Present status of the black rot disease of tea in Bangladesh

A survey was carried out in 2015 at Bangladesh Tea Research Institute (BTRI) to ascertain the impact of varieties, topography, age of plants and shade condition on the incidence and severity of Black rot disease in tea. Data were collected monthly from 300 tea bushes selected randomly for each attribute with three replications. Data were recorded on the incidence and severity of the diseases by observing the typical symptom by using 0-5 scoring scale. Disease incidence and disease index was computed by following a referenced formula. In all the cases of attributes, the maximum level of incidence with Black rot disease was found in September but severity with the same was found in July. The incidence and severity of Black rot disease were significantly higher in hellock (23.52 and 16.64%) areas as compared to flat and tillah areas. Age of tea plants had no variation with severity of the disease but a significant variation was found with the incidence. Highest incidence (22.64%) was found in immature tea bushes. More amount of disease was observed in shaded areas as compared to unshaded areas. Seed tea bush (22.64 and 17.07%) was found to be more predisposed to Black rot disease than clones (20.59 and 16.38%). The findings of this study will help to understand the pattern of distribution of the disease in the commercial tea fields so that the planters can escape the diseases easily.


INTRODUCTION
Tea is an important cash crop of Bangladesh. It is one of the largest agro-based industries in the country. There are 169 tea estates having about 59.609 thousand hectares of land under tea plantation producing about 63.86 million kg of made tea with average yield of 1239 kg per hectare during 2014 (PDU, 2015). About 2.67% of this production is exported and Bangladesh earns a substantial amount of foreign exchange of US$ 2,867 thousand during 2013-2014(PDU, 2015. The geographical location of tea growing area is restricted only to some green specks between 21° 30 ∕ and 26° 15 ∕ north latitude and between 89° 0 ∕ and 92° 41 ∕ east longitude (Ahmed, 2005). Tea ecosystem is a complex agro-ecosystem. It comprises tea, shade trees, green *Corresponding author. E-mail: btrippsyeful@yahoo.com.
Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License crops, forest, etc. The intensive mono culture of a perennial crop like tea over an extensive and contiguous area in apparently isolated ecological zones in Bangladesh has formed virtually a stable ecosystem which provided unlimited opportunity for perpetuation and spread of endemic and introduced diseases (Alam, 1999). The architecture of tea plantation, variability of plant types and the systemic interaction of various agrotechniques, intercultural operation, etc. imposes a significant impact on development of diseases. In Bangladesh, the average yield of tea per hectare is quite low as compared to other tea growing countries of the world. Many factors are associated with such low yield. The loss of tea in Bangladesh tea due to various pests, diseases and weeds has been estimated to be about 10-15% (Sana, 1989). Tea being a perennial crop is prone to attack by many pests and diseases. The majority of the diseases in tea are of fungal origin. More than 400 pathogens cause various diseases in tea (Chen and Chen, 1990) viz. foliage, stem and root. Among the diseases, Black rot is a most destructive leaf disease of tea caused by Corticium theae Bernard (Ali, 1992). Black rot of tea is of primary nature and is responsible for direct reduction of crop yield (Ali, 1992). The disease attacks the maintenance leaves just below the plucking table. Infected leaves do not falloff but remain hanging and attached to the next leaf by means of small pads of mycelium at the point of contact. Dense shade, bad drainage and sanitation, high humidity etc. are usually considered predisposing factors for the prevalence of the disease. Hellock areas and North tillah slopes are also conductive to the disease (Ali, 1992). Tunstall and Sarmath (1947) recorded a loss in the yield up to 50% on a bush attacked by Black rot when left untreated for four seasons consequently. Tea zone of Bangladesh divided into three units, viz. Tillah-a low hill, are normally up to 300 ft high steeply rounded. Flat are categorised as high flat and low flat. These are slightly undulating to undulating (Sana, 1989). Hellock is the place in between two adjacent tillah.
Microclimate of an area under tea plantation is greatly influenced by the architectures of plantation. In the same area, there are tillah, flat and hellock. Such topographical diversities directly affect penetration of solar radiation, humidity, temperature and air circulation in an area, thus influences plant growth as well as diseases development (Islam and Ali, 2010). There still remain many gaps in the authors' knowledge on the process of different diseases in tea and understanding of the complex relationships between various dimensions. So, this research work was conducted to define the pattern of distribution and severity of Black rot disease in the commercial tea fields towards the appropriate control strategies.

METHODOLOGY
A survey was carried out during February to last November' 2015 at Islam et al. 3563 the main farm of Bangladesh Tea Research Institute (BTRI) and its research farm to ascertain the impact of varieties, topography, age of plants and shade condition ( Figure 2) on the prevalence and severity of the diseases. Surveyed site was situated under Moulvibazar district ( Figure 1). Data were collected monthly from 300 tea bushes selected randomly for each attribute. This survey was replicated thrice for each. Data were recorded on the prevalence and severity of the diseases by observing the typical symptom ( Figure 3). These were done by using the following 0-5 scoring scale (Islam and Ali, 2011). The severity of the disease was expressed in percent disease index (PDI), which was computed following a standard formula as described below (Singh, 2000).

×100
Weather parameter during the surveyed period was collected from the weather regional office, BTRI campus, Srimangal, Government of the People Republic of Bangladesh. Data was subjected to analysis of variance by MSTAT computer programe. Mean separation was done by Duncan's multiple range test (DMRT).

RESULTS AND DISCUSSION
The Black rot disease was seen in the field in the month of February with very negligible amount. In all cases of attributes, the maximum incidence was found in September but severity with the same was found in July. From the month of April to May, an increasing trend of disease development was found. During this time, temperature, relative humidity and rainfall were also high ( Figure 4). The result in Table 1a and b revealed that seed tea bush was found to be more predisposed (22.64 and 17.07%) to Black rot disease than clones (20.59 and 16.38%). All the year round, a significant protagonist of variety was found for infecting the disease only with the incidence (Table 1a and b). Age of tea plants had no variation with severity of the disease but a significant (p= 0.01) variation was found with the incidence. Highest incidence (22.64%) was found in immature tea bushes and severity (16.88%) in mature tea (Table 2a and b). Table 3a and b represents the result of topographical variation for infection of the Black rot disease. The maximum and statistically dissimilar incidence was found in hellock areas (23.52%) followed by flat areas (22.52%). Hellock areas has also a great impact on disease severity, which is 16.46% followed by flat areas. But the degrees of severities with all topographical areas were statistically (p= 0.01) identical. Another element shade revealed the significantly different impact on disease severity but similar impact on incidence. More amount of disease was observed in shaded areas as compared to unshaded areas. In shaded condition, both incidence and severity were high these were 20.83 and 16.44%, respectively (Table 4). At the end of the monsoon, the disease causing fungus, Corticium theae Bernard produced resting bodies in the soil, creaks and crevices of the infected stems. The active mycelium died during the dormant season which produces new mycelium again during the next monsoon (Ali, 1992). The disease became evident after about four weeks following the onset of rains. Mature leaves and adjacent stems were infected by the disease causing organisms (Sarmah, 1960). Islam and Ali (2010) specified as the highest incidence of Black rot was noticed only on seedling and highest in flat areas. Mooi (1965) and Stewart (1990) reported that very young plants are susceptible to late blight, plants of intermediate age are the most resistant, and old plants become more susceptible again. A study conducted on Chrysanthemum morifolium found that leaf age affected the number and size of lesions caused by Pseudomonas cichorii, with older leaves being less susceptible to infection than younger, immature leaves (Jones et al., 1985). Conversely, Sanchez et al. (2003) found that in coffee (Coffea arabica), infection by P. cichorii was more frequent on older leaves. In the case of cocoa, coffee or tea plantations, agroforestery systems could also modify pests and disease incidence when compared with mono specific plantations and the effect of shade trees on diseases could have several explanations (Beer, 1987;Beer et al., 1989;Guyot et al., 2008;Schroth et al., 2000). The shade reduces sunlight and particularly UV-B, which plays an important role in some plant diseases such as blister blight of tea (Gunasekera et al., 1997); shade modifies the micro-environmental conditions (reduced temperature, reduced temperature fluctuations, reduced air movements and increased humidity) and creates a "phylloclimate" able to perturb interactions between pathogens and target organs (Chelle, 2005); and shade can also work as a barrier and can limit the splash dispersal of the pathogen (Ntahimpera et al., 1998). Young fruit and leaf tissues of citrus are more susceptible to Xanthomonas axonopodis pv. citri (citrus canker) than mature tissues (Verniere et al., 2003). The    intimate mixing of varieties is a proven method in certain cropping systems of introducing diversity to reduce plant disease (Wolfe, 1985, Akem et al., 2000and Zhu et al., 2000. Variety mixtures may reduce disease in three basic ways (Finckh and Wolfe, 1998;Mundt, 2002)    genotype. Variety diversification may also help to reduce pesticide input in conventional systems (Andrivon et al., 2003). The research findings were reviewed with the above findings of other plantations and fruits crops. All these reviewed findings matched the present findings. Survival and dissemination of the pathogen, nature of infection, mode of penetration of the pathogen etc. are greatly influenced by the various environmental factors. Temperature, humidity, light penetration, ventilation, etc. are the important factors for aggressiveness of the pathogen as well as disease development. These factors varies from one tea section to another section with varies of tea plant architecture, shade condition, topography, age and varieties. The findings of this study will be helpful to understand the pattern of distribution of the disease in the commercial tea fields so that the planters can escape the disease easily. These findings met the objectives of the present study and are also helpful for further study regarding the control of the disease.