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Control measures for black goo and black foot of grapevines: preliminary results
Despite the actuality and importance of these diseases among grapevine pathologists and producers throughout the world, very little information has been published about their etiology, epidemiology and control. In South Africa knowledge of black goo and black foot is based on the limited amount of published information, non-published results from various research projects concerning these diseases, as well as observations made during consultations, vineyard visits and diagnostic analyses.
The Disease Management Division of ARC Infruitec-Nietvoorbij is currently involved in various research projects regarding the control of these diseases. The purpose of this article is to suggest preliminary control measures to the industry, based on available information and results from research projects.
Pro-active control in nurseries
Since black goo and black foot are distributed mainly through infected nursery plants, and considering the responsibility of nurseries to provide disease-free plants to the industry, it is extremely important for nurseries to take pro-active control measures. For the pro-active control of these diseases in nurseries, the following aspects must be emphasised:
Pathogen-free grafting material. Current information indicates that black goo fungi are distributed to nurseries in infected rootstock grafting material. No information about the relative occurrence of these fungi in grafting material of various cultivars, mother blocks or regions is currently available. It is therefore important that all rootstock mother blocks are monitored for the occurrence of black goo fungi. This information will shed more light on the distribution of the infection in various rootstock cultivars, plant ages, regions, soil types, etc., which will help researchers to understand the etiology of black goo decline better. It will also provide industry with the necessary information to implement measures to minimise the distribution of the black goo fungi. Researchers from Nietvoorbij, in co-operation with KWV, have already taken steps to conduct this survey.
The treatment of grafting material before grafting to reduce infection levels is one option that is currently being investigated by researchers in co-operation with KWV-Picardi. The effects of hot water treatment and immersion of grafting material in chemical or biological solutions on the occurrence of black goo fungi are being compared in a nursery trial. The preliminary results are discussed below.
Aspects that remain to be investigated are the possible infection of rootstock mother vines during and after harvest, the importance of pruning wound protection in rootstock mother blocks, as well as the infection of rootstock shoots during the harvest, hydration and storage.
Nursery practices. Wounds inflicted on vines during the grafting process may be infected by a variety of fungi during grafting, callus or planting. Researchers will also attempt to evaluate and compare existing sanitation practices with more recent chemical and biological processes. Wounds and callus tissue in the grafting joint and the vine foot must be protected, especially at planting. Soil treatment and preparation to reduce soil pathogen populations are other possibilities that must be studied.
From preliminary trials conducted in conjunction with Hygrotech Seed, grafting material was treated with Trichoderma fungi (Trichoflow-T?, Agrimm Technologies Ltd, New Zealand), instead of fungicides, to try and obtain biological control of decay fungi. The vines were planted in soil that was treated with Trichoderma (Trichopel?, Agrimm Technologies Ltd, New Zealand), and the root zone was treated with Trichoderma (Trichogrow?, Agrimm Technologies Ltd, New Zealand) on a monthly basis. These treatments resulted firstly in a considerable increase in root mass (from 25.7 g to 36.5 g), and moreover in a reduction in the number of black goo fungi isolated from the rootstock (from 8.6% to 5.2%), as well as black foot fungi isolated from the roots (from 2.8% to 1.6%) (Fourie et al., 2001). Consequently the production of hardier and less infected vines resulted from this treatment. However, it remains uncertain whether the mere reduction of infection levels will be sufficient to control black goo and black foot. These and other products will be tested on a bigger scale with various grafting combinations in different nurseries.
Hot water treatment of grafting material. Preliminary results of research conducted at Nietvoorbij indicated that hot water treatment of grafting material (30 min at 500C) resulted in a significant reduction in the occurrence of black goo fungi in vines. Infection levels, determined after nursery vines were uprooted (June), were reduced from 6.0% to 2.5%. Hot water treatment of grafting material (500C for 30 min) is also recommended in Australian nurseries (Waite et al., 2001). However, local research has indicated that hot water treatment reduced starch levels in grafting material (Hunter, 2001). This may result in a lower callus percentage, as well as reduced initial root and shoot development in certain cultivars (Waite et al., 2001). From the above-mentioned research it was therefore clear that only good quality grafting material should be used.
Hot water treatment of nursery vines. The immersion of nursery vines (after uprooting) in hot water (500C for 15 min) controlled Phytophthora (Von Broembsen and Marais, 1978) and nematode infestations (Barbercheck, 1986). Hot water treatment (30 min at 500C) of dormant nursery vines reduced the occurrence of black goo fungi in the vines from 7.2% to 0.8%. It is therefore clear that hot water treatment does not completely eradicate black goo fungi in nursery vines, but it does reduce the infection levels thereof significantly. Although hot water treatment of nursery vines is practically feasible, the question remains whether it is justifiable, considering the additional cost of this treatment and the large number of vines that must be grafted and sold annually. Plants that have been treated with hot water are also more susceptible to drying out during transport, storage and planting. The effect of hot water treatment on the starch levels of nursery vines is still unknown. Should it reduce starch levels, as in the case of grafting material, it may cause establishment problems in certain cultivars.
Based on the available information, hot water treatment (30 min at 500C) of grafting material, combined with soil and root treatments with Trichoderma products, is strongly recommended. These treatments did not eliminate the black goo or black foot fungi in vines, but at least reduced the occurrence of these fungi in vines, and ensured the production of stronger vines that would be more able to withstand the stress conditions associated with planting and establishment.
Control in vineyards
There is currently no test to certify that nursery material is free of black goo or black foot fungi. Producers must therefore be aware that even certified nursery vines might be infected with black goo and black foot fungi. Very low infection levels may be sufficient to allow the fungi to colonise the plant in times of stress and this may cause decay and possibly dieback. It is extremely important to ensure optimal growth of all vines when planting and managing vineyards. Grapevines, especially those younger than 5 years, must not be burdened with too heavy crop loads, since the vascular system and the developing root system will not be able to cope with this stress at a young age.
No chemical control measures for these diseases are currently known. Glasshouse experiments at Nietvoorbij have indicated that foliar sprays with fosetyl-Al (Aliette) or phosphoric acid (Phytex) [not registered for vineyards; use Rootmaster which is registered on grapevines] had an inhibiting effect on black goo lesion development in artificially inoculated vines (Ferreira, 1998). Foliar sprays of the affected vines may be repeated on a 3 weekly basis (2-3 times). This treatment is not fungicidal, but will increase the plant's disease resistance and also protect it against root pathogens (Phytophthora and Pythium spp.) (Marais and Hattingh, 1986). The post-infection action of these products against P. chlamydospora and Cylindrocarpon, and whether it will prevent dieback of heavily infected and/or diseased vines, remains uncertain.
For further queries about these diseases, contact Paul Fourie at tel: 021-809 3104, or Francois Halleen at tel: 021-809 3040.
Literature references
Barbercheck, M. 1986. Control of Meloidogyne javanica in dormant grapevine nursery stock. Phytophylactica 18: 39-40.
Ferreira, J.H.S. 1998. Phialophora terugsterwing - 'n Algemene probleem by jong wingerde. Wynboer Tegnies Maart: T6-T8.
Fourie, P.H., Halleen, F., van der Vyver en Schreuder, W. 2001. Effect of Trichoderma treatments on the occurrence of decline pathogens in the roots and rootstocks of nursery grapevines. Phytopathologia Mediterranea (submitted).
Hunter, J.J. 2001. Ondersoek na die gehalte van plantmateriaal en faktore wat 'n rol speel by entkombinasiesukses tydens die kallusperiode en in die kwekery. LNR Infruitec-Nietvoorbij Navorsingsverslag, Projeknommer WW 12/03.
Marais, P.G. en Hattingh, M.J. 1986. Reduction of root rot caused by Phytophthora cinnamomi in grapevines by chemical treatments. Plant Disease 70: 109-111.
Von Broembsen, S.L. en Marais, P.G. 1978. Eradication of Phytophthora cinnamomi from grapevine by hot water treatment. Phytophylactica 10: 25-27.
Waite, H., Crocker, J., Fletcher, G., Wright, P. en deLaine, A. 2001. Hot water treatment in commercial nursery practice - an overview. The Australian Grapegrower & Winemaker, Annual Technical Issue: 39-43.
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