In its 2018-19 fiscal year, the Oregon Wine Board of Directors granted $437,500 to researchers for nine projects with the potential to advance quality grape growing and winemaking in Oregon. The update below is the second in a series to let you know about the status of these projects.

Dr. Bhaskar Bondada is a grapevine physiologist at Washington State University’s Wine Science Center. His interests are in both basic and applied research, with a particular focus on integrative plant physiology and plant anatomy and using knowledge of structure-function relationships to improve vine health and fruit quality. He has prepared the update below.

Microscopy of Red Blotch: A whole new perspective for developing a mechanistic understanding of the disease and its prevention by examining cellular details of the infection

Project objectives:
This project is aimed at discovering grapevine responses to Red Blotch disease, which is fundamental to understanding the progression as well as the mechanisms by which the virus causes the infection. The accepted notion is that the disease is caused by Grapevine red blotch associated virus (GRBaV) and is transmitted to grapevines by the three-cornered alfalfa tree hopper (Spissistilus festinus). This led to the hypothesis that GRBaV enters grapevines when the treehopper feeds on the vines’ plumbing system, mostly the phloem (the sugar-carrying tubes feeding the berries). This hypothesis will be tested by examining GRBaV movement within the phloem and the relationship between GRBaV and the tissues of its host. These aspects can be illuminated by examining phloem tissues and phloem-specific processes in healthy and afflicted vines.

Therefore, the objectives of this project are to determine:

  1. How does the virus enter the grapevine and, once it enters the vine, how is it distributed throughout the vine?
  2. Does the virus change the plumbing system of the vine? What is the symptomatology of such infection at the cellular level?

Importance to the Oregon wine community:
The Red Blotch virus is a serious concern to grape growers and winemakers in Oregon. One key aspect of all viruses, including Red Blotch, is their intimate association with cell components and the formation of unusual structures following infection. Consequently, management of Red Blotch virus depends on the understanding of the means by which the virus enters the vines, its distribution and survival following entrance throughout the vine, the infection cycle, and the symptomatology of the infection at the cellular level. Such information will aid growers in implementing effective management practices towards minimizing the spread of Red Blotch disease.

Progress so far:
Last season, we found that the pathway designed for feeding the berries (phloem) was blocked by a carbohydrate-based substance in the infected vines. This impairment explains the frequently observed diminished sugar accumulation in the berries of infected vines. Such a blockage will also lower the building of reserves in roots, causing erratic bud break in the following spring season. Healthy vines also accumulate this substance in their sugar pathway, but they do so close to harvest while preparing to go into dormancy.

The path designed for transporting water (xylem) also appeared to be blocked by tissues that generally support early shoot growth following bud break in the spring season. Furthermore, the infected vines showed xylem vessels in radial multiples, a dense cytoplasm and particles on the periphery of phloem tissues, and numerous plasmodesmata (cytoplasmic channels or bridges to make connections with the neighboring cells). The healthy vessels were open, and the phloem cytoplasm was mostly confined to the periphery.