I had the pleasurable task of moderating the session on Vine Pests and Disorders, held at the American Society of Enology and Viticulture (ASEV) Annual Meeting in Sacramento from June 27-30. Four speakers delivered five presentations (Dr. Andrew Walker presented two) on subjects dealing with nematodes, phylloxera, Armillaria root disease, berry shrivel disorder and the genetic basis for resistance. Following is a summary of each presentation.

Control of Armillaria Root Disease

Dr. Kendra Baumgartner, plant pathologist with the USDA on the UC Davis campus, presented some interesting background about the fungal pathogen Armillaria mellea. Related species are also very prolific, including a 38-acre patch in Michigan that is believed to be a single organism. Armillaria mellea is very common throughout California and is a species of great interest to grape growers. The so-called "Oak Root Fungus" is actually a misnomer, according to Baumgartner, which surprised many of us who use that term frequently. In reality, the fungus will grow on many native Californian species, and oaks are not its preferred host. Therefore, a vineyard planted where native trees and shrubs have been cleared is in danger of being infected, and fumigation is not completely effective in eliminating the pathogen.

Armillaria root disease is a dreadful problem, when it shows up in a vineyard, because there is really no control measure that can alleviate it. Vines indicate infection by stunted growth and frequently just collapse and die within a short period of time. Commonly, the disease will spread down a row of vines since the fungus moves through root-to-root contact (the roots don't have to be alive). This is a diagnostic feature, though it may spread across rows if the rows are close together as many modern vineyards now are. Affected vines must be removed in addition to as many roots as possible. Replants are in danger of infection also because the fungus will remain viable in the soil for many years. Armillaria's only positive attribute is that it is very slow growing.

Because Armillaria is so commonplace and difficult to control, it would be highly desirable to know which rootstocks are resistant to the root disease. Baumgartner tested several rootstocks in a controlled environment to see which, if any, displayed some resistance. Eight rootstocks were tested by placing infected wood pieces in contact with their root systems. Of those tested, none showed significantly different propensity for plant death as a result of the fungal infection. However, there were some differences in the quantity of root systems that became infected with the fungus. The least infected rootings were found in Freedom, with St. George, 110R and Ramsey having intermediate levels of infection. The others, O39-16, 5C, Riparia Gloire and 3309, had the greatest amount of infection.

While there were differences in susceptibility to infection by Armillaria mellea, none of the rootstocks showed a high level of resistance. Freedom, the most resistant rootstock, is not desirable for most high-end viticulture applications as it is extremely vigorous. Baumgartner stated that more rootstocks need to be screened for resistance to Armillaria root disease than those tested in this recent work.

Berry Shrivel and Bunch Stem Necrosis

Dr. Mark Krasnow, post-doctoral researcher at UC Davis, clarified the differences in what are two commonly confused, but very different, disorders of grape clusters. Bunch Stem Necrosis (BSN) is a disease that affects the fruit through death (necrosis) of the rachis (bunch stem). There are numerous other names for this disease, including "waterberry." It is distinct from other disorders in that it usually affects only a portion of the clusters (usually the tips), and the berries dry up and become raisin-like. At inception, the fruit display tiny dimples, giving a golf ball type of appearance. As it progresses, the rachis turns brown and the fruit take on a raisin flavor.

Berry Shrivel affects the fruit in very different ways. The rachis stays green throughout the season, and the fruit become flaccid, causing deep creases. They have low sugar and high acidity and frequently have off-flavors that make you want to spit them out. (Their lack of flavor makes me want to attach the term "waterberry" to Berry Shrivel instead of BSN.) Pigmented varieties that are afflicted with the disorder lack color, and the berries take on a pinkish-gray color. This is a serious problem for growers and wineries since affected fruit must be dropped before harvest begins. The fruit becomes worthless and a possible detriment to wine quality should it ever be included in the load.

Observations were made at the UC Oakville Experiment Station in Oakville. In one of the Cabernet Sauvignon blocks, there are vines that commonly display Berry Shrivel side-by-side with vines that never display the disorder. Hence, they have a ready-made system for investigation of this disorder. They sampled fruit from numerous clusters and observed symptoms as they progressed. Krasnow found that the first symptoms of shrivel occurred 112 days after anthesis (about mid-September). Affected fruit, however, had lower sugar content before symptoms appeared. Non-symptomatic fruit on the same vines had higher sugar content than the symptomatic fruit but lower sugar content than the control fruit from the unaffected vines. Non-symptomatic fruit were more similar in composition to the symptomatic fruit on the same vine than they were to the non-symptomatic fruit on the unaffected vines. Hence, they concluded that the disorder was a whole-vine disorder and not a disorder of individual clusters.

Krasnow's presentation shows that a significant step has been taken toward the understanding of this disease. Yet, while we now understand that it is a whole-vine disorder, we do not know the root cause of the disorder.

Gene Expression Differences between Vitis vinifera, Vitis aestivalis

Dr. Daniel Schachtman, associate member and principal investigator at the Donald Danforth Plant Science Center, St. Louis, Missouri, discussed a comparison made at the genetic level between two species of grape: The old world V. vinifera and the American species V. aestivalis. The American species, which includes the popular variety Norton, is highly resistant to powdery mildew (Uncinula necator). Upon infection, the leaves display small necrotic spots, but the disease does not progress further and does not require treatment. The plant "recognizes" the fungus and stops its growth. As we all know, that is not the case with V. vinifera, which is highly susceptible to powdery mildew.

Using gene chip technology, the Schachtman lab investigated genes that were expressed in the two species. What does "expressed" mean? It means that the genes were "turned on" so that they could produce proteins (enzymes) that fulfilled some biological function, such as an aspect of disease suppression. Of the nearly 10,000 genes that they found to be expressed, 39 were uniquely expressed by V. vinifera while 183 were uniquely expressed by V. aestivalis.

They challenged vines with powdery mildew and then assessed genes that were "turned on" or "turned off" as a result of the infection. They found 626 genes that were regulated in V. vinifera but only two genes that were regulated in V. aestivalis. In other words, powdery mildew infection appeared to trigger a massive change in metabolism in V. vinifera at the gene level, yet it triggered a relatively minor change in metabolism in V. aestivalis at the gene level. It appears that the mechanism for powdery mildew resistance in V. aestivalis is in place already and does not require a large change in genetic transcription after infection.

Nematode Resistance

Dr. Andrew Walker, professor and geneticist at the UC Davis Department of Viticulture and Enology, has been breeding rootstocks in his lab since 1989, a process that is painstaking and requires a great deal of time and patience. Nevertheless, he has had some success in developing rootstocks with resistance to numerous pathogenic nematodes. Currently, our choices for nematode-resistant rootstocks are limited. Freedom is excessively vigorous, and Harmony's resistance breaks down against some strains of root-knot nematode. 1616, which has good viticultural characteristics, was shown to have some susceptibility to root-knot nematode under high-temperature conditions.

Walker has been evaluating numerous crosses of Vitis candicans, V. champinii, V. cinerea, V. rufotomentosa, V. riparia, V. rupestris and Muscadinia rotundifolia. He tested these against some particularly virulent root-knot nematode strains and the dagger nematode (Xiphenema index). To make the screening more selective, tests were done at a high temperature, which generally causes root-knot nematode resistance to fail. Of the crosses, six selections were found to be resistant to these nematodes. Some of the vines have poor nursery characteristics, such as bushy or weak mother vines, while others look like they have commercial promise.

Walker's lab then tested these selections for resistance to the ring nematode, to which few current rootstocks are resistant. 420A has some resistance but not much. The six selections all exhibited resistance to the ring nematode. One rootstock selection in particular was Walker's favorite: it had the designation of 8909-05, but a more recognizable name will be assigned before release. The beauty of this selection is that it not only displayed no galling from nematode feeding but that it contains M. rotundifolia, which will convey Fanleaf resistance to the scion. Currently, there is only one rootstock that conveys Fanleaf (a virus disease) resistance to the scion, O39-16, which has poor viticultural characteristics (vigor) and may be failing to phylloxera due to its V. vinifera parentage.

Walker is encouraged by the new so-called VM hybrids, which indicates Vitis (but not V. vinifera) and Muscadinia parentage as opposed to the earlier VR hybrids, which indicated vinifera and rotundifolia parentage. The Muscadinia rotundifolia parentage is essential for Fanleaf resistance as there is something, not yet understood, that the rootstock conveys to the scion that prevents disease expression.

Phylloxera Genetic and Phenotypic Variability in Nodosity Feeding

Walker presented a talk from Dr. Jeffrey Granett, professor of entomology at UC Davis, with whom there was a joint project. Firstly, it must be understood that the so-called "resistant" rootstocks now in common use are not failing to phylloxera; phylloxera will feed happily on both resistant and susceptible rootstocks. The difference is that the resistant rootstocks do not respond with tuberosities, or large galls that stop root growth. They do, however, exhibit nodosities, which are much smaller enlargements of the root that do not stop root development.

The crux of the presentation was that there are host-adapted strains of phylloxera. That means that the strains of phylloxera that develop on one root species do not care to feed on a different species. Types A and B phylloxera, which developed on own-rooted V. vinifera as well as on AxR#1 (which had vinifera parentage), do not proliferate on roots that do not contain vinifera, such as 101-14 rootstock. Conversely, phylloxera that feed on 101-14 (V. riparia x V. rupestris) do not proliferate on vinifera roots. Hence, the populations that develop on the two parentage groups are genetically distinct from one another.

They also found that the resistance characteristics of 101-14 are not the same at all locations; therefore phylloxera-resistance is a highly complex phenomena. While it was stressed that current rootstocks are not failing to phylloxera, it was also advised that vineyard replants should use rootstocks of different parentage groups than that of the preceding vineyard so that the resident phylloxera population will not immediately infest the new vine roots. wbm