The tea patches were masked out from the landuse/landcover maps for further analysis to find the percentage of healthy tea patches, moderately healthy tea patches and affected tea patches. The formula used for estimating the percentage of affected and non-affected tea is given below:
 Using the LANDSAT, LISS III and ASTER landuse/landcover image, the area for the individual classes were computed (in hectares). Among the classes, the area of healthy, moderately affected and diseased tea patches were taken into consideration from which the percentage of healthy, moderately healthy and diseased tea patches were calculated. From Table 8, the variations in tea plantations at three different dates can be observed. The LANDSAT image of December, 2001 showed 60.4% area under healthy tea, 23.6% area under moderately affected tea and 16.2% area under diseased tea. For the LISS III image of February, 2004, it was found that 43.9% area under healthy tea, 36.8% area under moderately affected tea and 19.3% area under diseased tea. Similarly for ASTER image of June, 2004, area under healthy tea was found to be 24.9%, moderately healthy tea was found to be 50.1% and diseased tea to be 25.1%. From the above statistics it is quite clear that the disease was more prominent in the month of June. From June to October, the weather remains hot and humid accompanied by heavy monsoon showers. This is the peak period for the outbreak of Red Spider Mite attack and the Helopeltis attack. The tea bushes are highly prone to these two diseases during this period and once the disease breaks out it spreads very rapidly. |