first increased with the feeding pressure and then gradually decreased to a minimal level corresponding to a yield loss of 43%. In 1989, thrips impact was immediate and best described by an exponential model with a maximal yield loss of 34,5%. / Economic thresholds of 964 and 251 cumulative thrips-days/plant were calculated for 1988 and 1989 respectively. Converted to thrips/leaf these correspond to 2,2 and 0,9 thrips/leaf. This conversion could facilitate their use in a field situation. / Spatial distribution of T. tabaci was analyzed in ten commercial onion fields. Within field, density was homogeneous in the majority of cases. In some fields, higher thrips densities were temporary (two consecutive weeks at most) in one or two field margins. / The Iwao method was used to calculate the acceptance boundaries of sequential sampling plans for the economic thresholds of 0,9 and 2,2 thrips/leaf and onion growth stages of 6, 7, 8, 9 and 10 green leaves. (Abstract shortened by UMI.)

No description available.

No description available.

No description available.

No description available.