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Researching black foot disease in South African vineyards and grapevine nurseries |
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 Francois Halleen1, Paul H. Fourie2 & Pedro W. Crous3
1 Plant Protection Division, ARC Infruitec-Nietvoorbij, Stellenbosch. E-mail:
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2 Department of Plant Pathology, University of Stellenbosch, Matieland
3 Centraalbureau voor Schimmelcultures, Utrecht, The Netherlands
Cylindrocarpon spp. which cause black foot disease of grapevine, were
recently found to be associated with declining grapevines in South
Africa. Results obtained from the diagnostic service at ARC
Infruitec-Nietvoorbij showed that Cylindrocarpon spp. were isolated
from 52%, 22% and 29% of diseased vines during the 1999/2002, 2002/2003
and 2003/2004 seasons, respectively. However, it was unclear how
and where these infections occurred as very little information was
available regarding the aetiology and epidemiology of these
fungi. Diseased plants display an array of decline
symptoms. In vineyards, the first visible symptoms are usually
delayed budding or absence of budding (Fig. 1).
Subsequently, shoots develop abnormally and may become chlorotic and
even die during summer. When symptomatic plants are removed,
inspection of the roots may reveal the presence of brown to black
necrosis, which ultimately leads to the underdevelopment of the entire
root system. Further root abnormalities include the development
of secondary root crowns with roots growing parallel to the soil
surface. Internal symptoms include brown-black discolouration of
xylem vessels originating from the base of the rootstock (Fig. 2),
necrosis from the bark to the centre of the rootstock (Fig. 3) and
brown-black discolouration in roots (Fig. 4).
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1. A Chardonnay vineyard showing severe decline symptoms
including delayed budding and abnormal, weak vegetation. |
Fig 2. Dark vascular streaking as seen in longitudinal section of a nursery vine.
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Research objectives
The primary aims of research during the past 5 years have been (1) to
conduct nursery surveys in order to determine which pathogens might be
involved in the decline phenomenon, (2) to identify the organisms
believed to be the causal organisms of black foot disease, and (3) the
development of management strategies to prevent or eradicate infections
by these pathogens.
1) Nursery surveys
Nursery surveys were conducted in three commercial nurseries in
Wellington during the 1999/2000 season by means of destructive
sampling. The first isolations were made in September from
callused cuttings prior to planting in nurseries. After planting,
asymptomatic rooted cuttings were selected from nurseries after 3, 6
and 9 months. The isolation studies clearly demonstrated that different
Cylindrocarpon spp. infected cuttings from soil once planted in the
nurseries. These species rarely occurred in rootstock propagation
material prior to planting. At the time of planting, the
susceptible basal ends (especially the pith area) of most of the
nursery cuttings are partly or even fully exposed. Callus roots
also break during the planting process, resulting in small wounds
susceptible to infection by soilborne pathogens. The isolation studies
revealed that the first infections occurred in the roots, followed by
infections of the rootstocks. Furthermore, these infections also
increased progressively during the course of the growing season
(Halleen et al., 2003).
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Fig 3. Cross-section through infected rootstock revealing necrosis extending from the bark to the pith.
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Fig 4. Cross-section of an infected root. |
2) Identification
According to literature, black foot disease is caused by Cylindrocarpon
obtusisporum and Cylindrocarpon destructans. However, the
Cylindrocarpon isolates obtained from the nursery survey, as well as
from the diagnostic service, showed substantial variation in cultural
and morphological characters and could therefore not be identified
using "standard identification keys" like those published by Booth
(1966). Morphological and phylogenetic studies were therefore
conducted to identify these species and to establish their association
with black foot disease. Phylogenetic analyses confirmed the
diversity observed among the isolates and four Cylindrocarpon-like
species were identified. One of these groups was identified as
Cylindrocarpon destructans. A second group was newly described in
this study as Cylindrocarpon macrodidymum. The sexual stage,
Neonectria macrodidyma, was initiated by means of mating studies.
The two remaining Cylindrocarpon-like species were placed in a new
genus, Campylocarpon. The two species were named Campylocarpon
fasciculare and Campyl. pseudofasciculare. A growth study to
determine the cardinal requirements for growth also revealed further
differences between the two genera. The Cylindrocarpon group did
not grow at 30°C, whereas 30°C is the optimum temperature for the
Campylocarpon group (Halleen et al., 2004).
Pathogenicity
Inoculation of 6-month-old potted grapevine rootstocks with C.
destructans, C. macrodidymum, Campyl. fasciculare and Campyl.
pseudofasciculare resulted in death (17-35%), as well as reduced root
and shoot mass of inoculated plants. Any of these species can
therefore be regarded as potential pathogens of black foot disease
(Halleen et al., 2004).
3) Control / Management
Knowledge obtained pertaining to the infection period and site in the
nursery survey suggested that any suitable control method will have to
focus on preventing or eradicating infection of the basal ends of
nursery cuttings. Field trials were therefore conducted in two
field nurseries. After callusing, the basal ends of grafted
cuttings were dipped in various treatments prior to planting.
Additional treatments involved soil amendments with Trichoderma
formulations and hot water treatment of uprooted dormant nursery
grapevines. Nursery plants were uprooted after eight months and
evaluated by means of destructive sampling. If all factors are
taken into consideration (for example the percentage fungal infection,
take percentages, root- and shoot mass), none of the chemical and
biological treatments prevented or reduced infection significantly
and/or consistently. However, hot water treatment of uprooted
dormant nursery grapevines (50°C for 30 min) completely eradicated
these infections (Halleen et al., 2005).
Conclusion
The diversity of species associated with black foot disease has been
confirmed by this study. The reduction of Cylindrocarpon and
Campylocarpon infection in uprooted dormant nursery grapevines caused
by hot water treatment clearly demonstrated the potential of this
control measure. Apart from these pro-active management strategies in
grapevine nurseries, no curative strategy is known for declining
grapevines in vineyards. Producers are therefore urged to heed general
recommendations to prevent and/or correct predisposing stress
situations, such as soil compaction and poor drainage. Stress is
one of the most important aspects of this disease, and the following
points are therefore very important. Soil preparation is of
utmost importance, and compacted layers should be broken up.
Drainage must be improved on wet or waterlogged soils.
Establishment or planting of new vineyards should be done according to
"best practice" procedures. This includes proper root
distribution, correct plant holes, etc. Prevent and/or control
other diseases and pests like Phytophthora, Pythium, nematodes,
etc. Planting of certified vines according to best practice
procedures, and thereafter carefully managing them to ensure proper
root development, should go a long way in ensuring successful
establishment of a new vineyard.
References
- Booth C., 1966. The genus Cylindrocarpon. Mycological Papers 104, 1-56.
- Halleen F., P.W Crous and O. Petrini, 2003. Fungi associated with
healthy grapevine cuttings in nurseries, with special reference to
pathogens involved with in the decline of young vines.
Australasian Plant Pathology 32, 47-52. [pdf]
- Halleen F., H-J. Schroers, J.Z. Groenewald and P.W. Crous, 2004.
Novel species of Cylindrocarpon (Neonectria) and Campylocarpon
gen. nov. associated with black foot disease of grapevines (Vitis
spp.). Studies in Mycology 50, 431-455. [pdf]
- Halleen F., P.H. Fourie and P.W. Crous, 2005. Proactive
management of black foot disease in South African grapevine
nurseries. Phytopathologia Mediterranea 44, 118.
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