An increasing threat to Italian vineyards: pathogenic fungi and esca complex

by Maria Chiara Rosace, Sara Legler¹, Vittorio Rossi² | January 15, 2021

^{1 HORTA.srl., Via Egidio Gorra 55, 29122 Piacenza, Italy}

^{2 Department of Sustainable Crop Production, Università Cattolica del Sacro Cuore, 29122 Piacenza, Italia}

Several writers from the past identify Italy, and particularly southern Italian regions, as Enotria tellus, the land of wine.

“There the Oenotrians lived: now the rumour is that a younger race has named it Italy after their leader”.

(The Aeneid, III, 165)

Talking about Italian wine means going on a journey made of history, culture, biodiversity, and traditions that no other country in the world can offer. All over the Italian peninsula, each region owns splendid vineyards and Italian grape species, and wines are undoubtedly among the most renowned and appreciated ones in the world. Producing high-quality wine is not simple at all. Nowadays, farmers are facing complex challenges, caused not only by the strong fluctuations of the market, by climate changes, or the lack of healthy soils (see the article “Protect our soil to preserve food safety”) but also by the presence of diseases in the vineyard which compromise the health of vines and sometimes even the production and the longevity of the vineyard itself. Moreover, the current pandemic has caused serious losses to the Eno-gastronomic sector that will be not easy to remedy.

Yes, also plants can be affected by several diseases. An entire branch of science, plant pathology or phytopathology, is dedicated to the study of diseases caused by harmful organisms (including bacteria, viruses and fungi) to plants. Even grapevines can be subject to several diseases: downy mildew, powdery mildew, grey mold, are just some of the most common ones and of higher agronomic and economic interest. Not less important are also grapevine trunk diseases, which represent an emerging and significant problem for Italian vineyards. Among these, the most common in many viticultural areas of the world and in Europe is esca.

Not all fungi are good… or visible

Esca is a pathology, or rather a syndrome, caused by several fungi that act on the plant at the same time^1,2. These fungi are not certainly those we are used to seeing on our tables: they are small, microscopic, spread by spores and colonise the lymphatic vessels and wood of the affected plants, hindering the transport of water and nutrients from the roots to the aerial parts of the vines. Furthermore, some of them produce enzymes capable of decomposing lignin (a fundamental component of wood which undergoes browning, decay or rot) and other toxic substances which cause symptoms on the leaves. Phaeomoniella chlamydospora, Phaeoacremonium aleophilum and Fomitiporia mediterranea are the most common esca pathogens that can be found in Italy. The real esca syndrome seems to be caused by the combined or successive action of P. chlamydospora and P. aleophylum, which cause a tracheomycosis, and F. mediterranea, which causes white rot.

P. chlamydospora and P. aleophylum produce secondary metabolites that are transferred to the foliage through the xylem (a tissue whose function is to transport water and dissolved minerals from the roots to stems and leaves) and are responsible for the formation of chlorotic and necrotic regions between the main veins or along the leaves (‘tiger stripes’) (Figure 1), often together with the appearance of purple spots on the berries. F. mediterranea, on the other hand, degrades the wood of the stem reducing it to a spongy and crumbly mass known as white rot. The shoots of the vines affected by esca can desiccate, the leaves are altered and can become eventually necrotic and fall. The berries on the bunches also wither. In case of high temperature and water stress, a severe form of esca known as “apoplexy” results in sudden dieback of the entire shoot or adjacent shoots and vines can die in a few days.

As Esca is caused by fungi, the disease is spread by spores, which can be transported by wind, water splashes or insect vectors. The spores can end up on plants’ open wounds (for example, pruning cuts or wounds caused by severe weather, such as hail) spreading the disease in the vineyard 3,4. Fungi can remain dormant in plants for years, and symptoms can occur in the vineyard inconsistently: a plant that exhibits symptoms during one season may appear asymptomatic in subsequent years, only to re-experience symptoms of infection again. This makes monitoring and managing of the disease in the vineyard a complicate issue.

Better safe than sorry

There are no products on the market able to effectively fight esca and therefore, it is so difficult to manage the disease. In the past, sodium arsenite was used for this purpose but it has been withdrawn from the Italian market because of its toxicity to humans and the environment. Today, plants affected by esca cannot be treated effectively by chemical products and they are themselves sources of infection for the spread of the disease.

As explained, pruning wounds are the main entry points used by esca pathogens. The correct management of the vineyard, especially (but not only) during the pruning phases, is an essential tool to decrease the incidence of esca. The disease must be faced following an integrated approach through:

• good agricultural practices,
• preventive practices,
• practices aimed at recovering infected plants.

Good agricultural practices allow more rational management of the vineyard and help to prevent infections of the trunk by pathogenic fungi. It is necessary to start from the correct planting of new vines, choosing the right pruning methods and times, disinfecting the tools used during pruning activities on diseased plants, managing correctly pruning residues, soil and irrigation: all these factors, and many others, contribute to the realisation of effective management of the disease.

Figure 2 shows in more detail how cultural practices fit within the complex networks of factors that help to explain the occurrence of the disease in the vineyard.   

By preventive practices, we mean the use of protective products on pruning wounds, such as pruning sealers or (bio)fungicides. For example, in recent years the use of fungi of the genus Trichoderma has become popular to protect wounds. Trichoderma spp. are well known as fungi that exhibit antagonistic activity regarding other microorganisms such as those that may cause esca. How? Trichoderma-based products are applied directly on the plants’ wounds; here the fungi grow, colonising and physically occupying space in pruning cuts. This action prevents the growth of other pathogenic organisms. Scientific research also offers further hope. In 2019, researchers from Crea (the Council for Agricultural Research and Agricultural Economics Analysis) evaluated the use of viruses that can infect esca pathogens. These viruses, called mycoviruses, could be used in future as a biological control tool for the sustainable protection of plants⁷ as it has been already done for the management of other diseases, such as chestnut cancer.

Finally, interventions for recovering infected plants also exist. Curetage, for example, has led to positive and significant results in the cure of diseased plants. It is a very ancient practice, also mentioned in Latin bucolic lyrics, which involves the removal of diseased wood to save the vine and make it productive again. The part of the vine with decayed wood is removed by using tools such as small chainsaws⁸.

Positive results can also be obtained with the practice of pollarding, which consists of removing fractions of the infected plant until the healthy wood is found. It may sometimes be necessary to cut the vine below the infected portion of the trunk and re-graft with a new plant.

Taking care of the vineyard: conclusions

Many factors contribute to determine the infection of healthy plants. Some of these factors, such as climatic ones, cannot be controlled by winegrowers, while others, such as fertilisation, irrigation, pruning or other forms of management, can help them to prevent infections. Although there is no definitive solution to the problems caused by the esca complex, it is necessary, as already mentioned, to address the problem in an integrated way, starting from the choice of the rooted cuttings and throughout the life of the plant. The only valid weapons to limit the spread of the disease and manage it effectively consist in monitoring carefully and implementing appropriate strategies of prevention (and where possible curative/recovery practices) through agronomic and cultural techniques less prone to the formation of wounds. We also hope that scientific research in agriculture will continue to provide significant results, validating the already existing solutions or creating new ones, in order to ensure the protection of vineyards and to preserve the priceless Italian and worldwide agricultural heritage.

Acknowledgment

A special thanks to Vittoria Bardelloni, working in the grapevine sector technical area at Horta S.r.l., for providing the photographic material.

References

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