Abstract
Double pruning of grapevines was evaluated as a cultural management practice for reducing infections by Eutypa lata in spur-pruned vineyards. Double pruning involves two pruning passes through the vineyard. Vines are first prepruned by nonselectively trimming canes to a uniform height, generally 30 to 45 cm above spur positions, then pruned to traditional two-bud spurs in late winter when Eutypa infections are less likely to occur. In large vineyards, double pruning may allow for more final pruning in late winter, potentially reducing the number of infected spurs if Eutypa infections on prepruning wounds do not develop beyond the point where final pruning will occur. To evaluate double pruning, Chardonnay and Merlot grapevines were prepruned monthly from October to February with final pruning in March for three seasons. Eutypa lata spores were inoculated onto wounds at the time of prepruning in two seasons. Examination of inoculated canes after final pruning showed mean lengths of vascular discoloration below prepruning wounds of 0.29 to 4.67 cm in Chardonnay and 0.14 to 3.07 cm in Merlot, with maximum length of discoloration of 14 cm and 12 cm, respectively. Isolations for E. lata were made at various locations along the length of the discolored vascular tissue. Eutypa lata was not recovered from isolations made more than 4 cm below the point of inoculation, indicating that infections of prepruning wounds should all be eliminated during final pruning, as long as prepruning occurs 30 to 45 cm above the final pruning cuts. No impacts on viticultural performance were observed, including yield, pruning weight, and time of budbreak. By allowing for more final pruning to occur in March, double pruning should greatly reduce the number of Eutypa infections that occur each year.
Eutypa dieback, caused by the Diatrypaceous fungus Eutypa lata (Pers.) Tul. & C. Tul (= E. armeniacae Hansf. & M.V. Carter), affects grapevines (Vitis spp.) in many parts of the world (Carter et al. 1983). Ascospore infections of fresh pruning wounds lead to the development of perennial cankers that kill portions of the vine. The production of phytotoxins is responsible for foliar symptoms including stunting and distortion of shoots and leaves (Molyneux et al. 2002, Mahoney et al. 2003). The economic consequences of Eutypa dieback include productivity loss, management costs, and reduced longevity of vineyards (Munkvold et al. 1994). Losses to California growers from Eutypa dieback were estimated to exceed US$260 million in 1999 (Siebert 2001).
Effective control measures for Eutypa dieback are still lacking. Treatment of pruning wounds with the fungicide benomyl was a recommended practice in California for many years (Gubler and Leavitt 1992); however, multiple applications were required to protect most wounds until fully healed (Munkvold and Marois 1993a). Because of the costs of application, growers rarely treated more than once and generally saw little reduction in disease. Production and sale of benomyl was halted by the manufacturer in 2001.
Boron was proven to be effective as an alternative control method against Eutypa dieback (Rolshausen and Gubler 2005), although some applications of boron correlated with bud failure at the first node below the treated wound. The use of biological control agents to protect pruning wounds has been investigated (Munkvold and Marois 1993b), but few commercial products are registered for this use.
Growers in California rely in large part on cultural practices to reduce the chances of infection by E. lata, primarily by delaying pruning as long as possible. Ascospores are released from perithecia soon after rainfall. By delaying pruning until late February or March, the chances for rainfall to occur following pruning are reduced. Spore loads will likely be reduced due to depletion of ascospores from discharges during the previous months. In addition, pruning wounds made in February or March are susceptible to infection for a shorter period of time than infections made earlier in the winter (Petzoldt et al. 1981, Chapuis et al. 1998). All of these factors make delayed pruning a general recommendation for Eutypa management in California.
However, late pruning is not possible in all vineyards. Vineyard managers with a limited labor force often need to start pruning early in the winter to ensure completion of work before budbreak. Many vineyards are farmed by management companies that need to schedule their labor force throughout the winter months in order to complete pruning for all their clients. Finishing all pruning before budbreak may be difficult in larger vineyards if vines are left unpruned until late in the winter.
Double pruning is a practice that can be used in spur-pruned vineyards to allow more final pruning to occur late in the winter (late February and March). It involves two pruning passes through the vineyard. The first pruning (prepruning) is usually made in November or December after leaf fall but before heavy rains begin. At this time, the vines are trimmed to a uniform height, generally 30 to 45 cm above the spurs, removing most of the previous year’s growth, and leaving short canes containing 6 to 10 buds. This pruning is nonselective and can be done by hand with pruning shears, with hand-held power trimmers, or by tractor-mounted equipment. A second pruning is made in late February or March, at which time vines are rapidly and selectively pruned to their final desired bud count. These final cuts are made late in the winter when opportunities for Eutypa infections are reduced.
Because most of the brush is removed in the first pruning pass, the final pruning can be completed in considerably less time than if the vines had not been prepruned. If prepruning has already occurred, then a given number of workers can do more of the final pruning in late winter. By having more of the final pruning cuts made late in the dormant season, the number of Eutypa infections that occur each year should be greatly reduced.
We initiated double-pruning trials to evaluate the utility of this practice for reducing the risk of Eutypa infections and to assess its viticultural impacts, if any. Our goal was to determine how far E. lata could move down one-year-old canes in the time between prepruning and final pruning. If, during the intervening months, E. lata could move beyond the point where final pruning cuts were to be made, then double pruning would not be useful as a disease management tool. On the other hand, if the pathogen could not move the distance to the final two-bud spur position, then this cultural practice should be an excellent method to reduce the potential for E. lata infections.
Additionally, we were interested in learning how early vines can be safely prepruned without impacting the following year’s growth and productivity. Prepruning in October or November before the onset of heavy rains could be accomplished by tractor-mounted equipment while soils are still relatively dry. The efficiency of this mechanical operation could reduce the overall costs of pruning. Some possible consequences of fall prepruning include earlier budbreak the following spring (and a greater risk of frost damage) and lowered growth or productivity in future years due to reduced carbohydrate storage in cases where early prepruning eliminated leaves before natural leaf fall.
Materials and Methods
Trials were established in 2000 in two mature vineyards in the Napa Valley, California. The first site was a Chardonnay vineyard planted in 1990. The second site was a Merlot vineyard planted in 1989. All vines were cordon trained and spur-pruned. Two trials were established at each vineyard location: one to study the movement of Eutypa lata in canes following prepruning, the other to evaluate viticultural impacts of double pruning.
To study the movement of E. lata following prepruning, trials were established in the Chardonnay and Merlot vineyards, each with a randomized complete block design. The five treatments were prepruning and inoculation with E. lata midmonth in October, November, December, January, or February. These trials were conducted in the winter of 2000/01 and repeated in 2002/03.
On each treatment date, four vines in each of four replications were prepruned. Prepruning involved nonselectively cutting the previous year’s canes to a height ~45 to 60 cm above the spurs. All prepruning wounds (~25 per vine) were inoculated with E. lata by applying 10 μL of an ascospore suspension (200 ascospores/μL) directly onto the surface of freshly cut one-year-old canes using a micropipette. Inoculum consisted of ascospores obtained from stromata of E. lata collected from grapevine using the technique described by Carter (1991). Stromata were soaked in water for 1 to 2 hr and fixed on the lid of a sterile petri dish so the discharged ascospores could accumulate in sterile water in the base of the dish. Ascospores were then transferred to a sterile Eppendorf tube and the concentration was adjusted using a haemocytometer.
A final pruning of each of the inoculation treatments was made in mid-March to traditional two-bud spurs. The pruned-off cane pieces were bundled and brought to the lab for examination and isolation of E. lata. Ten cane pieces from each replicate of each treatment date of both varieties were examined in both years of the trial and the extent of vascular discoloration was determined. After removing bark and phloem tissues, canes were dissected longitudinally to reveal xylem necrosis. The length of vascular discoloration was measured from the top of the pruning wound down through the cane.
The progression of the pathogen was estimated by the frequency of recovery of E. lata from along these necrotic lesions. Necrotic tissues were isolated on potato dextrose agar amended with 100 mg/L tetracycline. In 2001/02, isolations were made from necrotic tissue collected in the first cm below the inoculated prepruning wound and at the bottom of the observed discoloration. In 2002/03, isolations were made at 1-cm intervals along the length of the vascular discoloration starting at the inoculated pre-pruning wound. Positive recovery of E. lata was based on development of typical E. lata colonies in petri dishes following isolations. The percentage of infected canes was calculated for each prepruning date for both varieties.
To study the viticultural effects of prepruning, two additional trials were established in the same Chardonnay and Merlot vineyards. At each site, a randomized complete block design was used with five replications of six pruning treatments, with four vines in each individual replicate. The six pruning treatments included prepruning (as previously described) midmonth in October, November, December, January, or February (all with final pruning in March) and a control treatment of March pruning without any prepruning.
Viticultural trials began with prepruning in October 2000 and continued through harvest 2003. Individual vine pruning weights were measured on the date of the prepruning treatments and at the final pruning in March. Prepruning and final pruning weights were combined for the pre-pruning treatments. If leaves were still present at the time of prepruning, they were stripped off before weighing. Prior to harvest, a 100-berry sample was collected from each replicate, then weighed and analyzed for Brix, pH, and total acidity. At harvest, vines were individually picked, clusters per vine were counted, and crop weight per vine was measured. Average cluster weights and berries per cluster were calculated. Vines were observed in early spring each year to see if prepruning influenced the date of budbreak.
Data were analyzed with SAS system software (SAS Institute, Cary, NC) using ANOVA and Tukey’s (HSD) t-test to compare means.
Results
Nearly all cane pieces of Chardonnay and Merlot collected following final pruning showed vascular discoloration; however, we were not able to isolate E. lata from many of these samples. Eutpa lata was reisolated from Chardonnay in 40% of canes inoculated in October 2000, 50% from November and December inoculations, 60% from January 2001, and only 10% from February 2001 (Figure 1A⇓). From Merlot samples, E. lata was reisolated from 65% of canes inoculated in October 2000, 47.5% from November, 52.5% from December, 60% from January 2001, and only 7.5% from February (Figure 1B⇓).
In the 2002/2003 trial, E. lata was reisolated from Chardonnay in 57.5% of the canes inoculated in October and November 2002, 15% from December, 47.5% from January 2003, and 10% from February (Figure 2A⇓). From Merlot, E. lata was reisolated from 82.5% of the canes inoculated in October 2002, 52.5% from November, 5% from December, 12.5% from January 2003, and 2.5% from February (Figure 2B⇓). Variance analysis on the percentage of reisolation revealed a strong date effect for 2000/01 and 2002/03 on both Merlot and Chardonnay (F value < 0.0001).
Mean lengths of vascular discoloration for the various pruning and inoculation dates in 2000/01 varied from 2.70 to 4.67 cm in Chardonnay (Figure 3A⇓) and from 1.98 to 3.07 cm in Merlot (Figure 3B⇓). In 2002/03, mean lengths of discoloration varied from 0.29 to 2.83 cm in Chardonnay (Figure 4A⇓) and 0.14 to 1.85 cm in Merlot (Figure 4B⇓).
In Chardonnay, the maximum length of vascular discoloration observed in 2000/01 was 14 cm. However, E. lata was never reisolated farther than 4 cm down from the inoculated prepruning wound in any of the samples. In 2002/03, the maximum length of discoloration in Chardonnay was 4 cm, and E. lata was not reisolated farther than 3 cm down from any inoculated prepruning wound. In Merlot, the maximum length of vascular discoloration was 12 cm in 2000/01. However, E. lata was not recovered from isolations made more than 4 cm below the inoculation point. In 2002/03 the maximum length of discoloration in Merlot was 3 cm, and E. lata was not reisolated farther than 3 cm away from the inoculated prepruning wound.
Among all the positive recoveries in both Chardonnay and Merlot, E. lata was primarily reisolated from the first 1 to 2 cm below the inoculated prepruning wound. The farthest distance from a prepruning wound that we were able to recover E. lata in Chardonnay cane pieces was 4 cm from the inoculations in October, December, and January 2000/01 and 3 cm from inoculations in October 2002. In Merlot, the maximum distance of recovery of E. lata was 4 cm in October 2000 samples and 3 cm from inoculations in October 2002.
There were no significant differences in mean yield and pruning weight data for the Chardonnay and Merlot trials among the treatments in any year (Table 1⇓, Table 2⇓). The date of prepruning did not influence vine growth the following year (as measured by pruning weights) or crop yield. Similarly, there were no significant differences among treatments in any year in either variety for clusters per vine, average cluster weight, berries per cluster, average berry weight, or in any of the berry sample juice analyses (data not shown).
Weekly observations of vines following final March pruning in 2001, 2002, and 2003 revealed no differences in the date of budbreak and early shoot growth among the pruning treatments in either variety (data not shown). All vines within each trial began growing at approximately the same time.
Discussion
The Merlot vineyard was harvested in mid-September each year and the October prepruning treatment occurred three to five weeks later. Green leaves were still present on the Merlot vines when prepruned in October 2000 and 2001. In October 2002, some green leaves were present at prepruning but most had begun to senesce and were turning yellow. The canes in October were all brown and lignified where prepruning cuts were made, although the ends of some canes were still green. No functioning Merlot leaves were still present on any of the November pre-pruning dates and canes were fully lignified. The Chardonnay vineyard was located in a cool site and historically has been one of the last to be harvested each year. The time between harvest and the October prepruning was only one to six days during the course of this trial. Green leaves were still present on the Chardonnay vines on all October prepruning dates. Canes were mostly lignified, but some were still green near the ends. By the November prepruning dates, the Chardonnay leaves had fallen or were yellow and senescing and the canes were fully lignified.
Despite early leaf removal from the October prepruning treatments, no viticultural impacts from double pruning were observed. There were no reductions in fruit production or pruning weights in subsequent years. There was no impact on budbreak, presumably because all vines had final pruning occur on the same date in March.
New Zealand Chardonnay vines that were defoliated (75%) at 4, 8, and 12 weeks after bloom had reductions in overwintering carbohydrate reserves and lower yields the following season (Bennett et al. 2005). In Australia, harvest pruning of Sultana vines for raisin production reduced leaf area by ~60% and led to yield reductions of ~10% the following season (May and Scholefield 1972, Scholefield et al. 1977). However, this loss of leaf area occurred three months before leaf fall.
In our study, vines prepruned in October lost 50 to 60% of their leaf area, but this defoliation occurred very late in the growing season (~22 weeks after bloom). Normal leaf fall occurred in these vineyards three to five weeks after the October prepruning. If there were reductions in overwintering carbohydrate reserves as a result of defoliation from October prepruning, they were not large enough to affect the following year’s growth or productivity. The loss of leaf area resulting from double pruning in October was of a relatively short duration, and it occurred late in the growing season. In October and early November, carbohydrate accumulation rates should be lower than in previous months because of shorter day lengths, cooler temperatures, and a preponderance of older leaves that are less photosynthetically active (Kriedemann et al. 1970, Alleweldt et al. 1982).
In 2000/01, E. lata was recovered from 40 to 65% of the wounds inoculated early in the winter (October through December) as compared with 7.5 to 10% of the vines inoculated in February (Figure 1⇑). Similar results were seen in 2002/03 with the exception of low recovery rates in December (both varieties) and in January for Merlot (Figure 2⇑). These data concur with previous work (Petzoldt et al. 1981) that indicates delayed pruning alone can be an effective cultural means of reducing the number of Eutypa infections.
Vascular streaking and discoloration were observed in almost every cane examined and extended as far as 14 cm from prepruning wounds. However, E. lata was not recovered from any sample farther than 4 cm below the pre-pruning wound. Xylem vessels in grapevines can be longer than 30 cm (Thorne et al. 2006). Vascular discoloration may be part of the vine’s physiological response to wounding and alone is not indicative of E. lata infection.
Final pruning in March left two-bud spurs 7 to 10 cm in length. On prepruned vines, the cane pieces removed at final pruning were ~35 to 50 cm long. Since the farthest downward growth of E. lata that we detected was 4 cm, double pruning should effectively eliminate all E. lata infections that may have occurred during the period between prepruning and final pruning.
Double pruning will not provide complete protection against Eutypa infections because final pruning wounds are still susceptible to infection, although at a much reduced rate than wounds made earlier (Petzoldt et al. 1981). If traditional pruning can be delayed until March, then double pruning would not provide any additional protection. Additionally, spore-trapping data and scouting for E. lata perithecia (Trouillas and Gubler 2000, unpublished data) showed that in years with light winter rainfall, Eutypa inoculum was still available for release with March and April rains.
Treatment of pruning wounds with a fungicide may still be necessary following final pruning in March, especially in years with light rainfall in February that would otherwise help to deplete E. lata ascospores. Because of the rapid wound healing that occurs during the spring, wound treatments should be very effective at protecting vines since susceptibility to infections occurs for only a short period. In many cases, bleeding (heavy oozing of xylem fluid) occurs following pruning in March, making effective application of wound treatments difficult. However, bleeding alone may provide some wound protection by flushing away spores.
Double pruning is not appropriate in vineyards that are cane-pruned because of the need to retain long one-year-old canes for adequate fruit production.
Conclusions
If prepruning maintains cane pieces that extend 30 to 45 cm beyond the point where final pruning will occur, then any Eutypa infections that might occur on these wounds will be fully removed during final pruning, thus indicating that double pruning can be an effective cultural practice to reduce the incidence of Eutypa infections in spur-pruned vineyards. In large operations or in vineyards with a limited labor force, double pruning should allow for more final pruning to be delayed until late February or March, thereby reducing the likelihood that Eutypa infections will occur.
This study indicates that growers could begin pre-pruning in the late fall before the onset of heavy rains, even if leaves are still present on the vines. Because access to vineyards with heavy equipment is still possible at this time, mechanical prepruning with tractor-mounted equipment becomes a more feasible option. Growers could likely use this equipment on enough acreage in late October and November in most years to justify the purchase price. Growers concerned about fall wood maturity due to high crop levels or potential cold climate effects should delay prepruning until after leaf fall.
Footnotes
Acknowledgments: The authors thank Martin Mochizuki and Walsh Vineyards Management for their assistance and access to vineyards.
- Received June 2006.
- Revision received August 2006.
- Copyright © 2007 by the American Society for Enology and Viticulture