Christine Hlavka’s scientific contributions

Grape Phylloxera damage grapevines by feeding on roots. Vines with damaged roots may be less able to replenish their leaves with water than vines with healthy roots. If these vines with damaged roots continue to transpire at the rate of healthy vines, their leaves should exhibit lowered water potentials.

Grape Phylloxera will cause the California wine industry to lose over one billion dollars by the year 2000. Grape growers with grape phylloxera- infested soils graft scion varieties onto what are popularly termed 'resistant rootstocks.' Rootstocks, however, differ in their suppression of phylloxera: some do not support the insect at all, while other support low populations. In addition, phylloxera biotypes vary in their growth on different rootstocks. In California's Napa and Sonoma county vineyards, about 75% of the vines have AXR#1 rootstock that tolerates phylloxera biotype A. In the early 1980's biotype B emerged there. It so devastates AXR#1 that the vineyards must be replanted with rootstocks resistant to biotypes A and B. Timing replanting is difficult because vineyards do not decline uniformly. A patchwork of uninfested vines, infested but asymptomatic vines, declining but productive vines', and unproductive vines typifies most vineyards. The grower must determine the proportion of vines in each category and estimate the yield loss the stressed vines will suffer. During 1993, 1994 and 1995 the NASA-Ames GRAPES study used remotely sensed leaf reflectance, temperature, and canopy size data and geographic information system (GIS) technology to study infestations in Napa County vineyards. As part of this study a vineyard with a range of phylloxera induced stress and accompanying symptoms -- reduced growth, less chlorophyll, and lower reflectance of near infrared:red light -- was investigated to determine the degree to which stress measurements predict the current and following season's yields from stressed vines relative to healthy vines. Such yield estimates could enable a grower -- before obtaining actual yields -- to calculate the economics of replanting. A grower who decided to replant would have 2-14 months additional lead time to plan and prepare.

Leaf/canopy model simulations and measured data were used to derive information on the form and strength of the nitrogen (N) "signal" in near-infrared (1100-2500 nanometer (nm)) spectra of fresh leaves. Simulations across multiple species indicated that in total, protein absorption decreased near-infrared reflectance and transmittance by up to 1.8% and 3.7% respectively, and all other inputs held constant. Associated changes in spectral slope were generally in the range of plus or minus 0.02% per nanometer. Spectral effects were about an order of magnitude more subtle for a smaller, though potentially ecologically significant, change in N concentration of 0.5% over measured. Nitrogen influence on spectral slope was fairly consistent across four empirical data sets as judged by wavelength dependence of N correlation. The observed and simulated data showed similar trends in sensitivity to N variation. Further, these trends were in reasonable agreement with locations of absorption by protein-related organic molecules. improved understanding of the form and strength of the N signal under differing conditions may allow development of reasonably robust spectral measurement and analysis techniques for "direct" (based strictly upon N-related absorption features) N estimation in fresh leaves. A pragmatic approach for remote sensing might additionally consider surrogate measures such as chlorophyll concentration or canopy biophysical properties.