Detection and Assessment of Blister Blight disease in Tea Canopies

Blister Blight, caused by biotrophic fungus known as “Exobasidium vexans Massee”, affects the young tea leaves and leave diseases are very significant in tea production. The disease is endemic to most tea growing areas of Asia. About 50% of the tea exported around the world is from Asia and the most important producing countries being Sri Lanka, India, China, Vietnam and Indonesia. The main raw material of the tea industry is young tea leaves and are plucked once every ten days. But the plants are grown for their young leaves and leave diseases are very significant in tea production.

Blister blight affects harvestable shoots and the newly emerging shoots of tea bushes and is capable of causing considerable enormous crop loss. The extent of crop loss is reported to be 32% in Sri Lanka.

Current practice is to use visual estimation to determine blister blight severity for cultivar screening, fungicide screening, crop loss assessment and other management activities. Several studies report the sources of assessment error inherent to the visual rating. Visual methods are subjective, often lead to over or under estimation. Since the estimations are influenced by rater experience and skill, the accuracy of visual estimates depends on the number of visual estimators involved per cultivation field.

Blister Blight severity assessments in nursery, young tea and fields recovering from pruning is not feasible because of non-destructive nature. Therefore, there is a need for a rater independent system useful for non-destructive assessment of Blister Blight disease enabling precise, accurate, reliable and repeatable measurements as well as monitoring of disease status in the field. This project aims at remote sensing Blister Blight detection and quantification to save the cost, manpower and time incurred in visual estimation in fungicide and cultivar screening trials.

Spectral reflectance data were collected using field-portable, handheld Spectroradiometer (PSR 1100f – SPECTRAL EVOLUTION) with 250 field of view in the spectral range 400 to 1100 nm and with spectral resolution of 3.0 nm @ 600 nm, under controlled laboratory conditions and field. 

Spectral reflectance data were collected from samples of healthy tea leaves, tea leaves with translucent spots (Initial stage of Blister Blight), tea leave with Blister Blight (Intermediate stage of Blister Blight) and tea leaves with necrotic spots (Final stage of Blister Blight).In this study it was used Tea Research Institute (TRI) recommended, up country clones TRI 2025, TRI 2043 and CY 9. Also spectral signatures were collected for TRI 2024 tea canopy which was recommended by TRI for mid country wet zone. CY 9 is known as estate clones and TRI 2043 cultivars is used for production of silvery tip teas. For data analysis of the collected spectral data, only the spectral range of 400 – 970 nm was used, because this range allows to find changes in leaf cells structure caused by disease or other factors (Sims and Gamon, 2002).It is identified the distinguishable differences of spectral signature of healthy tea leaves and the tea leaves which affected by Blister Blight disease at near infra-red region (700-1000 nm). Observation shows that the level of decrease of the reflectance percentage at near infra-red region depends on the severity of the Blister Blight disease and it is common for all the cultivars tested. Similar results were reported for Cercospora leaf spot, leaf rust and powdery mildew of sugar beet (Rumpf et al. 2010), Laurel wilt of avocado (Sankaran et al. 2012), etc.

The disease quantification by Spectroradiometer at ground based studies will open new window to investigate the potential use of remote sensed data in detecting early stage blister blight at tea canopies. This will also to help to quantify the disease severity for decision making in fungicide spraying and identifying disease resistance.