A RAPID AND NONDESTRUCTIVE METHOD TO ASSESS LEAF INJURY CAUSED BY THECASSAVA GREEN MITE, MONONYCHELLUS-TANAJOA (BONDAR) (ACARINA, TETRANYCHIDAE)

A relative scale of Leaf Damage Indices (LDI) from 0 to 5 describes th e visible injury to leaves of cassava, Manihot esculenta Crantz caused by the cassava green mite, Mononychellus tanajoa (Bondar). As the sca le is ordinal and thus not quantitative, the observed LDIs are convert ed individually to relative loss of chlorophyll on a ratio scale befor e an average injury is determined. This calibration is required becaus e the ordinal and the ratio scales are not linearly related. A calibra tion curve was established on the basis of laboratory experiments to d etermine the chlorophyll content, c, of leaves representing various le af damage indices. Several monotonously decreasing functions were fitt ed to the experimental data yielding the following relation c = c0 [1- LDI/5]a, where c0 is the chlorophyll content of uninjured leaf tissue and a is a constant describing the steepness of the curvilinear relati on. This means that LDIs could be converted to relative loss of chloro phyll, d, where d = [LDI/5]a. The photosynthetically active leaf area of plants can be estimated by combining the relative loss of chlorophy ll with leaf area assessments and adds the effects of defoliation and suspended growth to the chlorophyll depletion. The difference in photo synthetically active area that arises between uninjured and injured pl ants over a period of time provides a measure of spider mite injury th at can be related to growth and yield. The method integrates the injur y inflicted over a period of time, allows successive observations of t he same plants, and is rapid and reasonably precise considering the ti me savings.