David Doll

A detailed field study of direct correlations between ground truth crop water stress and normalized difference vegetation index (NDVI) from small unmanned aerial system (sUAS)

Aerial images with high spatial resolution and high temporal resolution were used to detect water stress based on canopy level normalized difference vegetation index (NDVI). We attempted to determine the correlation between stem water potential (SWP) and canopy NDVI with and without shade. Results indicated that removing the shade from the canopy improved the correlation between the NDVI of canopy and SWP with coefficient of determination (R2) from 0.001 to 0.0052. We further compared SWP and the NDVI of the canopy without shade over a period of one week to four weeks. The correlation between NDVI with SWP was highest in the time range of three weeks. However, both cases show that there is no obvious relationship between NDVI of canopy and SWP. Therefore, canopy level NDVI does not indicate water stress. Further research is needed beyond pretty pictures.

Climatic Constraints to the Potential of Microsphaeropsis amaranthi as a Bioherbicide for Common Waterhemp.

ABSTRACT The fungal plant pathogen Microsphaeropsis amaranthi is a candidate bioherbicide for the control of weedy Amaranthus species since it grows and sporulates readily in culture, is a pathogen of a number of important weed species, and is host-restricted to the family Amaranthaceae. This study was designed to determine the optimum and limiting environmental conditions for the efficacy of foliar applications of M. amaranthi for the control of common waterhemp. The greatest disease severity and the greatest plant biomass reductions were found when a prolonged leaf wetness period of >/= 12 h at 18 to 24 degrees C was provided after application. Disease severity and plant biomass reductions were greater when plants were treated at an earlier stage of development. Disease severity and impact were limited when shorter leaf wetness periods were provided, with negligible disease observed on plants provided with </=6 h leaf wetness. When common waterhemp seedlings were sprayed in the field weekly through the summer of 2004 with conidial suspensions of M. amaranthi, only 7 of 14 applications resulted in disease and only 3 applications resulted in more than slight disease flecks. Moderate levels of disease were observed when applications had been made on days with moderate temperatures and leaf wetness duration in excess of 12 h following application. We confirm the pathogenicity of M. amaranthi against common waterhemp and demonstrate that the unformulated pathogen can cause disease in the field. Nonetheless, the weather conditions of the Midwestern United States, where common waterhemp control would be desired, are infrequently conducive to infection. The continued candidacy of M. amaranthi as a bioherbicide for the control of common waterhemp in the Midwest-ern United States will require that its efficacy under suboptimal environmental conditions be improved.

The Role of Pest Control Advisers in Preventative Management of Grapevine Trunk Diseases

Vineyards with trunk diseases (Botryosphaeria dieback, Esca, Eutypa dieback, and Phomopsis dieback) can have negative returns in the long run. Minimizing economic impacts depends on effective management, but adopting a preventative practice after infection occurs may not improve yields. Pest control advisers may reduce grower uncertainty about the efficacy of and need for prevention, which often entails future and unobservable benefits. Here, we surveyed advisers in California to examine their influence over grower decision-making, in the context of trunk diseases, which significantly limit grape production and for which curative practices are unavailable. Our online survey revealed adviser awareness of high disease incidence, and reduced yields and vineyard lifespan. Advisers rated both preventative and postinfection practices positively. Despite higher cost estimates given to postinfection practices, advisers did not recommend preventative practices at higher rates. High recommendation rates were instead correlated with high disease incidence for both preventative and postinfection practices. Recommendation rates declined with increasing cost for preventative, but not for postinfection, practices. Our findings suggest that even when advisers acknowledge the risks of trunk diseases, they may not recommend preventative practices before infection occurs. This underscores the importance of clear outreach, emphasizing both the need for prevention and its long-term cost efficacy.

First Report of Phytophthora cinnamomi Causing Trunk Canker of Almond in California

Almond (Prunus dulcis) is the most economically important nut crop in California, accounting for

Deep injection and the potential of biochar to reduce fumigant emissions and effects on nematode control

5.9 billion value in 2016. During fall of 2015, declining 2-year-old almond trees on clonal Hansen 536 (peach × almond) rootstock were observed in a commercial orchard in Kern County, California. Affected trees expressed early defoliation with amber-colored gummosis on the trunk. Cankers developed at the bud union and extended into the scion in trees planted with their bud union placed at or below the soil surface. To isolate the causal agent, necrotic pieces of bark and subtending vascular tissue were cut from the canker margin, surface sterilized in 0.5% sodium hypochlorite solution for 3 min, rinsed twice in sterile water, and plated on corn meal agar amended with selective antibiotics (PARP) (Jeffers and Martin 1986). After several days of incubation at 25°C, Phytophthora-like colonies grew from the diseased tissues and were transferred onto acidified potato dextrose agar medium (APDA). The isolates were tentatively identified as Phytophthora cinnamomi Rands based on morphological characteristics. Sporangia were nonpapillate and ovoid to obpyriform or ellipsoidal to elongate-ellipsoid with an apical thickening, avg. 57.2 × 36.6 µm with a length-width ratio of 1.56; mycelium with abundant coralloid hyphal swellings. Globose chlamydospores formed abundantly in culture on the parent hyphae or on new hyphal branches (18 to 42 [avg. 27.65] μm in diameter) (Erwin and Ribeiro 1996). For all isolates, the internal transcribed spacer (ITS) regions of the rRNA gene and a portion of the β-tubulin (TUB) gene were amplified and sequenced with primers ITS1/ITS4 and TUBUF2/TUBUR1 (Lan et al. 2016), respectively. Two representative sequences for each locus were deposited in GenBank (ITS; KY924650 and KY924651, TUB; KY924648 and KY924649). BLASTn analysis against Phytophthora Database (www.phytophthoradb.org) showed that the ITS sequences had 99% identity and the β-tubulin sequences had 97 to 98% identity to reference sequences of P. cinnamomi (ITS: GU259227; TUB: EU079894). Two of the almond isolates of P. cinnamomi were tested for pathogenicity by wound inoculating 2-year-old potted almond trees of cv. Nonpareil on rootstocks of Hansen 536 and Nemaguard (Prunus persica × Prunus davidiana). Five-millimeter-diameter mycelial disks from PDA cultures of each isolate (and sterile PDA as a control) were used for inoculating wounds of the same size (5 mm) made in stems of: i) the almond scion on Hansen 536 rootstock, ii) the almond scion on Nemaguard rootstock, iii) Hansen 536 rootstock below the scion, and iv) Nemaguard rootstock below the scion. Five replicate trees were inoculated per combination of the three inoculants (two of P. cinnamomi, one control) and the four inoculation treatments in a randomized complete block design. After inoculation, the wounds were covered with petroleum jelly and wrapped with Parafilm. Two months after inoculation, all plants inoculated with the control remained healthy, whereas all scions and rootstocks inoculated with either isolate developed stem cankers. None of the trees inoculated on the rootstocks died during the experiment, but 40% of the trees inoculated on the scion were killed. P. cinnamomi was reisolated only from the infected plants. More than 10 other species of Phytophthora were reported to cause root and crown rots as well as aerial cankers on almond (Browne and Viveros 1999; Browne et al. 2015). To our knowledge, this is the first report of P. cinnamomi causing trunk canker disease of almond in California.

A new framework for UAV-based remote sensing data processing and its application in almond water stress quantification

Reducing fumigant emissions is essential for minimizing the environmental impacts of pre-plant soil fumigation. Low permeability plastic films are effective at reducing emissions but have high initial purchase, installation, and disposal costs. The objective of this study was to evaluate if deep fumigant injection and biochar soil amendments can reduce emissions, improve fumigant distribution in soil, and provide acceptable control of plant parasitic nematodes. A pre-plant soil fumigation trial was conducted in a commercial orchard in the San Joaquin Valley, CA, USA. Treatments included two rates of Telone® C-35 (a mixture of 1,3-dichloropropene and chloropicrin) under totally impermeable film or with no surface seal, two injection depths (45 or 65 cm), and two biochar rates (20 or 40 ton ha-1). Emission rates were generally low due to rain events encountered during the trial, but data clearly showed that the deep injection enhanced fumigant delivery to depths below 60 cm and resulted in significantly lower peak emission compared to the standard injection depth. Biochar applied at 40 ton ha-1 had the lowest emission rates during 1-month monitoring period. Although variability in nematode survival was high, tarped, deep injection, and biochar treatment showed lower survival of nematodes at various depths. Increase in fumigant persistence, especially chloropicrin, was observed in this study, likely due to the high soil moisture and low temperature. All data indicate that biochar amendments can help reduce fumigant emissions without reducing nematode control; however, additional research is needed to optimize treatments, determine the affordability of various biochar materials, and validate results under a range of field conditions.

Challenges in water stress quantification using small unmanned aerial system (sUAS): Lessons from a growing season of almond

With the rapid development of small imaging sensors and unmanned aerial vehicles (UAVs), remote sensing is undergoing a revolution with greatly increased spatial and and temporal resolutions. While more relevant detail becomes available, it is a challenge to analyze the large number of images to extract useful information. This research introduces a new general framework to process high-resolution multi- spectral images based on Principle Component Analysis (PCA) for crop stress quantification. As a case study, this framework is applied in almond water stress quantification using UAV-based remote sensing images. First, crop distributions of pixel value of sample trees are obtained as histograms consisted of 255 bins, assuming the stress information lies in the overall canopy pixels and ignoring the spatial relations among pixels. Second, PCA is applied to extract principle components out of histograms of 255 dimensions. This approach is advantageous in that it makes no assumption about the underlying canopy distribution of pixel values. It is shown that the first principle component has a significant correlation with stem water potential. This method is also compared with the traditional method of using the mean values of canopy Normalized Difference Vegetation Index (NDVI) as a baseline, and it shows improved performance in predicting the water stress.

Soil suitability index identifies potential areas for groundwater banking on agriculturallands

With water shortages and drought affecting many regions of the world, it becomes urgent to increase water use efficiency (WUE) by optimizing irrigation schedule. Proper irrigation scheduling, which includes integrating of soil moisture monitoring, surface evapotranspiration loss calculation, and plant based measurements is required for high WUE. Stem water potential (SWP) has become one of the more common methods to measure water status. It is, however, labor intensive and time consuming, and adoption has been slow. This study aims to build the link between SWP and canopy normalized difference vegetation index (NDVI) based on aerial multi-spectral images and ground-truth measurement of an almond orchard. Data suggests that the correlation between SWP and canopy NDVI can be improved by tuning canopy NDVI threshold, as indicated by the coefficient of determination (R2). Also, NDVI shows good correlation with SWP in different growing stages — fruit development and post-harvest. Finally, it is demonstrated canopy NDVI distribution from different missions are significantly different, even if the interval between two flights is less than one hour. This poses the challenge that further calibration is needed to conduct quantitative measurement in long flight missions. Meanwhile, quantitative consideration of characteristic of bi-directional reflectance distribution function makes it necessary to obtain stable performance of canopy NDVI.