Introduction

Grapevine trunk diseases are caused by various fungal groups. Botryosphaeria dieback is caused by species of the Botryosphaeriaceae, Petri disease by a combination of Phaeomoniella chlamydospora and Phaeoacremonium species and esca (‘beroerte’) by the aforementioned pathogens together with wood rotting Basidiomycetes. Eutypa dieback (‘tandpyn’) is caused by species of the Diatrypaceae with Eutypa lata being the most well known. Phomopsis dieback is caused by different Diaporthe species (previously known as Phomopsis).

 

Some of the fungal pathogens associated with grapevine trunk diseases have been associated with dieback or decline symptoms of various woody hosts. Economically important agricultural crops include pome fruit trees, stone fruit trees, olive trees, date palms and kiwifruit vines. However, very little is known about the trunk pathogens present on pome and stone fruit trees.

 

In South Africa, pome and stone fruit trees are often planted in close proximity of vineyards (Figure 1). Pome fruit trees are in close proximity of vineyards in the Grabouw/Villiersdorp and Wolseley/Ceres areas. Stone fruit trees are close to vineyards in different parts of the winter rainfall area of the Western Cape, as well as summer rainfall areas in the Limpopo Province. To investigate if pome and stone fruit trees can act as an inoculum source of grapevine trunk disease pathogens, a survey was conducted to sample trees from orchards close to vineyards showing dieback or cankers (Figure 2).

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FIGURE 1. Vineyards planted next to a) an apricot orchard in Montagu and b) an apple orchard in Villiersdorp.

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FIGURE 2. Outward symptoms observed on pome fruit trees included: a) dieback, b) a canker showing wedge-shaped dark brown internal necrosis and c) a canker that developed from a pruning wound.

 

Material and methods

Apple (cv. Granny Smith) and pear (cv. Packham’s Triumph) trees older than 15 years were sampled in 2006 and 2007 in Grabouw, Vyeboom, Villiersdorp, Wolseley and Ceres. Plum, peach, nectarine and apricot orchards in high (Stellenbosch, Paarl and Franschhoek) and low (Robertson, Bonnievale and Montagu) winter rainfall areas in the Western Cape Province and in summer rainfall areas in the Limpopo Province (Mookgopong and Modimolle) were sampled in 2004 and 2005. Isolations were made from fungi developing from surface sterilised wood pieces and from sporulating structures on the bark surface. Fungal species were identified according to cultural and morphological characters and DNA sequences.

 

The pathogenicity of selected species and isolates were determined on detached shoots. For isolates obtained from pome fruit trees, detached woody shoots of grapevine (cv. Sauvignon blanc), pear (cv. Packham’s Triumph) and apple (cv. Granny Smith) were used. Three fungal groups identified from stone fruit isolations were tested in separate pathogenicity assays. Isolates were inoculated onto detached nectarine (cv. Alpine) and plum (cv. Ruby Nel) shoots; detached apricot (cv. Belida) and plum (cv. Southern Bell) shoots; or detached apricot (cv. Belida), peach (cv. San Pedro) and plum (cv. Southern Bell) shoots. Shoots were inoculated with colonised agar plugs and lesions measured after 14 days of incubation.

 

Results

 

Survey of pome fruit orchards

Six different internal symptom types similar to those occurring in grapevines were identified from pear and apple samples (Figure 3). These were brown vascular streaking, black vascular streaking, wedge-shaped necrosis, watery necrosis, brown internal necrosis and soft rot.

 

The fungal species isolated from symptomatic wood included: four species of Botryosphaeriaceae, four species of Phaeoacremonium, two species of Diaporthe, Eutypa lata (Diatrypaceae), two species of Didymosphaeria and Pyrenochaeta-like fungi (Table 1). Of these the predominant species were Diplodia seriata and Phaeoacremonium minimum.

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FIGURE 3. Internal symptom types associated with trunk disease on pome fruit trees: a) watery necrosis, b) soft rot indicated by arrow, c) brown internal necrosis, d) black/brown streaking, e) brown streaking and f) wedge-shaped necrosis.

Table 1

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Survey of stone fruit orchards

In cross-section, the symptomatic wood had either irregularly shaped or V-shaped necrotic lesions and was situated close to old pruning wounds and/or cankers, sometimes also associated with gummosis (Figure 4). In plum wood, lesions from which Phaeoacremonium species were isolated were often reddish brown in the centre and greenish towards the margin.

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FIGURE 4. Internal symptoms observed on stone fruit trees included: a) wedge-shaped necrosis, b) black/brown streaking and c) central brown internal necrosis together with black/brown streaking (indicated by arrows). (Photos: Providence Moyo.)

 

A wide diversity of fungal species were isolated from stone fruit trees and 22 new species were described (Table 2). The fungal species identified included: nine species within the Botryosphaeriaceae (three new species), 14 species of Phaeoacremonium (five new species), at least five Diaporthe species, Eutypa lata (Diatrypaceae), two species of Didymosphaeria (one new species), one new species of Paraconiothyrium, three new species of Calosphaeriaceae, a new genus, Collophora, with three species, five species of Phaeomoniellales (four new species) and three species of Coniochaeta (two new species). The predominant fungal species were Diplodia seriata, Collophora rubra, Collophora paarla, Phaeoacremoniunium scolyti, Didymosphaeria variable and Celeloriella prunicola.

 

Multiple trunk disease pathogens were often isolated in different combinations from symptomatic tissues. A specific symptoms type could not be ascribed to any single pathogen. On stone fruit trees from many specimens (mostly peach and nectarine), especially Collophora rubra, was isolated as the only fungus.

Table 2

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Pathogenicity tests

Pome fruit

Isolates obtained from pome fruit trees that were inoculated on grapevine shoots showed that Neofusicoccum australe was the most virulent species. The Pyrenochaeta-like sp., Didymosphaeria variabile, Diplodia seriata and P. fraxinopennsylvanicum, could also be considered pathogenic, since their lesion lengths were significantly longer than the negative controls. On pear, the most virulent species were Diplodia malorum and Neofusicoccum australe. The lesion lengths of Diplodia seriata and Eutypa lata were also significantly different from the negative controls. On apple, Neofusicoccum australe and Phaeoacremonium ira­nianum formed the longest lesions and were the most virulent species. Diplodia malorum, Phaeoacremonium minimum, Neofusicoccum vitifusiforme, Diplodia seriata, Phaeoacremonium fraxinopennsylvanicum, Eutypa lata and Didymosphaeria rubi-ulmifolii s.l. could be considered pathogenic with lesion lengths significantly longer than the negative controls.

 

Stone fruit

Pathogenicity tests for stone fruit isolates were conducted only for species from the Botryosphaeriaceae, Coniochaeta/Collophora and Phaeoacremonium. All Botryosphaeriaceae species tested, except Dothiorella viticola, caused lesions on nectarine shoots, and all species except Diplodia viticola and Diplodia pinea caused lesions on plum shoots that were significantly longer than the controls. All Phaeoacremonium species, except Phaeoacremonium fuscum and Phaeoacremonium pallidum, caused lesions in the xylem of plum shoots that were significantly longer than the controls. Five species caused lesions on apricot shoots that were significantly longer than the controls: Phaeoacremonium parasiticum, Phaeoacremonium iranianum, Phaeoacremonium subulatum, Phaeoacremonium griseorubrum and Phaeoacremonium africanum. Collophora africana, Collophora rubra, Coniochaeta prunicola and Celeloriella dura caused lesions on the xylem of apricot shoots that were significantly longer than the negative controls. On peach shoots, Collophora paarla, Coniochaeta africana and Neophaeomoniella zymoides caused lesions that were significantly longer than the negative controls. Only Collophora paarla caused lesions on plum shoots that were significantly longer than those caused by the negative controls.

 

Discussion

The results of the isolations confirmed that pome and stone fruit trees are hosts to known grapevine trunk pathogens of the Botryosphaeriaceae, Diatrypaceae, Diaporthe and Phaeoacremonium. Most of the species from these fungal groups have been isolated from grapevines and are known as grapevine trunk disease pathogens (Table 1 and 2). The new species in the Botryosphaeriaceae and of Phaeoacremonium need to be taken note of and could in future also be found on grapevines.

 

The Botryosphaeriaceae species that was most frequently associated with symptomatic wood of pome and stone fruit trees was Diplodia seriata. This species has also been found to be the dominant species on grapevine wood in South Africa and is a known pathogen of grapevines. Neofusicoccum australe is one of the most pathogenic Botryosphaeriaceae on grapevine, now also found on pear, peach, apricot and plum trees in this study. Phaeoacremonium minimum was the dominant Phaeoacremonium species from pome fruit trees and Phaeoacremonium scolyti from stone fruit trees. Interestingly, Phaeoacremonium minimum is also the Phaeoacremonium species found most commonly associated with Petri disease and esca of grapevines. Eutypa lata was isolated for the first time from pome fruit trees in South Africa, whereas this pathogen is a known canker pathogen of stone fruit trees and grapevines. Interestingly, Diaporthe ampelina (previously known as Phomopsis viticola, causal agent of Phomopsis cane and leaf blight/‘streepvlek’ and Phomopsis dieback) have not been found on pome and stone fruit trees.

 

Several genera were found that are less known as trunk pathogens and have so far not been found on grapevines. For example, Didymosphaeria species were found both on pome and stone fruit trees. In the pathogenicity assay Didymosphaeria variable was shown to be also pathogenic to detached grapevine shoots. The unidentified Pyrenochaeta-like species isolated from apple trees caused significant lesions on grapevine, which might be an indication that this fungus could be a possible future grapevine trunk pathogen. Genera found only on stone fruit wood include Calosphaeria, Collophora (new genus), Coniochaeta, Jattaea and species originally described as Phaeomoniella species that have recently been transferred to other genera (Aequabiliella, Celeloriella, Minutiella and Neophaeomoniella). Unfortunately pathogenicity tests of these genera were not conducted on grapevines, but the results on detached stone fruit shoots provide an indication of their relevance as trunk pathogens. Three of the species of Collophora (Collophora africana, Collophora rubra and Collophora paarla), two species of Coniochaeta (Coniochaeta Africana and Coniochaeta prunicola), Celeloriella dura and Neophaeomoniella zymoides caused lesions that were significantly longer than the negative controls. Of these fungi, Collophora rubra was the most frequently isolated species, indicating the importance of this newly found species on stone fruit trees. As the pathogenicity assays were done on detached shoots, they provide a preliminary indication of pathogenicity. Further studies are necessary to investigate the impact of the species on tree health.

 

Results of this project confirmed the presence of several major grapevine trunk pathogens in pome and stone fruit wood in South Africa. Trunk disease pathogens usually form fruiting structures on dead branches. The removal and destruction of dead/diseased branches in fruit orchards is highly recommended to minimise the formation of inoculum of trunk disease pathogens and would also prevent the spread of trunk diseases.

 

Summary

Grapevines are often grown in close proximity to stone and pome fruit trees. Fungal pathogens causing Botryosphaeria dieback, esca (‘beroerte’), Eutypa dieback (‘tandpyn’), Petri disease and Phomopsis dieback of grapevines were isolated from dieback and canker symptoms of pome and stone fruit trees. Therefore, pome and stone fruit orchards should be considered as potential inoculum sources of grapevine trunk disease pathogens. The removal and destruction of dead branches in fruit orchards is highly recommended to minimise the development of fruiting structures of trunk disease pathogens and would also prevent the spread of trunk diseases to nearby vineyards.

For more information, contact Lizel Mostert at lmost@sun.ac.za.

 

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