Resistance gradient of black pod disease in cocoa and selection by leaf disk assay

To characterize the gradient of resistance to black pod disease experimentally, the leaf disk assay was applied to 217 cocoa genotypes. The parameters time interval, number of leaves evaluated per genotype and distribution of experimental treatments were evaluated. Significant differences between genotypes were found, resulting in the grouping in five homogenous groups (p < 0.01). In the assessments with the leaf disk test, five and seven days after inoculation, the F value was highest after seven days, clearly separating the resistant from susceptible genotypes. There was no difference between the clones behavior and the use of more than one box to group the genotypes (p = 0.56) and the use of different leaves of the same genotype (p = 0.08). The resistance gradient observed indicates variability enough for mapping and cocoa breeding in the population.

Sinking Leaf-Disk Test for Potato Variety Herbicide Tolerance

A sinking leaf-disk technique enabled detection of differences among potato (Solanum tuberosumL.) varieties in herbicide tolerance within 24 hr. Susceptibility of varieties to field application of metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazine-5-(4H)-one] was correlated (r = 0.76∗∗) to the number of sunken disks in a metribuzin solution in a growth chamber with continuous light at 30 klux. Varieties that were very tolerant, very susceptible, or intermediate in tolerance to field application of metribuzin were generally in these same categories when evaluated by the sinking leaf-disk technique. Speed of this response was dependent upon herbicide concentration and solution temperature, but the varietal relationships did not change. It was necessary to use leaves of nearly the same age as determined by relative size and position on the plant. Other factors influencing results included plant growth stage, soil moisture, and atmospheric temperature. Results from the sinking leaf-disk test of winter-grown greenhouse plants of the same clones were correlated (r = 0.86∗) to estimated metribuzin injury to field-grown plants. This method, with adequate modification of technique, may have potential use for studying other plant species and other relationships between plants and photosynthesis-inhibiting herbicides.