|
A Technical Guide for Wine Producers
|
|
RECENT ARTICLES | WYNBOER HOME
Diagnosis of fungal diseases and their involvement in dieback disease of young vines
Paul Fourie and Francois Halleen
Diagnostic service
The Disease Management Division of ARC Infruitec-Nietvoorbij offers producers and consultants a diagnostic service for vine diseases. Vine samples (preferably intact vines with soil around the roots) are collected during consultations by pathologists of this Division, or submitted to Nietvoorbij by producers or consultants. Information required about the specific samples or disease condition includes: scion and rootstock cultivar, date of planting / age of vine, nursery of origin, distribution patterns of diseased vines in the block, and any other remarks that may be useful to the pathologists. After being submitted, external and internal symptoms are noted and photographed, whereafter isolations are made from the symptomatic tissue. After an incubation period of about two weeks, fungi growing out of the isolated tissue sections are identified. Data obtained from the diagnostic analysis, as well as the background information provided by the client, are then used to explain the most likely cause for the decay or dying back. A report with a description and colour photographs of the symptoms, a description of the particular diseases, as well as a recommendation are then mailed to the client.
Apart from the light that such a diagnostic analysis sheds on the dark side of vine diseases, it also provides researchers with valuable information about new and unknown diseases. Moreover, it allows researchers to keep a finger on the pulse of the vineyard industry in South Africa.
In the past two seasons 232 vine samples were analysed in this way. Of these vines, more than 80% were 5 years old and younger. The majority of these vines were infected with black goo (Phaeomoniella chlamydospora and Phaeoacremonium spp.) and/or black foot fungi (Cylindrocarpon spp.). Other well-known vine pathogens were also isolated out of the internal wood symptoms. In Table 1, a summary is given of the fungal genuses that were isolated, frequencies at which they occurred, associated symptoms, organs from which they were isolated, as well as the disease caused by them. From the older vines (8 years and older) Eutypa and Phomopsis spp. were regularly isolated from the V-shaped necrotic sectors associated with Eutypa dieback (Fig 1). These fungi infect pruning wounds and in due course cause the dying back of arms.
Table 1: Vine pathogens isolated from 232 samples in the course of diagnostic analyses during the 1999/2000 and 2000/2001 seasons. Fungus
isolated (genus) Occurrence(%) Cylindrocarpon 51.7 Roots,
rootstock Vascular
discolouration; Black
foot Phaeomoniella 42.2 Roots,
rootstock; graft union; trunk; arms Vascular
discolouration; Black
goo Phaeoacremonium 11.6 Roots,
rootstock; graft union; trunk; arms Vascular
discolouration; Black
goo Botryosphaeria 9.1 Rootstock;
trunk; arms Vascular
discolouration; Black
dead arm Phomopsis 3.4 Rootstock;
trunk; arms Necrotic
sectors Phomopsis;
eutypa Eutypa 0.9 Trunk;
arms Necrotic
sectors Eutypa Verticillium 1.7 Roots;
rootstock Vascular
discolouration Wilting Phytophthora 5.2 Roots;
rootstock; (soil) Rotted
roots Collar
and root rot Pythium 6.9 Roots;
(soil) Rotted
roots Root
rot
The root pathogens, Phytophthora and Pythium spp., were mainly isolated from the soil, using baiting techniques.
Black goo and Black foot
Black goo dieback in South Africa is caused mainly by Phaeomoniella chlamydospora (Groenewald et al., 2001), but at least two Phaeoacremonium spp. are also associated with the disease. Repeated isolation from asymptomatic nursery vines (Halleen and Crous, 2001), rootstock shoots and rootstock mother plants point at the endophytic or dormant nature of these fungi. Consequently the main mechanism of distribution appears to be via infected propagation material. Stress conditions in vineyards result in symptom expression of infected plants. Symptoms involve the yellowing of foliage (Fig 3), at times accompanied by necrosis, dwarfed shoot growth, delayed or green budding (Fig 4), wilting and dieback. Internal wood symptoms entail brown to black discoloured xyleme vessels (Fig 5), as well as dark brown discoloured wood tissue adjacent to yellow-brown, soft wood rotting in the pith of older vines. The black goo fungi are also precursors of the esca disease complex. Esca is a problem in Europe especially, where it causes the decay and death of old vines. Although esca-like symptoms (internal yellow-brown, soft wood rotting of the stem and/or arms) have been observed in South Africa (Fig 2), the wood rotting fungus in this complex, Fomitiporia punctata, has not yet been identified here.
Cylindrocarpon species, a group of fungi commonly occurring in the soil, were also frequently isolated from the diseased vines. Black foot in vines is caused by C. destructans (Sweetingham, 1983; Rego et al., 2000) or C. obtusisporum (Grasso and Magnano-di-san-Lio, 1975; Scheck et al., 1998). Information derived from research projects at Nietvoorbij indicates that various closely related species of the Cylindrocarpon genus are involved in this disease. A survey of the fungi occurring in nursery material and vines indicated that Cylindrocarpon spp. rarely occur in the propagation material, but that the roots and rootstocks of the young vines are infected from the nursery soil (Halleen and Crous, 2001). This infection is visible as a black discolouration of the vascular tissue starting in the foot of the vine (Fig 6). The extent to which these light infections influence the plant is still uncertain. When the plant experiences stress conditions, however, it cannot offer sufficient resistance to further infection and colonisation, consequently the infection becomes worse. Advanced symptoms of black foot are also a watery brown-black rotting from the phloem (bark) to the pith of the vine (Fig 7 and 12). At times of high water requirement, infected plants will show growth arrest, decline and wilting symptoms. The blockage in the vascular tissue obstructs the transport of water and nutrition to the parts above the surface and the water requirement of seriously infected plants is not satisfied. The plants wilt and die back. The leaves turn brown and remain suspended on the vine (Fig 8). Phytophthora root rot causes similar symptoms.
The black goo or black foot infected vines submitted to Nietvoorbij for diagnostic analysis over the past few seasons had clearly been subjected to stress conditions in almost all instances, resulting in the expression of disease symptoms. Various stress conditions were observed: malnutrition (bad spots in vineyards), drought stress, poorly drained conditions, soil compaction, weakening of plants by other diseases, heavy crop yields, and poorly callused, infected or dried out graft joints. Planting young vines in poorly prepared soil, improper plant holes, or holes that are either too small or too shallow, results in a potbound effect which also causes various stress conditions.
Botryosphaeria
Botryosphaeria is a very complex fungal genus with various species associated with various symptoms (Fig 9, 10 and 11). Botryosphaeria stevensii (Lehoczky, 1988), B. obtusa (Mugnai et al., 1999; Larignon and Dubos, 2001), B. dothidea (Milholland, 1988; Phillips, 2000; Larignon and Dubos, 2001) and B. rhodina (Hewitt, 1988) occur on vines. These pathogens are associated with symptoms ranging from berry rot, leaf yellowing, poor budding, wilting and discolouration of vascular tissue to dieback of shoots, arms and even trunks. The role of each of these species is still not entirely clear. It is possible, however, that in some cases they may be primary pathogens of vines subject to stress conditions, while in other cases they may be considered secondary infectors of wood which has already been colonised by other pathogens (Pascoe, 1998). To further complicate identification of these fungi, various anamorphous (asexual phase) phases, inter alia Botryodiplodia, Dothiorella, Lasiodiplodia, Macrophoma, Macrophomopsis, Diplodia, Fusicoccum and Sphaeropsis, are also associated with Botryosphaeria (Phillips, 2000). Research under the supervision of Prof. Pedro Crous (Department of Plant Pathology, University of Stellenbosch) aims to characterise the various species occurring on vines in order to determine their significance in the South African vineyard context.
Verticillium
Verticillium is a well-known root pathogen of vines causing wilting of the entire, or only parts of the plant (Schnathorst and Goheen, 1988). Wilting occurs mainly during the warm times of the season when water requirement is high. This fungus also causes browning in the vascular tissue of the roots and trunk (Fig 12). Verticillium-wilt occurs particularly when vines are planted in soil in which susceptible agricultural crops have been cultivated. Cultivation of susceptible crops can result in the accumulation of the fungal population in the soil. Symptomatic plants are widespread and are noticed especially in the second year after planting. New infections will cause wilting in future seasons. After five or six seasons, the symptomatic plants that have not died, will recover. The disease will no longer be visible and the vines that have recovered do not show any significant side-effects.
Phytophthora and Pythium
At the moment the bulk of the information on these important pathogens is based on research conducted by Dr. PG Marais in the early 1970s. This work showed Phytophthora cinnamomi to be the most virulent vine root pathogen. Most losses occur on young vines, but also on vines as old as 10 years. The other Phytophthora and Pythium spp. can also kill young vines, but in older vines they will only result in decay and poor growth, due to the rotting of the fine roots and root tips. This infection can put stress on vines, however, making them more predisposed to other diseases, such as black foot and black goo. Results of root level isolations indicated Phytophthora-infection to be the result of planting infected material, rather than infection from the soil. However, in the case of Pythium both infected plant material and soil infection appear to play a role. According to Marais (1988) infected nursery material is therefore the single biggest factor responsible for the spreading of these pathogens. In a few cases only infected soil could be proven as the primary cause of dieback of vines in new vineyard plantings. According to Sandra Denman (Dept. of Plant Pathology, US) Phytophthora spp. is widespread in soil and irrigation water, which could also therefore act as sources of infection.
Phytophthora cinnamomi first infects the phloem vascular tissue (bark) and symptoms of infected roots are usually brown, rotted root bark separating from the root. The underlying tissue will then appear brown. The first symptoms of infected vines are the leaves appearing slightly wilted and also lighter in colour than healthy vines. Leaf browning also occurs in time, the vine wilts and when the vine dies, the leaves appear dark brown. A typical symptom is that the browned leaves as well as bunches remain hanging on the dead vine (Fig 8).
Conclusion
The past two season's diagnostic results indicate that decay and dieback of young vines can be ascribed mostly to black goo and black foot. It is also clear, however, that various pathogens cause similar symptoms in vines. Various pathogens have also been isolated from the same symptomatic tissue. It is possible, furthermore, that slow-growing, primary pathogens are overgrown by the fast-growing saprophytes. All these factors complicate accurate diagnosis and once again underline the importance of research into these diseases and/or disease complexes.
For further information about these diseases or the diagnostic service, contact Paul Fourie at tel: 021-809 3104, or Francois Halleen at tel: 021-809 3040.
Literature
Groenewald M., Kang, J., Crous, P.W. and Gams, W. 2001. ITS and ß-tubulin phylogeny of Phaeoacremonium and Phaeomoniella species. Mycological Research (in press). |
|
Wynboer is incorporated in WineLand, magazine of the SA wine producers.
Visit our sister sites:
|
| UP |
COPYRIGHT (C) 2000 WineLand |