Grapevine Trunk Disease
Figure 1
Causal Agents
Grapevine Trunk Disease (GTD) is caused by over 100 taxonomically unrelated fungal pathogens with broad host and geographical range. It is a chronic problem of vineyards in all viticulture areas worldwide. These pathogens affect many woody plants including fruit and nut crops, forest trees and ornamental plants.
Disease Symptoms
The pathogenic fungi are soilborne, airborne and waterborne, and affect all vine tissues. They use wounds as entry points, either below or above ground, to invade the host and compromise the integrity and biological functions of the vasculature. Wood decay of the xylem, phloem and cambium is commonly observed in cross or longitudinal sections (Figure 1). Many of these fungi are opportunistic latent endophytes and only cause disease when the host is under stress. As a result, water, nutrient and signal transports in the vine are impaired and the affected host declines overtime as the disease progresses. Early symptoms include decline loss of vigor and wood dieback often seen on a single spur position of the cordon (top picture). Leaf and fruit symptoms are seen later as the disease severity increases (Figure 2). Severe symptoms include vine apoplexy and death (Figure 3).
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Management
No grapevine is free of GTD pathogens. In nurseries, grapevines are trained to yield high quality and quantity of wood cuttings, whereas in commercial vineyards the goal is to achieve high quality (and sometimes quantity) of fruit. Dormant pruning of grapevine in both nursery and vineyard creates entry points for GTD pathogens (Figure 4). In nurseries, infections also occurs from contaminated soil and water during the plant propagation phase because vines are exposed after wounding induced by grafting, pruning and disbudding events (Figure 5). Early adoption of pruning wounds protection practices with fungicides in both commercial and nursery vineyards is a must to reduce infection levels in vines and extend vineyard longevity and productivity. In nurseries, implementing sanitation practices that limit pathogen spread and contamination are recommended. In vineyards, minimizing practices that cause stress to grapevines (over cropping, early cropping post planting, over or under fertilizing) or mitigating environmental stress (drought, heat, salt), are also recommended to reduce factors that are conducive for opportunistic fungi to become pathogenic.