Prior, C. 1978

A method of inoculating young cocoa plants with basidiospores of Oncobasidium theobromae. Annals of Applied Biology 88: 357-362

Notes

Papua New Guinea - Lowlands Agricultural Experimental Station, Keravat Datafile: DIS_VSDS - Disease reaction to VSD Descriptors: Resistance to Vascular Streak Dieback based on infection following inoculation Environment: Lowlands Agricultural Experimental Station, Keravat, East New Britain, Papua New Guinea Germplasm: Local PNG selections Experimental conditions: Sporophores on pieces of infected stem were collected during the day, moistened and stored under damp conditions in the laboratory. In the early evening of the same day, the sporophores were laid across open plates of 35% w/v sucrose agar overlaid with disks of boiled cellophane, so that the spores were shed onto the cellophane. The plates were placed on a tray with a damp cloth in the bottom, the tray enclosed in a large polythene bag to protect against rain and insects and then put outside overnight. Sporophores stayed moist and usually produced abundant spores under these conditions. The plates were brought inside early the next morning before the spores could receive direct sunlight, the lids replaced and they were stored in the dark for 12 h. The following evening the spores were washed from the cellophane into a watch glass of sterile water and then centrifuged at 140g for 5 min. Withdrawal of some of the supernatant left a concentrated spore suspension; haemocytometer counts revealed up to 500,000 spores/ml. This suspension was then diluted to the required concentration. Inoculation of young plants - Young seedlings or cuttings were inoculated by placing 0.02-0.03 ml of the aqueous suspension containing 100-10,000 spores onto the upper surface of a young expanded leaf (1.0 cm long) and onto the stipules of an apical bud using a Pasteur pipette. Spore viability was assessed by putting a few drops of suspension onto water agar and assessing percentage germingation after 18 h. The importance of dew in the initiation of infection was tested on five groups of 27 seedlings of clone K 5 exposed to the open sky; four groups were inoculated at 1800 h, each plant receiving a drop containing c. 7,500 viable spores, and the fifth served as a control to assess natural infection. The four inoculated groups were returned to the shade house after 0, 2, 5.5, and 13 (all night) h exposure of dew. Inoculation of seedlings of known female parent. Seedlings were raised from a single pod of five clones, selected for their range of susceptibility to VSD. Each seedling was inoculated with a drop containing 1000 viable spores, and subsequently kept in a shade house until symptoms developed. Inoculation of rooted cuttings of known resistance to VSD. Cuttings of five clones were selected from the distribution nursery at Keravat, and placed in a remote part of the nursery well away from possible sources of infection. Each cutting was inoculated with c. 10000 (in the first experiment) and 1200 (in the second) viable spores. Inoculation of seedlings with successive dilutions of spores suspension. Seven boxes each containing 49 seedlings of a well-randomised seed sample of the susceptible clone K 5 were inoculated with successive dilutions of spore suspension ranging between 2200 and 0 viable spores/plant.

 Disease