Infections that occur during spring lead to chronic disease ; however, infections that occur during late summer and fall may cause disease symptoms in the current year, but a high proportion of vines lack symptoms of X. fastidiosa infection in the following year . Nonetheless, models that incorporate low temperatures have substantial explanatory power in predicting rates of winter curing of X. fastidiosa infections in grapevine . Infections that occur early in the season may have a longer period during which X. fastidiosa can colonize and reach high infection levels, which may increase the likelihood of the disease surviving over the winter. Following this rationale, if most late-season infections remain in the distal ends of shoots and have lower infection levels, removing the symptomatic portion of the vine might eliminate X. fastidiosa. In other words, the efficacy of pruning infected grapevine tissue could depend on both the time of year in which the plant was infected and on winter temperature. A potential benefit of severe pruning versus replanting is that pruning leaves a mature root stock in place, which is likely to support more vigorous regrowth compared to the developing root stock of a young transplant . Recent attempts to increase vine productivity by planting vines with more well-developed root systems are based on this presumption. However, even if severe pruning can clear vines of infection,grow bag for blueberry plants it removes a substantial portion of the above ground biomass of the vine. Thus, a method for encouraging rapid regrowth of the scion after aggressive pruning is needed. We studied the efficacy of pruning infected vines immediately above the root stock graft union—the most aggressive pruning method—for clearing grapevines of infection by X. fastidiosa.
We reasoned that if such severe pruning was ineffective at clearing vines of infection, less severe pruning would not be warranted; if severe pruning showed promise, less severe pruning could then be tested. We use the term “severe pruning” to refer to a special case of strategic pruning for disease management, analogous to the use of “remedial surgery” for trunk diseases . To test the efficacy of clearing vines of X. fastidiosa infection, we followed the disease status of severely pruned versus conventionally pruned vines over multiple years, characterized the reliability of using visual symptoms of PD to diagnose infection, and compared two methods of restoring growth of severely pruned vines.Pruning trials were established in Napa Valley, CA in commercial vineyards where symptoms of PD were evident in autumn of 1998. The vineyards used for these trials varied in vine age, cultivar, and initial disease prevalence . All study vines were cordon-trained and spur-pruned. We mapped the portions of the six vineyards selected for study according to evaluation of vines for disease symptoms. The overall severity of PD symptoms for each vine was recorded as follows: 0 = no symptoms, apparently healthy; 1 = marginal leaf scorch on up to four scattered leaves total; 2 = foliar symptoms on one shoot or on fewer than half of the leaves on two shoots on one cordon, no extensive shoot die back, and minimal shriveling of fruit clusters; and 3 = foliar symptoms on two or more shoots occurring in the canopy on both cordons; dead spurs possibly evident along with shriveled clusters. To test the reliability of the visual diagnosis of PD, petiole samples were collected from the six vineyard plots when symptom severity was evaluated for vines in each symptom category; these samples were assayed using polymerase chain reaction . Petioles were collected from symptomatic leaves on 25, 56, and 30 vines in categories 1, 2, and 3, respectively.
Next, severe pruning was performed between October 1998 and February 1999 in the six vineyard plots by removing trunks of symptomatic vines ~10 cm above the graft union. Cuts were made with saws or loppers, depending upon the trunk diameter. During a vineyard survey, severe pruning was conducted on 50% of vines in each symptom category; the other 50% of vines served as conventionally pruned controls. Sample sizes for control and severely pruned vines in each disease category ranged between six and 62 vines depending on the plot, with at least 38 total vines per plot in each control or pruned treatment. In spring 1999, multiple shoots emerged from the remaining section of scion wood above the graft union on severely pruned vines. When one or more shoots were ~15 to 25 cm long, a single shoot was selected and tied to the stake to retrain a new trunk and cordons, and all other shoots were removed at this time. We evaluated the potential of severe pruning to clear vines of infection, by reinspecting both control and severely pruned vines in all six plots for the presence or absence of PD symptoms in autumn 1999 and 2000. In all plots, category 3 vines were inspected in a third year ; in plot 6, vines were inspected an additional two years . Finally, in plot 6 we investigated chip-bud grafting as an alternate means of ensuring the development of a strong replacement shoot for retraining. To do this, 78 category 3 vines were selected for severe pruning, 39 of which were subsequently chip-bud grafted in May 1999. An experienced field grafter chip budded a dormant bud of Vitis vinifera cv. Merlot onto the root stock below the original graft union, and the trunk and graft union were removed. The single shoot that emerged from this bud was trained up the stake and used to establish the new vine. The other 39 vines were severely pruned above the graft union and retrained in the same manner as vines in plots 1 to 5. Development of vines in plot 6, with and without chip-bud grafting, was evaluated in August 1999 using the following rating scale: 1) “no growth”: bud failed to grow, no new shoot growth; 2) “weak”: multiple weak shoots emerging with no strong leader; 3) “developing”: selected shoot extending up the stake, not yet topped; and 4) “strong”: new trunk established, topped, and laterals developing. All analyses were conducted using R version 3.4.1 .
We used a generalized linear model with binomial error to compare the relative frequency of X. fastidiosa-positive samples from vines in the different initial disease severity categories . Next, we analyzed the effectiveness of chip budding versus training of existing shoots as a means for restoring vines after severe pruning. This analysis used multinomial logistic regression that compared the frequency of four vine growth outcomes the following season: strong, developing, weak, or no growth. This main test was followed by pairwise Fisher exact tests of the frequency of each of the individual outcomes between chip budded-trained and trained vines . We analyzed the effect of severe pruning on subsequent development of PD symptoms using two complementary analyses. First, we compared symptom return between severely pruned and control vines in the three symptom severity categories for two years after pruning. To appropriately account for repeated measurements made over time, our analysis consisted of a linear mixed-effects model with binomial error, a random effect of block, and fixed effects of treatment , year ,blueberry grow bag and symptom severity category . Next, we analyzed the rate at which PD reappeared in only severely pruned vines from category 3 in subsequent years using a survival analysis. Specifically, we used a Cox proportional hazards model with a fixed effect of plot .Accurate and time- or cost-efficient methods of diagnosing infected plants are important elements of a disease management program, both with respect to roguing to reduce pathogen spread , and the efficacy of pruning to clear plants of infection . Accurate diagnosis of PD in grapevines is complicated by quantitative and qualitative differences in symptoms among cultivars and other aspects of plant condition . Our results suggest that a well-trained observer can accurately diagnose PD based on visual symptoms, particularly for advanced cases of the disease. The small number of false positives in disease category 1 and 2 vines may have been due to misdiagnosis of other biotic or abiotic factors . Alternatively, false positives might indicate bacterial populations that are near the detection limit; conventional PCR has at least as low a detection threshold as other methods that rely on the presence of live bacterial cells . Regardless, although scouting based on visual symptoms clearly captured most cases of PD in the current study, some caution should be used when trying to diagnose early disease stages to ensure that vines are not needlessly removed. There is no cure for grapevines once infected with X. fastidiosa, except for recovery that can occur in some overwintering vines . The virulent nature of X. fastidiosa in grapevines, and the corresponding high mortalityrate for early season infections, increases the potential value of any cultural practices that can cure vines of infection. Moreover, new vines replanted into established vineyards generally take longer to develop compared to vines planted in newly developed vineyards, potentially due to vine-to-vine competition for resources that limits growth of replacement vines. As a result, vines replanted in mature vineyards may never reach full productivity . Thus, management practices that speed the regeneration of healthy, fully developed, and productive vines may reduce the economic loss caused by PD . A multinomial logistic regression showed significant differences in the relative frequency of different grapevine growth outcomes between the two restoration methods .
Chip-budded vines showed significantly lower frequency of strong growth and significantly higher frequencies of vines with developing growth and, especially, of no growth . Nearly 30% of chip-budded vines showed no growth in the following season, compared to 0% of vines on which established shoots were trained. These results indicate that training newly produced shoots from the remaining section of the scion was more likely to result in positive regrowth outcomes. As a result, of the two methods we evaluated, training of shoots that emerge from the scion of a severely pruned trunk is recommended for restoring growth. However, it is important to note that the current study did not estimate the amount of time required for severely pruned vines to return to full productivity. Moreover, the study did not include mature vines, in which growth responses may differ from young vines. Additional studies may be needed to quantify vine yield, and perhaps fruit quality, in severely pruned vines over multiple seasons. The usefulness of pruning for disease management depends on its ability to clear plants of pathogen infection . A comparison of symptom prevalence among severely pruned and control vines from different disease severity categories showed significant effects of the number of years after pruning , pruning treatment , and initial disease symptom category . The analysis also showed significant interactions between year and treatment and between treatment and symptom category , a non-significant interaction between year and symptom category , and a marginally significant three-way interaction . Overall, more vines had symptoms in the second year compared to the first , and there was a higher prevalence of returning symptom in vines from higher initial disease categories . Severe pruning showed an apparent benefit to reducing symptoms of PD after the first year, but this effect weakened substantially by the second year, with no differences for category 1 or 3 vines, and a slightly lower disease prevalence for severely pruned category 2 vines . A survival analysis of severely pruned category 3 vines showed a significant difference in the rate of symptom return among plots . All vines in plots 1 to 3 had symptoms by autumn 2000, two years after pruning . In plots 4 and 5, more than 80% of vines showed symptoms after three years. Only plot 6 showed markedly lower disease prevalence; in plot 6, ~70% and 50% of severely pruned category 3 vines showed no symptoms after two and four years, respectively, versus ~36% of control vines overall, after two years. It is important to note that at the time of this study, disease pressure may not fully explain the return of symptoms in severely pruned vines.