WINETECH Technical Yearbook 2019

A total of 176 BFD and 108 CRR pathogen isolates were obtained. The predominant BFD pathogens isolated from grapevines in these nurseries were Dactylonectria macrodidyma , Campylocarpon pseudofasciculare , and Ca. fasciculare . Symptoms associated with BFD could be seen on the vines isolated from (Photo 2). Other BFD pathogens, such as D. novozelandica , D. toressensis , D. alcacerensis , D. pauciseptata and I. liriodendri , were isolated as well. The predominant CRR pathogens were Pythium irregulare followed by Phytopythium vexans . The other CRR pathogens isolated included Pythium sylvaticum , Py. ultimum , Py. heterothallicum , Py. rostratum , and Phytophthora niederhauserii . The rotation crops sampled included Canola, white mustard, forage radish, Triticale and lupines. The weeds sampled included Johnson grass, rye grass, winter grass, Cape marigold and corn spurry. No pathogens were isolated from Canola, white mustard and lupines. Of the different rotation crops Triticale had the greatest diversity of pathogens. Four isolates of Py. irregulare , two isolates of Py. ultimum , D. pauciseptata , D. novozelandica , and one isolate of D. macrodidyma were obtained from Triticale roots. One isolate of Py. irregulare was obtained from forage radish. Pathogens were also isolated from the weeds corn spurry, Cape marigold, rye grass, winter grass and Johnson grass. One isolate of

D. macrodidyma was obtained from the weed, corn spurry, while one and two isolates of Py. irregulare were obtained from the weeds, winter grass and rye grass, respectively. Two Py. ultimum isolates were also obtained from rye grass and Cape marigold. DISCUSSION No correlations could be made between DNA concentrations and occurrence of pathogens in vines or rotation crop. However, one nursery had no pathogens present in the vines isolated from, and the concentration of the Dactylonectria and Ilyonectria DNA was significantly less than two other nurseries for all three years. This nursery is situated on the slopes of a hill and makes use of ridging as a method to improve soil drainage. Other factors that could also contribute to this, could be that it does not have a long history of grapevine nursery cultivation and implements a three-year rotation system, whereas two-year rotation systems were used in the other nurseries sampled. The soil characteristics determined by the laboratory tests were all considered to be within the normal ranges (Dr. F. Ellis, personal communication). Therefore, stress as a result of nutrient deficiencies or salinity were deemed to be negligible. The soil wetness index is an important characteristic which indicate the potential of a soil to become waterlogged. Four of the nurseries are

situated in a production area with low lying alluvial soils which is prone to wetness and also had higher soil wetness index. These soil conditions can contribute to the occurrence of BFD or CRR pathogen infections. The presence of the pathogen DNA in the soil, as well as in grapevine plants, shows that pathogen inoculum persists in the soil during the crop rotation year. The DNA concentrations in the crop rotation year (2014) for Dactylonectria and Ilyonectria species, as well as Phytophthora species, were equal or higher than in the first grapevine year (2013) investigated. This, together with the fact that the DNA concentrations for these pathogens were higher in the second grapevine year investigated (2015), indicate that the pathogens survive successfully in the soil or in the roots and basal ends of specific weeds and rotation crops. This study has highlighted the possible effects of soil wetness, rotation crops, crop rotation system (two- vs. three-year rotation systems), and cropping history on the incidence of BFD and CRR pathogens in grapevine nurseries in the Western Cape. The complexity of interactions and diversity of factors involved, preclude the drawing of concrete conclusions with regard to crop rotation system to be used to suppress soilborne pathogens. A more extensive study over a longer period of time should be conducted on the suitability of different rotation crops currently used

in grapevine nurseries in South Africa and especially the contribution of Brassica crops for biofumigation. CONCLUSIONS • This is the first study in South Africa to detect BFD and CRR pathogen DNA from the soil. DNA extraction and qPCR protocols were optimised for soil samples. • BFD and CRR pathogen DNA were detected from soil of all five nurseries investigated. From four of the nurseries these pathogens were also isolated from nursery vines collected in year one and year three. • Of the rotation crops, only Triticale and forage radish harboured black foot and crown and root rot pathogens, whereas no pathogens were isolated from Canola, white mustard and lupines. The latter rotation crops would be recommended. • The weeds corn spurry, Cape marigold, rye grass, winter grass and Johnson grass, harboured pathogens, and may serve as an alternative host in the absence of grapevines (during crop rotation and fallow periods). Weed control would aid in reducing the carry over of pathogens from one vine cropping year to another. • Since nursery vines get infected from BFD and CRR pathogens present in nursery soils, the most viable solution is hot water treatment of nursery vines.

– For more information, contact Lizel Mostert at lmost@sun.ac.za.