Alternative rootstocks: how will they perform?

Since the discovery of AXR#1's decline to a more aggressive type of phylloxera, grape rootstocks have received much needed attention. The most obvious area of concern continues to be the availability of alternative rootstocks and how will they perform at a given site. Just as important is the increasing awareness that new rootstocks are needed for a variety of sites with soil-borne pests and problems such as: root knot and nematode complexes; fanleaf degeneration; Armillaria root rot (oak root fungus); phytophthora; drought; and saline soils. Breeding rootstocks capable of resisting or tolerating these pests and problems is possible, but a suitable rootstock must have additional characteristics. New rootstocks must root and graft with reasonable ease, they must induce appropriate levels of vigor in cultivars grafted onto them, and they must be resistant to phylloxera. The vast majority of currently used rootstocks are composed of three Vitis species, berlandieri, riparia and rupestris. These species will not provide us with the genes to resist or tolerate all of the above listed soil-borne problems. Fortunately, sources of resistance to these problems are likely to exist in one or more of the many other Vitis species.

The phylloxera resistance found in berlandieri, riparia and rupestris is broad and durable. It has been tested for over 100 years, during which time no rootstock composed of these species, singly or in hybrid combination, has collapsed to phylloxera in the field. The mechanism and genetic control of phylloxera resistance in these species is not understood, but it is functional. However, the phylloxera resistance of the many other Vitis species we need for specific characters, such as drought and salinity tolerance or nematode resistance, has not been time-tested. We must be sure that rootstocks bred to address specific soil-borne problems will not produce AXR#1-like consequences.

A major hindrance to our breeding program is testing a given species or seedling for phylloxera resistance. Highly susceptible species, such as vinifera, may exist in infested fields for many years without showing decline for a variety of reasons including phylloxera distribution, population size, soil types or combinations of these factors. Testing vines for resistance to phylloxera in a greenhouse is also difficult because of problems in maintaining optimal conditions for phylloxera in the pot and preventing phylloxera spread particularly if the greenhouse is used for other grape experiments or is nearby other grape plantings. We are developing a tissue culture-based system that will allow us to have phylloxera and grapevine in small containers so that we can readily observe plant/pest interactions. Thus far we have established phylloxera and vinifera together in culture and have seen reproduction and typical feeding damage. We are now testing the system with other susceptible and resistant species. If the reactions to phylloxera with susceptible and resistant species appear normal then we can use this system to screen seedling populations for resistance.

We are also studying the genetic and mechanistic controls of resistance to phylloxera. Crosses designed to help clarify the genetics of resistance in berlandieri, riparia and rupestris have been made. The resulting seedling populations are established in the field and are ready for testing. Each seedling from these populations will be tested for phylloxera resistance and the variability in their responses will indicate the number of genes involved in resistance. These tests will also allow us to identify the most susceptible and most resistant seedlings and then examine them for differences in root periderm (bark) or phenolic composition which may confer resistance. Differences in the DNA of these seedlings will be examined for markers (consistent DNA differences between resistant and susceptible) that may correlate with resistance. DNA markers would allow seedlings to be rapidly screened for resistance since a seedling's DNA could be examined, rather than a lengthy test in which a seedling's response to phylloxera infestation was observed.

Variability within phylloxera populations resulted in the decline of AXR#1--a more aggressive strain appeared and overcame AXR#1's low level of resistance. We are not yet sure whether this strain appeared from preexisting variation in phylloxera or whether it evolved from another strain to overcome AXR#1. Phylloxera variation is important to rootstock breeding; rootstocks must resist all forms of phylloxera and breeders need to be aware of phylloxera's ability to change and adapt to rootstocks. This second interaction is likely to be unique for each Vitis species and perhaps to each rootstock. We have studied phylloxera variability by observing differences at the protein and DNA level. The DNA studies may allow us to investigate the co-evolution of phylloxera and Vitis species and result in a better understanding of phylloxera resistance within the species and also a better understanding of Vitis species evolution.

(The author, M. ANDREW WALKER is a scientist at the Department of Viticulture and Enology, U.C., Davis.)

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