Bark split frost injury of tea plant occurs to young trees in early winter and early spring. If the damage is severe, the tea plant can wither and die. This led us to investigate the mechanism of bark split frost injury. Bark splitting caused by the formation of ice, was observed near the cambium layer of tea branch within 2cm above the surface of rock wool fully absorbed with water, when treated at -5℃. Bark splitting was not observed when the tea branches were inserted into dry rock wool. These facts indicate that bark split frost injury is caused by water. Liquid leaks were observed at several places of tea branches when the temperature was returned from -5℃ to room temperature. This phenomenon is possibly involved in bark split frost injury of tea plant. Although a split in cortex at the part of damage was confirmed with MRI, the blockage of sap-flow could not be confirmed.

We estimated the densities of Anthracnose damaged leaves by Colletotrichum theae-sinensis (Miyake) Yamamoto using aerial image data of tea fields, taken by a multirotor-type UAV (Drone) . We used DJI Phantom 4 for aerial photography and the commercial photo editing software, Adobe photoshop element 13, to analyze the image data. The aerial photographs of the tea field where Anthracnose occurred after autumn skiffing clearly showed the points where the disease severely occurred. We analyzed the correlation between the densities of diseased leaves and the values of several arithmetic expressions combining mean values of three primary colors (RGB), luminescence (Y), and normalized RGB (NR, NG, and NB) extracted from the image data. There was a significantly high correlation (absolute value of r > 0.75) between the eight types of calculated values, NR, (R+G)/G, R/Y, G-R, NG-NR, (G-R)/Y, (G-R)/(G+R), and G/R, and the densities of diseased leaves. Furthermore, at other field points, we verified the suitability of the eight linear regression equations obtained from the relationship between the calculated values and the densities of diseased leaves. Our results suggest the potential use of drones to collect aerial image data in tea fields for estimating the densities of Anthracnose damaged leaves.

We tried monitoring the seasonal occurrences patterns of several species of mites in tea fields by using small suction devices driven by dry cell batteries, which were developed for capturing Pseudaulacaspis pentagona larvae, from April to November 2010. The devices captured Tetranychus kanzawai, phytoseiidae mites and two species of eriophyid mites: Acaphylla theavagrans, and Calacarus carinatus. The number of captured individuals of the two species of eriophyid mites was larger. Simultaneously, after replacing the trap cartridge in the device, we collected 20 tea leaves weekly from areas near the place where the capturing device was set, and investigated the density of each mite species on the collected leaves. We compared the densities of the mites and the number captured by the device. For the two species of eriophyid mites, A. theavagrans, and C. carinatus, the patterns of seasonal occurrence revealed that the number of mites captured and their density on leaves were almost synchronized. Moreover, a significant positive correlation was found between the number of mites captured and their density. Although the device had some problems in outdoor durability, the findings suggested that it is suitable for monitoring of eriophyid mites in tea fields.