Recognition Of The Rust

When an asparagus field is badly infested with the rust the general appearance is that of an unusually early maturing of the plants (Fig. 46). Instead of the healthy green color there is a brown hue, as if insects had sapped the plants



or frost destroyed their vitality. Rusted plants, when viewed closely, are found to have the skin of the stems lifted, as if blistered, and within the ruptures of the epidermis the color is brown, as shown in Fig. 47. The brown color is due to multitudes of spores borne upon the tips of fine threads of the fungus, which aggregate at certain points and cause the spots. The threads from which the spores are produced are exceedingly small and grow through the substance of the asparagus stem, taking up nourishment and causing an enfeebled condition of the victim, which results in loss of the green color and the final rustiness of the plant, due to the multitude of spores formed upon the surface. These spores are carried by the wind to other plants, where new disease spots are produced; but as the autumn advances a final form of spore appears in the ruptures that is quite different in shape and color from the first ones produced through the summer. The spores of late autumn, from their dark color, give an almost black appearance to the spots. There is another form which the rust fungus assumes not usually seen in the asparagus field, but may be found in early spring upon plants that are not subjected to cutting. This is the cluster-cup stage, so named because the fungus produces minute cups from the asparagus stem, and in small groups of a dozen to fifty, making usually an oval spot easily seen with the naked eye. This stage of the fungus comes first in the order of time in the series, and is met with upon volunteer plants that may grow along the roadside or fence row, or in a field where all the old asparagus plants have not been destroyed.

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