The effect of five rootstock cultivars and two pruning methods with two crop loads each on Botrytis cinerea Pers. rot of Vi tis vinifera L. cv. Chenin blanc was investigated. The effect of these factors on bunch compactness, berry skin strength, pedicel strength, total soluble solids and nitrogen content of berries was also investigated to determine the correlation between these parameters and botrytis rot. The most rot occurred with Chenin blanc on Ramsey, 110 Richter and 101-14 Mgt when spur pruned, while the least rot occurred with Chenin blanc on 99 Richter when cane pruned.Rootstock cultivar had a significant effect on bunch compactness, berry skin strength, pedicel strength, cane mass and crop mass while rootstock cultivar and pruning method had a significant effect on total nitrogen and total soluble solids of berries. Except for bunch compactness none of the other parameters appear to have any direct effect on botrytis rot.Control of bunch rot of grapes caused by Botrytis cinerea Pers. is achieved mainly by chemical means. This is an expensive operation; the total cost in South Africa for wine and table grapes is estimated at R2,0 million per season. As a result of the rise in cost of chemical control as well as the fact that strains of B. cinerea resistant to some of the fungicides have occurred, it is unlikely that practical control of botrytis bunch rot can be achieved by the use of fungicides alone.It is known that many cultivation practices which are likely to increase vine vigour, e.g. excessive nitrogen fertilisation, vigorous rootstocks and irrigation, will increase the susceptibility of grapes to B. cinerea (Branas, 1960;Champagnol, 1969;Delas, 1972;Dry & Smart, 1982). Christensen (1981) found that by increasing the number of nodes per vine from 40 to 60 in the case of Chenin blanc, a significant decrease in botrytis bunch rot occurred. Practical experience in European viticultural areas has shown that choice of rootstock and scion cultivar as well as fertiliser application level, are very important factors with regard to control of botrytis bunch rot (Dry & Smart, 1982).The present study was carried out to determine the effect of certain cultivation practices, viz. the use of certain rootstock cultivars, pruning method and crop load on the incidence of botrytis rot of Chenin blanc. The effect of these factors on bunch compactness, as well as pedicel strength, berry skin strength, nitrogen and sugar content of berries was also evaluated in order to determine if a relation exists between these parameters and botrytis bunch rot. MATERIALS AND METHODSA fourteen-year-old vineyard at Robertson, consisting of five rootstock cul ti vars viz. Ramsey, 99 Richter, 110 Richter, 101-14 Mgt and Jacquez with Chenin blanc as scion, was used in this study. Vines were trellised ac'cording to the Perold system as described by Zeeman (1981).A randomized block design in which each rootstock cultivar was replicated four times was used. Each plot consisted of four rows with seven vines per row. Two pruning methods v...

The technical assistance of J. Sutherland and H. S. van der Walt is gratefully acknowledged.

The mechanism of penetration of 99 Richter grapevine roots by Phytophthora cinnamomi was studied by light, scanning and transmission electron microscopy. Zoospores encysted on roots within 30 min after inoculation. More spores encysted and germinated near roots than further away, indicating a response to some stimulus exuded by the roots. Germ tubes were usually not swollen at the point of entry into the roots. Penetration occurred mostly down the anticlinal walls separating epidermal cells and hyphae developed intercellularly. Swollen germ tubes were sometimes observed where intracellular hyphal growth was preceded by direct penetration of epidermal cells. Some evidence of hydrolysis of epidermal cell walls was found, whereas hydrolysis of cortical cell walls occurred more frequently. Shortly after penetration a plug of amorphous material formed in the germ tube sealing off the penetration peg within the roots. Within 48 hours after inoculation hyphae were observed in the endodermis. Invaded epidermal, cortical and endodermal cells were disrupted and underwent plasmolysis.Phytophthora cinnamomi Rands is the causal agent of root rot associated with the decline and ultimate death of grapevines grafted on 99 Richter (Van der Merwe, Joubert & Matthee, 1972;Marais, 1978a). Different grapevine rootstock cultivars have been shown, both in the field and pot trials, to vary in susceptibility to P. cinnamomi (Marais, 1979). The pathogen apparently attacks susceptible as well as tolerant rootstock (Marais, 1978b ), but the mechanism of penetration and infection in grapevine roots is largely unknown. Additional information is required to broaden existing knowledge of resistance to P. cinnamomi. Infection and penetration of 99 Richter roots by P. cinnamomi zoospores and vegetative hyphae were, therefore, studied. MATERIAL AND METHODSZoospore production: To obtain sporangia of P. cinnamomi an isolate from grapevine roots was initially cultured on potato dextrose agar. Inoculum disks were cut from the advancing margin of the culture after 7 d, placed in petri dishes containing V-8 juice broth (Chen & Zentmyer, 1970) and incubated for 2 d at 25°C. The resulting mycelial growth was washed with sterile distilled water and incubated at 20°C in petri dishes containing liquid soil extract (James, 1958). To achieve synchronized release of zoospores, mycelia bearing sporangia were chilled at 10°C for 15 min and returned to room temperature. Zoospore concentration was determined with a haemacytometer.. Preparation and inoculation of plant material: Lengths (40 mm) of actively growing shoots were cut from 99 Richter grapevines, surface-sterilized in 0,5% sodium hypochlorite, washed in sterile distilled water, transferred aseptically onto filter paper bridges in 2,5 cm diameter test tubes containing 0,5% Hoagland's nutrient solution, and incubated at 22°C to induce root formation. Root tips (20-30 mm) were excised from rooted shoots, washed in distilled water and placed in a suspension containing 10 2 zoospores/ml in an observation ce...