Source: http://www.avf.org/category/rootstocks/phylloxera-resistant/
Timestamp: 2019-04-23 14:23:15+00:00

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Approximately 1,170 of the 1,540 seed from the controlled crosses to create population 1 were germinated from June until August. About 980 plants completed germinating and were planted in 2Vi” pots and then inoculated when they were one month old. They were evaluated in January, 2000 and only one plant was found infected with phylloxera. The poor results may have been due to the small seedling roots. The seedlings have been re-potted and inoculated again. For population 2, cuttings were taken from 950 plants in the field. Four leafy cuttings per plant were mist propagated for a total of 3,800 cuttings. All cuttings that rooted were transplanted to 2V4″ pots and inoculated with phylloxera eggs at that time. The first cuttings were taken June 21 and the last cuttings taken August 12, 1999. The rooted cuttings have much stronger roots than plants started from seed and it seems easier for the phylloxera to become established on them. Good phylloxera infection was obtained for the crosses 1 to 9 resulting in good evaluation of resistance and susceptibility. Poor results were obtained for crosses 15 to 20 because they were inoculated after the first of September. Plants were inoculated again after their evaluation to fully test their resistance and eliminate the possibility of escapes. The occurrence of phylloxera on the roots is positive identification of susceptible individuals. There was good correlation between the tests conducted in the 6″ pots and the 2Vi” pots. Susceptible parents generally produce susceptible offspring except when Kober 5BB was the resistant parent. Based on this progeny test, Kober 5BB has the highest level of resistance that is passed to its offspring. Because of the low number of individuals in certain crosses, they were repeated this spring. A total of 237 seed were produced from the four crosses made. They have been stratified and are ready for planting. Work on molecular markers was not started. The resistant/susceptible nature of the seedlings needs to be determined first.
Phylloxera resistant grape rootstocks which also resist nematodes, viruses and fungi and tolerate salt and drought are needed in California. To breed such stocks we will need to know biological and genetic mechanisms of phylloxera resistance in parental grape species to be used for the breeding of the rootstocks. We proposed to initiate work on determining the biological and genetic mechanisms of V. rupestris resistance by studying a population from the genetic cross V. rupestris x V. vinifera hybrid vines. These vines have been growing in the U.C. Davis vineyard for several years. We started by doing bioassays leading to a knowledge of the genetics. We used a modification of our 25-day biotyping bioassay for assessment of the resistance of 235 vines to biotype A grape phylloxera. 132 vines were hybrids of V. vinifera cv. ‘Aramon’ x V. rupestris, 50 were selfed ‘Aramon’ x ‘Aramon’ seedlings, and 53 were selections or crosses of selections of V. rupestris. We estimated survival and developmental rates (a developmental index) at day 25 and reproductive rates from day 25 to day 29, then calculated a combined estimate of the growth rate of the colony, the ‘vulnerability index’. All Aramon vines had high (1.49 to 7.33) vulnerability indexes, and except for cultivar ‘Geant’ (vulnerability = 1.36), all V. rupestris vines had very low vulnerability indexes (0.00 to 0.37). With the average vulnerability index of the hybrids at 0.47, most appeared to have inherited resistance to biotype A from the V. rupestris parent, but resistance was compromised to varying degrees by inheritance of susceptibility from the Aramon parent. Eleven of the hybrids had vulnerability indexes from 1.46 to 2.69 at or near the lower end of the Aramon range, and another at 6.39 appeared to be highly susceptible to biotype A. These results suggest that inheritance of resistance is multigenic with some genes suppressing survival or fecundity and others retarding development. The next step is to begin characterizations of a sample of these vines that span the vulnerability spectrum. Characters which are quantitatively, negatively correlated with the level of vulnerability, may in fact be related to the resistance mechanism. We have chosen 15 accessions with relatively high survivorship for this based on variation in the developmental index. These criteria will allow us to focus on mechanisms which affect nutrition rather than cause decreased survival. We have also tested a method for evaluating the link of fungal and phylloxera resistance in these stocks. The preliminary results suggest that fungal and phylloxera resistance is not linked in a number of rootstocks.
Characterize virulence and life cycles of new phylloxera strains ‘Non A/non B’ strains were tested in the laboratory and shown to have increased aggresivity on some rootstocks. Based on the population growth data, overall aggresivity was too low for us to predict that these strains would cause economic losses to strongly resistant rootstocks with no V. vinifera parentage. A ‘Harmony’ adapted strain (strain 4) from Napa Co. had high population growth in the laboratory tests and was observed to cause damage to field vines. This strain represents a new biotype and this finding supports recommendations not to use this rootstock for phylloxera control. A German strain, reported to be damaging to 5C there, has been established in the laboratory and the first tests indicate that it is probably not virulent enough to cause field losses. The tests completed to date were plagued by technical problems and will be repeated. We can not guarantee the permanent stability of phylloxera resistance in rootstocks; understanding variation in phylloxera aggresivity from California and the world will help us evaluate the risks for the future. People from Andy Walker’s and our laboratory, made comparisons of phylloxera DNA. Results indicate that at least several strains of biotypes A and B exist. Our work failed to identify markers for biotypes or for geographical origin of strains. One interpretation of these data is that biotype B was selected more than once (rather than spreading from a single epicenter). We conclude from this interpretation that AXR#1 is not safe to use anywhere in California and quarantines will not prevent new occurrences of biotypes.
To determine the potential for control tactics other than rootstocks Mocap tests in large planter boxes failed to control phylloxera or protect vines. Greenhouse studies demonstrated that Fusarium and Pythium fungi contribute to vine damage associated with phylloxera feeding. Although vine damage is caused by phylloxera alone, presence of secondary fungi increased damage about two-fold. The two most common fungi observed in the greenhouse studies were also found to be present in phylloxera-caused feeding wounds on field vines. The contribution of fungi to phylloxera based vine damage makes economic injury levels difficult to establish for this insect. This work also suggests that curative insecticide treatments in the field will not result in rapid reversal of vine-damage symptoms. We are collecting field data in a Sonoma County experiment to determine whether prophylactic insecticide treatments will slow spread of phylloxera or prevent damage. The first year’s data are encouraging. We have been screening new chemical and biological control agents in the laboratory. Some manufacturers are encouraged by the tests and are planning to test some with field trials. Others will not.
To evaluate the factors that affect phylloxera populations, damage and spread Work on temperature thresholds for grape phylloxera suggests that temperature dependency of population growth is complex. Temperature thresholds vary with the insect stage and change with pulses of unseasonable temperatures. These results argue against use of a day-degree model for this insect.

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