The aim of this study was to determine the occurrence of Eutypa-like species associated with dieback and canker-affected grapevines and to determine their status as grapevine pathogens.
The trunk disease complex which comprises of grapevine canker diseases, including Eutypa dieback, Phomopsis dieback, as well as Botryosphaeria dieback and canker, is responsible for shortened life span of vineyards and severe losses as a result of vineyard re-establishment and reduced yields. For many decades, canker diseases on grapevine have been attributed to the fungal pathogen Eutypa lata. Recent studies, however, have reported the occurrence of several Eutypa-like species (family Diatrypaceae) on grapevines affected with cankers, worldwide. These findings led to several investigations where many of the newly discovered species were found to occur more abundantly and widely distributed than E. lata.
Similar to other grape-growing countries, canker pathogens are important in South Africa, but little information is currently available about the occurrence and identity of Eutypa-like species in South African vineyards.
Materials and methods
Surveys were carried out in different grape-growing areas in the Western Cape Province to identify Eutypa-like species associated with dying spurs, wedge-shaped cankers, as well as those that produce fruiting bodies on grapevine.
Spur dieback and poor budburst had been identified as a problem within Sauvignon blanc and Cabernet Sauvignon vineyards. To identify pathogens associated with this phenomenon, spurs on four- to eight-year old Sauvignon blanc and Cabernet Sauvignon vineyards were sampled. Ten dying spurs were collected from each of 19 Sauvignon blanc and 17 Cabernet Sauvignon vineyards surveyed. To identify species associated with wedge-shaped cankers, older vineyards (≥ 10 years old) were selected based on the presence of symptoms including dying spurs, stunted shoots, shortened internodes and cankers. Cordons and trunks of such vines were cut in cross section and isolations were conducted from wedge-shaped necrosis typical of Eutypa infections. Samples were collected from 10 cultivars, namely Cabernet Sauvignon, Chardonnay, Chenin blanc, Merlot, Pinot noir, Pinotage, Sauvignon blanc, Sémillon, Shiraz and Viognier. Dead grapevine wood with Eutypa-like fruiting bodies were also collected from vineyards for identification.
Three different trials were conducted to determine the pathogen status of the Eutypa-like species found on grapevine from the current surveys, as well as similar species found recently on other woody hosts in close proximity to vineyards in the Western Cape. In Trial 1, pruning wounds of a nine-year old Cabernet Sauvignon vineyard were inoculated with spores of eight Eutypa-like species. In Trial 2, green shoots of a 14-year old Sauvignon blanc and nine-year old Cabernet Sauvignon vineyard were wounded and inoculated with mycelial plugs of 15 Eutypa-like species. In Trial 3, one-year old lignified canes of a nine-year old Cabernet Sauvignon vineyard were inoculated with mycelial plugs of the same 15 Eutypa-like species used in Trial 2.
Seven Eutypa-like species, namely Cryptovalsa ampelina, Cryptovalsa rabenhorstii, Eutypa consobrina, Eutypa lata, Eutypella citricola, Eutypella microtheca and a new species, Eutypa cremea, were isolated and identified in this study. Cryptovalsa ampelina was the most commonly isolated species from the 360 dying spurs collected (Figure 1A and B), comprising 46.4% of the total number of isolates obtained. Eutypa lata was the most common species (89.2% of all isolates) isolated from the 269 grapevine samples showing wedge-shaped necrosis (Figure 1C). A total of 81 grapevine wood pieces with Eutypa-like fruiting bodies (Figure 1D) were collected from vineyards. Fruiting bodies of E. lata were the most frequently encountered. Fruiting bodies of Cryptovalsa ampelina were the second most abundant, followed by Eutypa cremea, Eutypella citricola and Eutypella microtheca in much lesser numbers. The total number of isolates obtained for each species from the different symptom types and fruiting bodies are presented in Table 1. In many cases, more than one species were found to occur in one symptom and species were also isolated from spurs collected from vineyards as young as four-years old.
FIGURE 1. Symptom types targeted during grapevine surveys. (A) Dying spur (arrow) observed in a Cabernet Sauvignon vineyard. (B) Dying spur cut through the pruning wound revealing internal symptoms. (C) Wedge-shaped necrosis in grapevine cordon. (D) Stromata, showing fruit body cavities of Cryptovalsa ampelina, on dead grapevine wood. Fungal spores which are able to infect healthy vines are produced within these structures.
Inoculations of Cabernet Sauvignon and Sauvignon blanc grapevines with Eutypa-like species resulted in wood necrosis. In all three field trials, the various Eutypa-like species, whether originating from grapevine or non-grapevine hosts, were able to infect, colonise and cause lesions on grapevine. Several species, including C. ampelina and Eutypella citricola which are the second and third most frequently isolated species, respectively, caused lesions similar to Eutypa lata, the well-known grapevine pathogen.
This study identified several Eutypa-like species associated with spur dieback and wedge-shaped necrosis on grapevine and also identified species which are able to complete their life cycle on grapevine by producing fruiting bodies. Overall, seven Eutypa-like species were identified from the vineyard surveys. Except for E. lata and C. ampelina, all the other species are reported for the first time on grapevine in South Africa. Furthermore, this is the first study to associate E. cremea and E. consobrina with dieback of grapevine, worldwide. The isolation of several Eutypa-like species in vineyards as young as four-years old, suggests that these species are involved, singly or synergistically, in the process of dieback and canker development and ultimately, shorten the productive life of vineyards.
Pathogenicity of various Eutypa-like species was confirmed in the study. Regardless of the origin of these species, whether from grapevine or non-grapevine hosts, several species consistently produced lesions that were similar to those produced by E. lata, a known grapevine canker pathogen.
Eutypa lata and most other trunk pathogens infect grapevines primarily through susceptible pruning wounds. The fact that all these species were able to infect grapevine pruning wounds, green shoots, as well as lignified canes, adds another dimension to the epidemiology of these pathogens. Very little is known about the infection of green and lignified shoots during the season.
Eutypa lata and several Eutypa-like species were identified as grapevine pathogens and should be taken into consideration in integrated management strategies to ensure sustainable grapevine production in South Africa. The fact that Eutypa-like isolates collected from non-grapevine hosts are pathogenic to grapevine, further highlights this point. Management practices should include sanitation practices to reduce inoculum sources within and around vineyards. Such practices include the removal of dead wood in vineyards and surrounding fruit orchards, but also in other woody hosts in close proximity to vineyards. Dead wood attached to trellis wires is a significant source of inoculum. When removed, the wood must immediately be destroyed. If the wood is chipped, it must be done as fine as possible and preferably composted. However, the most important control measure is the protection of pruning wounds with fungicides and/or biological control agents. These treatments must be applied as soon as possible after pruning, but in the case of sapflow, the optimum time is six hours after pruning. The occurrence of Eutypa-like species in dying spurs of plants as young as four years clearly highlights the fact that pruning wound protection must start when the first wounds are made in new vineyards.
Eutypa lata and six other Eutypa-like species were isolated from grapevines. Five of these, namely Cryptovalsa rabenhorstii, Eutypa consobrina, Eutypella citricola, Eutypella microtheca and a new species, Eutypa cremea, were isolated for the first time from South African grapevines. These species, together with eight other Eutypa-like species recently isolated from other woody hosts in close proximity to vineyards were subjected to pathogenicity studies. All 15 species, whether originating from grapevine or non-grapevine hosts, were able to infect, colonise and cause lesions on grapevine. Eutypa lata and several Eutypa-like species were identified as grapevine pathogens and should be taken into consideration in integrated management strategies, based on sanitation and pruning wound protection practices, to ensure sustainable grapevine production in South Africa.
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