Grapevine trunk diseases

What management strategies are available to table-grape growers and nurseries?

By Anna Mouton

“Grapevine trunk diseases are among the most important phytopathological problems in wine and table grapes,” says researcher Dr David Gramaje, head of the BIOTIVIS group at the Institute of Grapevine and Wine Sciences in La Rioja, Spain. The BIOTIVIS group focuses on fungal pathogens of grapevines, with an emphasis on trunk diseases. As many as 135 fungal species in 35 genera can cause trunk diseases. The signs of infection differ depending on the pathogen and the age and condition of the grapevine but typically include stunted growth, reduced vigour, dieback, and loss of production. Affected vines may die or need early replacement. Internal changes visible in cross-sections of the woody tissues may include dead and discoloured areas, black spots in xylem vessels, and white rot. “These internal symptoms can compromise the translocation of nutrients and water from the roots to the aerial parts of the plant,” notes Gramaje. He explains that grapevine trunk diseases are complex because the many fungi involved differ in their biology and epidemiology. “They can live in different places and spread in different ways. Several live in soil and infect through roots, others spread aerially and infect through pruning wounds, and still others specialise in infecting plant material in nurseries.” Although infected grapevines can’t be cured, viticulturists can follow an integrated management strategy to protect their plants.
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Measures for mature vines Integrated management starts at the mother blocks used to produce plant material. Healthy mother vines are critical for establishing healthy commercial vineyards. “In Spain, it’s quite common to cultivate rootstock mother vines sprawling on the soil surface,” says Gramaje. “This can be detrimental because soil-borne fungal pathogens can contaminate the plant material.” He has seen South African nur-series place a woven fabric between vines and soil but thinks trellising vines is safer. He also encourages drip irrigation in mother blocks, as flood irrigation can lead to excessively wet soils that promote infections, and sprinklers may stimulate several pathogenic fungi to release spores. Gramaje illustrates management practices for established commercial vineyards by using the example of Botryosphaeria dieback, emphasising that most diseases affecting mature grapevines have similar life cycles. “Botryosphaeria produces resistant fruiting bodies on infected pruning debris and dead wood, especially during winter,” he elaborates. “In spring, when they have adequate temperature and humidity, they release spores, which are disseminated by wind, rain, or arthropods. When they reach a susceptible pruning wound, the spores germinate and cause internal symptoms.” Removing pruning debris and dead wood is one way to stop trunk diseases from spreading. Burning and composting are effective disposal methods, although not always practical or economical. Shredding material finely and applying it as a mulch or incorporating it as a soil amendment could allow the survival of fruiting bodies – this needs further research. “Another factor that makes this pathosystem more complex is that these pathogens can also infect alternative woody hosts,” says Gramaje. In SA, grapevine trunk pathogens have been identified from apple and pear trees.

Protecting pruning wounds Pruning wounds are one of the main points of entry for trunk disease pathogens. Gramaje discusses two classes of pruning-wound protectants: chemical and biological. “The evaluation of different products in several countries mostly concluded that mastics or pastes, plus fungicides belonging to the benzimidazole carbamate mode of action group or the demethylation inhibitors, are the most effective in protecting pruning wounds against grapevine trunk disease pathogens,” says Gramaje. Recent work also demonstrated the efficacy of a mixture of pyraclostrobin and boscalid. However, as Gramaje points out, restrictions on chemical use in viticulture continue to increase, especially in the European Union. “There is now a frantic search for alternatives – one is biological control, especially with different species of the genus Trichoderma.” In his opinion, biological control agents such as Trichoderma work best in the controlled or semi-controlled environments of laboratories or greenhouses and less well in the field. Their performance in preventing grapevine trunk diseases has been unreliable. Gramaje cites studies in Italy and California that showed promise when evaluating Trichoderma asperellum and T. gamsii in controlled experiments. His group assessed these biocontrol agents under natural conditions in several vineyards in Spain and France over three years and achieved inconsistent results. South African studies have demonstrated that T. atroviride can significantly reduce Eutypa lata infections. Canadian researchers also showed it to be effective against Botryosphaeria infection of pruning wounds. “One issue is the cost of treatment,” says Gramaje. “Manual painting is much more effective than spraying, but three or four times more expensive. Another issue is when to apply the protective treatment. I strongly recommend applying biological control agents immediately after pruning.” Chemical products such as tebuconazole, fluazinam, and pyraclostrobin are protective when applied up to six days after pruning. If all else fails and vines develop trunk diseases, remedial surgery may be possible. “It consists of removing visibly infected parts of the vines until getting apparently healthy wood,” says Gramaje, “covering the wound by a mastic or paste, and trying to renew the plant using basal shoots.” Surgery is not guaranteed to work, and it’s only indicated for localised infections such as Botryosphaeria and Eutypa dieback. Surgery is unlikely to succeed in diseases such as Esca, where the infection can reach the rootstock, because removing all the affected wood isn’t possible.

Make a clean start “Unfortunately, nurseries provide nearly optimal environments for fungal development,” says Gramaje. “And the production practices in most nurseries provide many opportunities for fungal infection.” His message to nurseries: start clean and keep it clean. Nurseries should make every effort to use pathogen-free plant material. They should also clean grafting machines frequently, maintain a high standard of general cleanliness, never reuse callusing media, and avoid lengthy soaking of cuttings. South African nurseries still have access to agricultural disinfectants, but many other countries don’t allow chemical products. Biological control of trunk diseases using Trichoderma atroviride can work well in the semi-controlled environment of a nursery. And hot-water treatment is partially effective, albeit with a risk of reduced grapevine viability, if misapplied. “Nursery fields are also potential sources of soil-borne pathogens, especially black foot pathogens,” says Gramaje. “In Portugal and South Africa, researchers demonstrated an increase in black foot disease following crop rotation, for example with cereals.” Where chemical fumigation isn’t an option, biofumigation with mustard seed meals can be considered. Different Trichoderma species did not prevent black foot, although Gramaje has seen reductions in fungal infections when combining biocontrol agents. In future, mycorrhiza may become part of integrated control, but more research is needed. In young vineyards, Gramaje cautions against overcropping. “There is a hypothesis that stress factors such as overcropping can be related to disease development and symptom expression of grapevine trunk diseases.” Grapevine trunk diseases are a complex pathosystem with a significant economic impact on viticulturists. To mitigate this risk, table-grape growers should implement an integrated management system in every step of vine production, from nursery mother blocks to mature vineyards.
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