This review illuminates key findings in our understanding of grapevine xylem resistance to fungal vascular wilt diseases. Grapevine (Vitis spp.) vascular diseases such as esca, botryosphaeria dieback, and eutypa dieback, are caused by a set of taxonomically unrelated ascomycete fungi. Fungal colonization of the vascular system leads to a decline of the plant host because of a loss of the xylem function and subsequent decrease in hydraulic conductivity. Fungal vascular pathogens use different colonization strategies to invade and kill their host. Vitis vinifera cultivars display different levels of tolerance toward vascular diseases caused by fungi, but the plant defense mechanisms underlying those observations have not been completely elucidated. In this review, we establish a parallel between two vascular diseases, grapevine esca disease and Dutch elm disease, and argue that the former should be viewed as a vascular wilt disease. Plant genotypes exhibit differences in xylem morphology and resistance to fungal pathogens causing vascular wilt diseases. We provide evidence that the susceptibility of three commercial V. vinifera cultivars to esca disease is correlated to large vessel diameter. Additionally, we explore how xylem morphological traits related to water transport are influenced by abiotic factors, and how these might impact host tolerance of vascular wilt fungi. Finally, we explore the utility of this concept for predicting which V. vinifera cultivars are most vulnerable of fungal vascular wilt diseases and propose new strategies for disease management.

Fungal wilt diseases are a threat to global food safety. Previous studies in perennial crops showed that xylem vessel diameter affects disease susceptibility. We tested the hypothesis that xylem vessel diameter impacts occlusion processes and pathogen compartmentalization in Vitis vinifera L. We studied the interaction between four grape commercial cultivars with the vascular wilt pathogen Phaeomoniella chlamydospora. We used qPCR and wood necrotic lesion length to measure fungal colonization coupled with histological studies to assess differences in xylem morphology, pathogen compartmentalization, and fungal colonization strategy. We provided evidence that grape cultivar with wide xylem vessel diameter showed increased susceptibility to P. chlamydospora. The host response to pathogen included vessel occlusion with tyloses and gels, deposition of non-structural phenolic compounds and suberin in vessel walls and depletion of starch in parenchyma cells. Pathogen compartmentalization was less efficient in wide xylem vessels than in narrow diameter vessels. Large vessels displayed higher number of tyloses and gel pockets, which provided substrate for P. chlamydospora growth and routes to escape occluded vessels. We discuss in which capacity xylem vessel diameter is a key determinant of the compartmentalization process and in turn grape cultivar resistance to disease caused by P. chlamydospora.

Cytospora species are ubiquitous pathogens of numerous woody plants, causing dieback and wood cankers in agronomic crops, timber trees and wildland trees (e.g. Prunus, Eucalyptus and Salix, respectively). Cytospora chrysosperma, C. cincta and C. leucostoma have been reported from grapevines in Iran showing symptoms of one or more recognized trunk diseases (esca, botryosphaeria-, eutypa-and phomopsis diebacks); however, only C. chrysosperma was shown to be pathogenic to grapevine. To understand the potential role of Cytospora species in the grapevine trunk-disease complex, 21 Cytospora isolates were examined that were recovered from dieback and wood cankers of Vitis vinifera and Vitis interspecific hybrids in seven northeastern U.S. states and two Canadian provinces. Phylogenetic analyses of ITS and translation elongation factor 1-a identified two novel species: Cytospora vinacea sp. nov. and Cytospora viticola sp. nov. Differences in culture morphology and conidial dimensions also distinguished the species. When inoculated to the woody stems of potted V. vinifera 'Thompson Seedless' in the greenhouse, both species were pathogenic, based on development of wood lesions and fulfilment of Koch's postulates. Cytospora viticola was the most virulent based on lesion length at 12 months post-inoculation. As cytospora canker shares some of the same general dieback-type symptoms as botryosphaeria-, eutypa-and phomopsis diebacks, it may be considered part of the grapevine trunk-disease complex in eastern North America.

Grapevines are sensitive to a wide range of fungal pathogens, including agents such as Phaeomoniella chlamydospora and Phaeoacremonium aleophilum that cause tracheomycosis. In the present study, a procedure for DNA extraction from grapevine woody tissue is first evaluated and shown to be suitable for quantitative analysis. Next, a multiplex real-time PCR method targeting the β-tubulin gene of the pathogens and the actin gene of plant material is developed and its quantitative capability is verified. This protocol was evaluated in inoculated grapevine-wood samples and in young vines from a nursery and was found to be reliable and highly specific. Results obtained from inoculated cuttings show that the fungal colonization process must be considered regardless of the wood phenotype. An analysis of samples of young vines from the nursery shows that a high rate of contamination occurs at the basis of plants and that this contamination is associated with low quantitative values. This finding provides evidence that in vine nurseries, these fungi may be efficient soil-borne pathogens.