Black rot, caused by the fungus Guignardia bidwellii is one of the most serious diseases of cultivated grapes in the eastern United States, especially in warm, humid areas. As evidenced by these micrographs, oblique illumination adds a pseudo three-dimensional effect to images when compared to unaided brightfield microscopy.

Crop losses can be devastating, ranging from 5-80% depending on the weather, the variety of grape being grown, and the amount of disease in the vineyard. The fungus can infect all green parts of the vine but the most damaging effect is on the fruit, which shrivel up into dark-colored mummies.

This fungus reproduces with two types of spores, ascospores and conidia (pycniospores). Ascopores are produced in the grape mummy and forcibly discharged into the air, often traveling considerable distances. Conidia are vegetatively reproduced spores that the fungus uses to propagate, and are spread through rain or irrigation water splashing on the plants.

Transmitted oblique illumination was obtained with a Hoffman-style substage condenser fitted with an offset aperture slit, which provides illumination from an oblique angle with respect to the optical axis of the QX3 microscope. Digital images were recorded with the QX3 interactive software operating from the Live View menu. In the case of birefringent specimens, a polarizer was added beneath the Hoffman condenser and an analyzer was placed in front of the QX3 lens cover. In some instances, a full-wave retardation plate was inserted between the specimen and the analyzer before capturing images.

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