In agriculture, early detection of plant stresses is advantageous in preventing crop yield losses. Remote sensors are increasingly being utilized for crop health monitoring, offering non-destructive, spatialized detection and the quantification of plant diseases at various levels of measurement. Advances in sensor technologies have promoted the development of novel techniques for precision agriculture. As in situ techniques are surpassed by multispectral imaging, refinement of hyperspectral imaging and the promising emergence of light detection and ranging (LIDAR), remote sensing will define the future of biotic and abiotic plant stress detection, crop yield estimation and product quality. The added value of LIDAR-based systems stems from their greater flexibility in capturing data, high rate of data delivery and suitability for a high level of automation while overcoming the shortcomings of passive systems limited by atmospheric conditions, changes in light, viewing angle and canopy structure. In particular, a multi-sensor systems approach and associated data fusion techniques (i.e., blending LIDAR with existing electro-optical sensors) offer increased accuracy in plant disease detection by focusing on traditional optimal estimation and the adoption of artificial intelligence techniques for spatially and temporally distributed big data. When applied across different platforms (handheld, ground-based, airborne, ground/aerial robotic vehicles or satellites), these electro-optical sensors offer new avenues to predict and react to plant stress and disease. This review examines the key sensor characteristics, platform integration options and data analysis techniques recently proposed in the field of precision agriculture and highlights the key challenges and benefits of each concept towards informing future research in this very important and rapidly growing field.

Fruit skin colour is one of the most important visual fruit quality parameters driving consumer preferences. Proximal sensors such as machine vision cameras can be used to detect skin colour in fruit visible in collected images, but their accuracy in variable orchard light conditions remains a practical challenge. This work aimed to derive a new fruit skin colour attribute—namely a Colour Development Index (CDI), ranging from 0 to 1, that intuitively increases as fruit becomes redder—to assess colour development in peach and nectarine fruit skin. CDI measurements were generated from high-resolution images collected on both east and west sides of the canopies of three peach and one nectarine cultivars using the commercial mobile platform Cartographer (Green Atlas). Fruit colour (RGB values) was extracted from the central pixels of detected fruit and converted into a CDI. The repeatability of CDI measurements under different light environments was tested by scanning orchards at different times of the day. The effects of cultivar and canopy side on CDI were also determined. CDI data was related to the index of absorbance difference (I_AD)—an index of chlorophyll degradation that was correlated with ethylene emission—and its response to time from harvest was modelled. The CDI was only significantly altered when measurements were taken in the middle of the morning or in the middle of the afternoon, when the presence of the sun in the image caused significant alteration of the image brightness. The CDI was tightly related to I_AD, and CDI values plateaued (0.833 ± 0.009) at I_AD ≤ 1.20 (climacteric onset) in ‘Majestic Pearl’ nectarine, suggesting that CDI thresholds show potential to be used for harvest time decisions and to support logistics. In order to obtain comparable CDI datasets to study colour development or forecast harvest time, it is recommended to scan peach and nectarine orchards at night, in the early morning, solar noon, or late afternoon. This study found that the CDI can serve as a standardised and objective skin colour index for peaches and nectarines.

Some cultivars of Pyrus communis develop mature fruit with a distinctive red blush. Investigating the patterns of pear colour development in response to sunlight has implications for orchard management of these pears. The objectives of these experiments are to study the seasonal patterns of colour development and investigate the influence of shade and sunlight exposure on the red colour and harvest quality of blush pears “ANP-0118” and “ANP-0131”. Several long, medium and short shading treatments were applied at different stages of fruit development from 28 (“ANP-0131”) and 29 (“ANP-0118”) days after full bloom (DAFB) until harvests at 119 DAFB (“ANP-0118”) and 175 DAFB (“ANP-0131”). Fruits were measured every three weeks for colour parameters (a*, hue angle, chroma) and at harvest for quality parameters (fresh weight, visual assessments of percentage blush coverage and blush intensity, flesh firmness and soluble solids concentration). In the unshaded control, red colour increased during the growing season (increase in a* value and decrease in hue angle), as well as increasing in chroma value. Periods of shading during the season negatively affected red colour in both cultivars, as evidenced by significant decreases in a* value and increases in hue angle. Shaded fruits that were subsequently re-exposed to sunlight reacted with a dynamic increase in a* value and decrease in hue angle. Fruit shaded for the length of the experiment or prior to harvest had significantly lower a* values than the control at harvest. Visual assessment at harvest of percentage blush coverage and blush intensity were significantly affected by shading in both cultivars. Shading treatments applied early in the experiment had a negative effect on the fresh fruit weight of “ANP-0118”.

The triggers of biennial bearing are thought to coincide with embryonic development in apple and occurs within the first 70 days after full bloom (DAFB). Strong evidence suggests hormonal signals are perceived by vegetative apple spur buds to induce flowering. The hormonal response is typically referred to as the floral induction (FI) phase in bud meristem development. To determine the metabolic pathways activated in FI, young trees of the biennial bearing cultivar ‘Nicoter’ and the less susceptible cultivar ‘Rosy Glow’ were forced into an alternate cropping cycle over five years and an inverse relationship of crop load and return bloom was established. Buds were collected over a four-week duration within 70 DAFB from trees that had maintained a four-year biennial bearing cycle. Metabolomics profiling was undertaken to determine the differentially expressed pathways and key signalling molecules associated with biennial bearing. Marked metabolic differences were observed in trees with high and low crop load treatments. Significant effects were detected in members of the phenylpropanoid pathway comprising hydroxycinnamates, salicylates, salicylic acid biosynthetic pathway intermediates and flavanols. This study identifies plant hormones associated with FI in apples using functional metabolomics analysis.

The aims of this study were to investigate the sunlight requirements during floral initiation and differentiation for the development of flower buds in ‘Autumn Bright’ nectarine and to explore its source–sink relationship. In early January 2019 (111 days after full bloom), prior to floral initiation and differentiation, 12 new shoots were tagged on 14 trees, with four shoots in each of the low (0–1.2 m), middle (1.2–2.0 m), and high (>2.0 m) canopy heights. Three treatments (bud shading; leaf pluck; bud shading and leaf pluck) were applied to three shoots in each canopy height on the fourth and eighth bud, in addition to a fourth control shoot. Light penetration was measured at the different canopy heights. Buds were assessed in Spring for floral transition, number of floral buds per node, and fruit set. The treatments at the node level had no effect on floral initiation, indicating that sink strength was not promoted by additional light. Light penetration decreased with decreasing canopy height and corresponded with lower floral buds in the low zone. Fruit set was uninfluenced by all treatments. The results of this study emphasised the importance of the availability of photosynthetic assimilates for floral initiation in peach and nectarine trees. Balanced crop load management and summer pruning to enhance canopy sunlight distribution would increase the availability of nutrients for improved floral transition in this cultivar.

This study aimed to evaluate the effects of fruit position, light exposure and fruit surface temperature (FST) on apple fruit colour development and fruit quality at harvest, including sunburn damage severity. This was achieved by undertaking two experiments in a high-density planting of the dark-red apple ANABP 01 in Tatura, Australia. In the 2020–2021 growing season an experiment was conducted to draw relationships between fruit position and fruit quality parameters. Here, sample fruit position and level of light exposure were respectively determined using a static LiDAR system and a portable quantum photosynthetically active radiation (PAR) sensor. At harvest the sample fruit were analysed for percentage red colour coverage, objective colour parameters (L*, a*, b*, hue angle and chroma), sunburn damage, fruit diameter (FD), soluble solids concentration (SSC), flesh firmness (FF) and starch pattern index (SPI). A second experiment was conducted in the 2021–2022 growing season and focused on how fruit shading, light exposure and the removal of ultraviolet (UV) radiation affected the FST, colour development and harvest fruit quality. Five treatments were distributed among sample fruit: fully shaded with aluminium umbrellas, shaded for one month and then exposed to sunlight until harvest, exposed for one month and then shaded until harvest, covered with a longpass UV filter and a control treatment. The development of colour in this dark-red apple cultivar was highly responsive to aspects of fruit position, and the intensity and quality of light exposure. The best-coloured fruit were exposed to higher quantities of PAR, exposed to both PAR and UV radiation simultaneously and located higher in the tree canopy. Fruit that were fully exposed to PAR and achieved better colour development also displayed higher FST and sunburn damage severity.