Studies on the Fire-Blight Disease of Udo Salad Plant

Abstract

1. This paper gives the results of studies on the morphological, physiological, and pathological characters of a new fungus that causes the fire-blight disease of the Udo Salad Plant (Aralia cordata THUNB.).2. This disease begins to appear in the late spring on the leaves, stems, and flowers. On the stem appear brown blisters and ellipsoidal spots; at first small, but gradually enlarging under favourable conditions, sometimes reaching 3-11 x 1-2 mm. in diameter, their shape being usually ellipsoidal or spindle. In very advanced stages of the disease, there appear in the blister a brown to grayish white center, bordered by violet-brown, and small black pycnidia produced on a lesion. The spots on badly affected stems coalesce, often forming a corky surface and atrophy, after which they dry up. On the leaf blade, mid-rib, and petiole of the leaves appear red-violet spots, they wither or shrivel and then dry up. The flowers also wither and die.3. The pycnidia of the fungus are usually formed under the epidermis of the upper surface of the spot, and the epidermis is perfolated by minute ostioles. The pycnidia, which are spherical or subspherical in shape, measure about 37.5-150.0 μ in diameter and are dark brown in colour. The pycnospores are ellipsoidal, or slightly slender, at one pole, and are hyaline. Their measurements are 3.75-6.25×2.5-3.75 μ. In culture media, the pycnidia are 75.0-300.0 μ in diameter, and their colour black or brownish black, while the external character of the pycnospores are the same as those observed on the affected stems.4. A single spore isolation was made from an affected stem, and the cultural characters of the fungus were studied with ten different media. It was found that the most suitable medium for mycelial growth was potato decoction agar, while that for the formation of pycnidia and pycnospores was potato decoction agar, Richard's solution agar, and apricot decoction agar.5. The relation of temperature to the growth of the fungus was studied on apricot decoction agar, Richard's solution agar, and soy agar. It was found that the optimum temperature for mycelial growth was 25°C in apricot decoction agar and Richard's solution agar, and at about 22°C in soy agar, the minimum being below 10°C, and the maximum between 30°C and 35°C.6. The relation of hydrogen-ion concentration to mycelial growth was studied under different pH for Richard's solution agar, which were prepared by using caustic soda and sulphuric acid. The growth occurred in media, the pH of which were from pH 2.2-3.0 to 8.6<, and at pH 6.8. the most favourable growth was observed.7. The effect of concentrated sucrose on mycelial growth was studied with Richard's solution to which various proportions of sucrose were added. The most suitable concentration was the addition of 2% sucrose both to the liquid and solid, followed by concentrations of 1% and 5%, whereas any change in the proportion, whether increase or decrease, interfered with growth.8. The relation of temperature to growth of germ-tubes from pycnospores was studied by sowing them on apricot decoction and kept for 20 hours at different temperatures. The optimum percentage of germination was about 25°C, followed by 28°C and 22°C. At 35°C and 15°C no germination was observed.9. The relation of hydrogen-ion concentration to growth of germ-tubes from pycnospores was studied by sowing them on Richard's solution of different pH, and keeping for 18 hours at 22°C. Growth occurred in media the pH of which were from pH 1.8-2.2 to 8.6<, pH 6.8 furnishing the best growth.10. The effect of concentration of sucrose on the growth of germ-tubes from pycnospores was studied by sowing them on Richard's solution and adding to it various proportions of sucrose.