IntroductionSpecies within the Botryosphaeriaceae, Diaporthe and Phaeoacremonium, as well as Phaeomoniella chlamydospora and Eutypa lata have been shown to be part of a pathogen complex causing trunk diseases of grapevines. Pruning wounds are known infection portals for all of these pathogens. Several studies have indicated that air-borne inoculum of the respective pathogens is present in vineyards for long periods of time, especially when weather conditions are favourable for spore release and dispersal and therefore available for infection of susceptible wounds. Due to the lack of curative control measures for trunk diseases, pruning wound protection is of critical importance. For the development of a successful pruning wound protection strategy, knowledge about the duration of pruning wound susceptibility to infection is important; especially to determine the timing of treatment and duration of protection required. The aim of this study was therefore to determine the duration of pruning wound susceptibility to natural and induced infection by the major pathogens associated with grapevine trunk diseases. This knowledge would aid in the development of pruning wound protection strategies against trunk disease pathogens in South African vineyards. Materials and methodsPruning and inoculationGrapevines in an 18-year-old vineyard, cv. Chenin blanc, in the Stellenbosch region were spur-pruned to three buds per cane at two stages [mid- (July) and late winter (August)] during the dormant seasons of 2004 and 2005. Individual pruning wounds were spray-inoculated with a spore suspension of either E. lata, Pa. chlamydospora, Neofusicoccum australe or Diaporthe ampelina (previously known as Phomopsis viticola) directly after pruning, and 1, 2, 3, 7, 10, 14, 17 and 21 days after pruning. Two non-inoculated treatments (sprayed with water or painted with wound sealant) were also included. |
 FIGURE 1. Pruning wounds of Chenin blanc showing distinct dark brown to black streaking developing from wounds (right hand side) inoculated with spore suspensions of: a. Neofusicoccum australe, b. E. lata; c. Pa. chlamydospora and d. Diaporthe ampelina. |
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Trial evaluation and statistical analysis
Eight months after treatment, the distal internodes containing the treated pruning wounds were removed and taken to the laboratory for pathogen isolation. The occurrence of the inoculated pathogens in the tissues directly beneath the pruning wound scar was subsequently determined. This was done by removing tissue sections from the woody tissues and plating them out onto Petri dishes containing potato dextrose agar. Petri dishes were incubated at 25°C for two to four weeks before morphological identification of the isolated fungi. The incidence of the inoculated pathogens was subsequently calculated for the xylem and pith tissues and the whole pruning wound stub. Infections from aerial spores present in the vineyard were determined from the paint and water treatments. Lesions observed as vascular browning in the pith and xylem tissue of the split pruning wound stubs were also measured and recorded. The pathogen incidence data was subsequently subjected to appropriate statistical analyses.
Weather data
In the trial vineyard block a weather station recorded hourly measurement of rainfall, temperature, relative humidity and wind speed during the trial periods of 2004 and 2005.
Results
Pathogen incidence in treated pruning wounds
Inoculated pruning wounds showed distinct dark brown to black streaking developing from the wound (Figure 1). Wounds inoculated directly after pruning in July 2004, yielded significantly lower mean pathogen incidences compared with wounds made in August 2004 (Figure 2). Results from both months in 2004 indicated a slow decline in pathogen incidence with increasing wound age at treatment (Figure 2, Table 1).
In 2005, wounds inoculated directly after pruning yielded similar pathogen incidences in wounds made in July compared to August (Figure 2, Table 1). Results indicated a more rapid decline in pathogen incidence with increasing wound age at treatment compared to 2004. The decline was also significantly more pronounced in wounds made in July 2005, compared with August 2005 (Figure 2).
When looking at individual pathogen groups’ occurrence in 2004, the mean Diaporthe spp. incidence in inoculated pruning wounds increased slightly with increasing wound age at treatment. The mean pathogen incidence in wounds inoculated with Pa. chlamydospora remained almost constant with increasing wound age at treatment. Only in the case of pruning wounds inoculated with Botryosphaeriaceae spp. and E. lata did the mean pathogen incidence show a decline with increasing wound age at treatment (Figure 3). However, opposed to 2004, in 2005 all pathogens showed a decline in the mean pathogen incidence in inoculated pruning wounds (Figure 3). The mean pathogen incidence in Diaporthe spp. inoculated pruning wounds declined at the slowest rate with increasing wound age at treatment while the rate of decline in pruning wounds inoculated with Pa. chlamydospora, Botryosphaeriaceae spp. and E. lata were much faster (Figure 3).
Weather data
Average temperature for the 21 days after pruning in July 2004 was 13.1°C and for the same period after pruning in August 2004 was 13.5°C. This is in comparison with 14.8°C for the period after the July 2005 pruning and 14.6°C for the August 2005 pruning time. Rainfall recorded for the 21 days after the respective pruning times was 2.6 mm for July 2004, 1.4 mm for August 2004, 3.3 mm for July 2005 and 3.3 mm for August 2005.
Discussion
Previous studies on the susceptibility of grapevine pruning wounds reported that wounds made early in the dormant season were more susceptible, and remained susceptible for longer, to infection by E. lata, Pa. chlamydospora and Phaeoacremonium minimum as opposed to pruning wounds made later in the dormant season. All of these studies also found that wound susceptibility declined with increasing wound age.
The present study is the first study comparing pruning wound susceptibility (at various wound ages) with different pruning times during the pruning season and to a wider variety of trunk disease pathogens. Results support the previous studies as pruning wound susceptibility declined with increasing wound age irrespective of the time of year pruning took place. However, differences in the rate of decline in wound susceptibility were noted between 2004 and 2005. The variation observed could be explained by the mechanisms involved in wound repair and the effect climatic conditions have on these mechanisms.
At the trial site weather data showed that the average temperature and total rainfall for the 21 days after pruning in July and August 2005 was higher compared to the same periods for the July and August 2004 pruning times. Therefore, the pruning wound repair processes might have been slower in 2004 compared to 2005, due to the lower average temperature and total rainfall during the trial months of July and August. These higher temperatures and rainfall in 2005 could also have contributed to a faster colonisation of pruning wounds by naturally occurring epiphytes, therewith reducing pruning wound susceptibility.
Despite the variation observed between the two years, wounds remained susceptible to infection by all inoculated pathogens for at least three weeks. Pruning wounds that were made and inoculated during August in both years, generally had higher incidence of infection compared to the pruning wounds inoculated in July, which suggests that wounds made later in the dormant season are more susceptible to infection than wounds made earlier. This could also indicate that it is not necessarily the time of year that pruning is done that determines the period of wound susceptibility, but rather the climatic conditions experienced after pruning.
As in previous studies, pruning wounds remained susceptible to infection to all major trunk disease pathogens for an extended period after pruning, regardless of when pruning was done. It is therefore of utmost importance that grapevine pruning wounds should be protected for prolonged periods after pruning against infection by all pathogens implicated in the trunk disease complex by means of chemical and/or biological agents.
Summary
Eutypa lata and Phaeomoniella chlamydospora, as well as several species in Botryosphaeriaceae, Diaporthe and Phaeoacremonium are trunk pathogens of grapevines that infect via pruning wounds. Duration of pruning wound susceptibility to some of these pathogens was largely unknown. Plants of the cv. Chenin blanc were pruned at two stages and spray-inoculated with spore suspensions of four important trunk pathogens directly after pruning, and 1, 2, 3, 7, 10, 14, 17 and 21 days after pruning. Results indicated that, irrespective of pathogen inoculated, pathogen incidence in the inoculated pruning wounds of both mid- and late winter declined with increasing wound age. Wounds remained susceptible for 3 or more weeks after pruning. Late winter (August) wounds were more susceptible to infection than wounds made earlier (July) in the season.
This article originates from research funded by Winetech and the final report of project US PP 04 2004, “Diagnosis, epidemiology and integrated management of trunk diseases of grapevines”, can be downloaded from http://www.sawislibrary.co.za/dbtextimages/Winetech2010_12.pdf.
– For more information, contact Lizel Mostert at lmost@sun.ac.za.
