1. Introduction
Grapevine trunk diseases (GTDs) are the leading cause of vineyard deterioration, causing a decrease in the viability of vineyards, an increase in production costs, and major economic losses in the wine industry [
1,
2,
3,
4,
5,
6]. The three main GTDs are esca disease, Eutypa dieback, and (BD) (BD), which generally attack the structural parts of plants [
2,
6,
7,
8,
9,
10]. (BD) is present on all continents where wine or table grapes are grown [
2,
5,
7,
11,
12,
13,
14,
15,
16]. Regarding the pathogens associated with BD, 26 species have been identified [
10], mainly within the genera
Botryosphaeria,
Diplodia,
Lasiodiplodia, and
Neofusicoccum [
7,
10,
11,
17]. Some genera are more common in different climates, for example,
Diplodia in temperate regions or those with cold winters and
Lasiodiplodia in tropical and subtropical areas [
18,
19,
20,
21,
22,
23,
24].
Diplodia seriata is the most prevalent species in vineyards in Central Chile [
5,
15,
22] as well as in other parts of the world, such as New South Wales [
25,
26,
27], Western Australia [
28], California [
29] and South Australia [
26].
(BD) causes essential losses in the production of grapes for wine, with a decrease from 30 to more than 50% of production depending on the severity of the disease [
5]. The loss of production is associated with the damage it causes to the vines due to the death of the wood where fruit develops, such as cankers in the wood, dead spurs, dead arms, and brown discolorations in the vascular area [
2,
5,
7,
15,
16,
30,
31,
32,
33,
34]. In the field, the prevalence and severity of (BD) increase proportionally with the age of the vineyards [
5,
6,
7,
10,
15,
33]. However, many available pathogenicity studies have been carried out on one-year-old shoots or two-year-old canes. Another critical issue to consider is the susceptibility and importance of some varieties for the wine industry. In this case, Cabernet Sauvignon is the most planted variety in Chile, and it is appreciated for good quality wines [
35].
For this reason, in this study, a pathogenicity test was carried out with five Chilean isolates of D. seriata in 2-year-old detached canes of Vitis vinifera cv. Cabernet Sauvignon, and with the two most representative isolates, a pathogenicity test was carried out on vine plants under field conditions, with inoculations in 1-year-old shoots, 2-year-old wood, and 10-year-old wood of grapevine. In this study, we observed that D. seriata is more aggressive in old tissue of grapevine cv. Cabernet Sauvignon than in young tissue, which contributes to the knowledge of the epidemiology of this disease in Mediterranean climates.
3. Discussion
This study reports for the first time the comparison of the pathogenicity of the fungus
Diplodia seriata on tissues of different ages in cv. Cabernet Sauvignon plants and demonstrates its aggressiveness in old tissue under field conditions.
Diplodia seriata is one of the most prevalent pathogens isolated from grapevines with symptoms of (BD), mainly in wine or table grape-producing areas with Mediterranean climates [
5,
6,
15,
17,
23,
25,
26,
28,
36]. Of the isolates obtained from diseased tissue,
D. seriata is the most widely distributed in studies carried out from more than 20 years to the present. In a sampling of 11 sites distributed in different vineyards in the Hunter Valley of New South Wales (NSW, Australia),
D. seriata was isolated in 90% of them [
25]. In California (USA), a survey on different varieties of grapevine (table grapes and wine) indicated that
D. seriata had the broadest geographical distribution within the
Botryosphaeriaceae found between the wine-growing areas located between Mendocino and Kem [
12]. From collections in the wine-growing regions of Western Australia and the Hunter Valley and Mudgee of NSW [
17,
28],
D. seriata accounted for more than 50% of the total number of isolates collected and present in all the regions surveyed. Pitt et al. [
26] reported that
Diplodia was the most predominant genus. The species
D. seriata represented almost 80% of
Botryosphaeriaceae isolates collected during the survey in NSW and South Australia. In Catalonia (Spain), the results of GTD-associated fungal isolates showed that
D. seriata was isolated in up to 68.4% of the samples in diseased plants, being the most prevalent species isolated [
36]. In central Chile, with a Mediterranean climate, results of studies carried out on table grapes and wine have been consistent with those of the above studies. Morales et al. [
15] reported an 83.3% prevalence of
D. seriata in table grape plants affected by (BD). Additionally, in Chile, Díaz et al. [
16] reported that
D. seriata (unlike other pathogens found) was present in all the valleys studied in a national survey of Chilean decayed vineyards. Valencia et al. [
22] recorded the dissemination of
Botryosphaeriaceae conidia, and
D. seriata was the main species detected in vineyards of cv. Cabernet Sauvignon. Specifically, in cv. Cabernet Sauvignon, a sampling carried out in the main producing area of this wine variety in Chile (O’Higgins and Maule regions), reported that in 2010,
D. seriata was the most prevalent species isolated from symptomatic vines (68%). Eight years later (2018), when sampling the same area,
D. seriata continued to be the most prevalent species, increasing its prevalence (91%), and the overall yield losses associated with (BD) were 39% in 2010 and 46% in 2018 [
5]. Based on the works mentioned above, most isolates of
D. seriata were obtained from symptomatic old wood tissues. However, pathogenicity tests for
D. seriata are mainly carried out on young tissue (shoots, one- or two-year-old canes, rotted plants, or young tissue on old plants). In these cases,
D. seriata virulence was weak or less significant than that of other
Botryosphaeriaceae species, such as
Lasiodiplodia theobromae,
L. viticola,
Neofusicoccum parvum or
Diplodia mutila [
8,
33,
37,
38], or when compared with other causal agents of GTDs, such as
Eutypa lata [
39]. This difference in virulence could be due to several factors, including the differential susceptibility of the vine varieties, the conditions, age and tissue type of the host plant, the inoculation method used, the geographic origin of the isolates, and the different incubation periods of the experiments [
7,
31,
32,
40,
41].
In our work, the pathogenicity of
D. seriata isolates was verified in plants, with inoculations on young tissue (one- or two-year-old tissue age) and on old tissue (10-year-old wood) in vine plants. The vascular lesions developed by
D. seriata on old tissue (10-year-old wood) were more than twice as long as those in the young tissue. Our results coincide with those described by Morales et al. [
15] in old table grape plants in a commercial vineyard. When inoculating shoots <one year, canes >five years old, and mature arms in a 25-year-old vines in commercial vineyards,
D. seriata increased the damage by more than 40% in mature arms with respect to the other two ages evaluated, contrary to
D. mutila. Additionally, our results are consistent with those reported in naturally affected vineyards. In the field, symptoms of (BD) increase with the age of the vineyard, both in incidence and severity [
5,
12,
15,
30]. The extension of the advance of this pathogen in the wood can be explained by the microenvironmental theory [
42], where pathogen reaction zones can alone explain the progression of fungi within tree trunks, as fungi follow dehydrated, oxygen-rich zones that occur along the reaction zones near the wounds. The high moisture content and the associated aeration restriction limit the activity of mycelial fungi in felled wood. Additionally, the most significant damage in old tissue can be explained concerning the plant microbiome, which evolves over time and with plant tissues. Microbiome diversity is higher in young vines [
1,
43,
44,
45] and in visually healthy tissues than in diseased tissues [
46].
In contrast, studies with
Neofusicoccum on walnut plants show that the relationship between age and damage is inverse. One-year-old shoots were significantly more sensitive to
N. parvum than two-year-old to four-year-old shoots wood [
47]. Therefore, correctly identifying the causal agent and studying the interaction with the host is essential.
In our work, when comparing the results of pathogenicity tests on two-year-old tissue, detached cuttings or canes of plants (attached), our study shows a difference in the severity of
D. seriata damage. This may be due to the type of inoculum, mycelium (in the first case) or conidia (in the second case), and/or the state of the tissue (detached or attacched). The vascular lesion was much more aggressive in detached cuttings inoculated with mycelium, reaching up to 12 cm, than in canes on plants, where the lesions were up to 5 cm tissue. In the literature, it is described that during the infection of the vine, a series of molecules are activated that provide protection against the spread of the pathogen [
46]. The first active defense is the formation of tylosis within the vessels. Responses to GTD fungi are thought to occur due to PAMP-activated immunity. Phytoalexins, compounds of the phenylpropanoid pathway such as resveratrol or viniferin, are found in higher amounts in tissues after GTD fungal infection [
48,
49,
50,
51,
52]. Phytoalexins can inhibit the growth and colonization of fungi, block some metabolites produced by fungi during infection, or interfere with oxidation‒reduction reactions [
52]. In addition to the active defense implemented by the vines, the anatomy of the wood could also be essential. The density of parenchymal rays and their arrangement in space could enhance active responses to infection [
54] and the diameter of the vessels could also be a factor [
55,
56,
57]. Contrary, Amponsah et al., [
58], showed a faster rate of germination of
N. luteum conidia on attached shoots and leaves wounded than on detached and unwounded leaf surfaces, this may be due to the ability of
Neofusicoccum species to attack on spring and summer time associated to shoots and leaves damage, unlike
D. seriata which mainly affects woody tissue. Regarding the type of inoculum, our results are similar to those reported by Moral et al. [
59]. They found that inoculation with a suspension of
Botryosphaeria dothidea conidia resulted in more severe disease than inoculation with a plug of mycelium in olive fruits, probably due to the presence of water in the inoculum. In addition, in other
Botryosphaeriaceae species,
B. dothidea in this case, the physiological characterization of the isolates that cause Dalmatian disease showed that the optimal temperature was 26°C for mycelium growth and 30°C for germination of conidia. Both factors can influence the aggressiveness of the symptoms, which should be studied for
D. seriata.
The results obtained in detached canes (two years old) show that, in general, all the isolates increased their pathogenicity after activation on grape berry cv. Red Globe, being higher in the oldest isolates. For example, with nonactivated isolates, the PUCV 1472 isolate did not cause damage to the cuttings (no vascular lesions and no canker), and in the assay with activated isolates (after inoculation on grape berries), vascular lesions and canker developed. The PUCV 1549 isolate increased the length of the canker by 67% after inoculation in grapes (
Figure 1a,b). Concerning this point, studies carried out with postharvest pathogens (i.e.,
Botrytis cinerea,
Colletotrichum, and
Alternaria) can explain these results. In this sense, pathogen enzymes are not all constitutive and may require induction by proper substrates [
41]. Reveglia et al. [
60] demonstrated that the
Botryosphaeriaceae family associated with (BD) in Australian grapevine leaves generates secondary metabolites. Martos et al. [
61] demonstrated that
D. seriata can produce secondary metabolites with phytotoxic properties, and it is possible that it has other mechanisms besides mycelial growth in the host that could be related to its virulence and that this phytotoxicity was strictly dependent on acidic pH substrates. This could explain the lower virulence variability of
D. seriata isolates when inoculated on fruits rather than on canes, although this requires further study. In addition, according to the phylogenetic tree obtained, the
Diplodia seriata isolates analyzed, all isolated from
Vitis vinifera, presented greater variability in the ITS-BT segment with the available
D. mutila isolates (also from
Vitis vinifera), requiring further analysis.