The mechanism of infection of potatoes by Oospora pustulans causing skin spot
El-Nagdy, Galal El-Din Aly
1. Tubers of 30 commercial potato varieties growth under two different soil conditions were subjected to standard laboratory inoculation with a spore and mycelial suspension of Oospora pustulans and thereafter stored in two different conditions. The experiment was carried out in two successive years, the varieties differed in susceptibility to skin spot in both surface and eye, and the differences were in line with those previously recorded in field experiments. The soil type in which the potato was grown did not affect susceptibility. The storage conditions used were apparently not sufficiently precise to give significant differences in the infection of the skin, but eye infection did tend to be increased by the lower or less variable temperatures.2. It was found that varieties differ widely in the structure of the periderm; in the total thickness, the thickness of the suberised layer, the number of rows of cells and the type of cells and cell walls.Varieties also differed in the crude fibre content of the akin, but the ran .e was not very wide, ho outstanding varietal differences were found in the structure of the eye. he eyes were classified into deep, intermediate and shallow, and according to the shape of buds into open and closed buds.The types of soil did not seem to affect the periderm thickness or the crude fibre content or the structure of the eye. A general relationship occurred between the periderm thickness and the crude fibre content. Varieties with the thickest periderm had the highest crude fibre content, and varieties with the thinnest periderm had the lowest crude fibre content.3. Periderm thickness and susceptibility to skin spot wore found to be strongly associated, varieties with the thick periderm were least susceptible, whereas varieties with thin periderm were most susceptible. Varietal resistance Increased gradually with the gradual increase of the perlderm thickness.The same relation existed between crude fibre content and susceptibility. The resistant varieties had the highest crude fibre content and the susceptible varieties had the lowest content, No relation was found between the structure of the eye and susceptibility to eye infection.4. There appeared to be a relation between susceptibility to surface Infection and eye infection. The varieties highly resistant to surface infection were highly resistant to eye infection, (2nd varieties susceptible to surface infection were also susceptibile to eye infection.5, Varieties did not differ in the method of pustule formation or eye infection. Infection caused a thickening of the cell walls and the cells to be disorganized. The infected areas started to erupt after three months of infection and the pustule matured four months after inoculation, Eye infection was found to destroy the central bud, the axillary buds and the scales gradually. Eye tissue became nectrotic and the tubers became useless for planting as seed after four months of inoculation.6, Immature tubers were found to possess the same periderm thickness as the mature tubers and, except in one case, there was no difference in susceptibility between mature and immature tuberfs.7, Infection was increased by small abrasions in the tubers made prior to inoculation, and thus entry of the fungus in this way was confirmed.8. Oospora pustulans was detected from soil taken from a field at least six years after a potato crop, Tomato seedlings were used to detect the presence of the fungus in the soil.