Since grapevine leafroll disease (GLD) is the most important viral disease impacting wine grapes in Washington State, its symptoms are the main focus here, but short descriptions and links to pictures of grapevines infected with both RW complex and nepoviruses are included.
What to look for—and keep in mind
GLD. In general, GLD-infected grapevines show less vigor (Figure 1 and Figure 2), have smaller leaves (Figure 3), and produce lower yields (Figure 4) compared to healthy grapevines. In addition, berries from GLD-infected vines show uneven fruit size and maturity, lower sugar accumulation, and poor color development due to lower accumulation of anthocyanins. The uneven ripening of berries is more striking in red-berried grapes (Figure 5) than in white-berried grapes (Figure 6). Other stress (such as water deficit) imposed on infected vines is likely to exacerbate disease symptoms.
The complexity of GLD is compounded by the fact that foliar symptoms are highly variable in different cultivars. And, although all European wine grape cultivars of Vitis vinifera L. are susceptible to GLD, symptoms are more dramatic in red-berried cultivars than white-berried cultivars. Typical signs of GLD infection in red-berried cultivars consist of green veins and inter-veinal reddening (Figure 7), with downward rolling of leaf margins (Figure 8) in the advanced stages towards the end of the season. General yellowing or chlorotic mottling (Figure 9 and Figure 10, as in Chardonnay varieties) is more commonly associated with white-berried cultivars.
In addition to the variability of GLD symptoms based on cultivar, differences in the age of the vine, stage of infection, time of the year when symptoms were observed, number of grapevine leafroll-associated viruses (GLRaVs) present in infected vines, and environmental conditions affect how the disease exhibits itself. To complicate matters, visual diagnosis may not necessarily be accurate since abiotic conditions like nutritional deficiency, physical damage, and herbicide injury may result in discolorations that mimic GLD symptoms.
Also problematic for keeping the disease in check is the healthy appearance of grapevines infected with GLD during the early part of the season. It is virtually impossible to visually differentiate virus-infected grapevines from those that are healthy during the dormant season because dormant wood does not exhibit abnormalities. Infected vines do not begin to show symptoms until veraison on mature leaves near the basal part of the shoots and extending upwards to other leaves as the season advances.
RW complex. Unlike GLD, RW complex is characterized by alterations to the woody trunks or stems of grapevines resulting in pits, grooves, and other disorders. Hence, RW complex is also known as trunk disease.
The 4 types of RW disorders are characterized by the following features:
- Rupestris stem pitting disorder, which shows basipetal stem pitting symptoms extending downward below the graft unions in V. rupestris cv. St. George, but remains symptomless in cv. LN 33 and Kobe 5BB;
- Kober stem grooving disorder, which exhibits severe grooving on the grafted stems of cv. Kober 5BB, but remains symptomless in cvs. St. George and LN 33;
- LN 33 stem grooving disorder, which shows grooves on grafted stems of cv. LN 33 but no symptoms on cvs. St. George or Kober 5BB; and
- Corky bark disorder, which produces grooving and pitting on the entire surface of grafted cvs. St. George and LN 33 stems, but no symptoms in Kober 5BB. In LN 33, corky bark disorder also causes severe stunting of grafted plants with internodal swelling, leaf rolling, and reddening.
All these disorders remain latent or symptomless in own-rooted or ungrafted wine grape cultivars, American Vitis species, and rootstock hybrids. Depending on the rootstock-scion combination, woody grapevine cylinder alterations occur on rootstocks, scions, or both. Any type of swelling above the graft union, marked difference between the relative diameter of the scion and rootstock, poor bud take, graft incompatibility or decline, slow growth, or delayed bud opening are manifestations of RW complex.
Grapevine Leafroll Disease Symptoms Video
Nepovirus disease. One of the characteristic features of nematode-borne viruses is their infection of grapevines in patches. In most cases, infected grapevines show foliar symptoms early in the spring and become less distinct with increased ambient temperatures in summer. Different strains of GFLV, the most common nepovirus, cause 3 distinct symptom types: infectious malformations, yellow mosaic, and vein banding.
Juice and table grapes
Although American Vitis species and French-American hybrid varieties and rootstocks can become infected with GLD, they are not impacted by the negative effects on plant growth and yield that plague wine grapes. However, juice grapes like Concord, the most widely grown in Washington State, can be infected with GLRaVs and serve as a potential source of infection for wine grapes.
Contact the grape virologist at WSU for advice or questions.