SPARAGUS STEMS AFFECTED WITH RUST]

[Illustration: FIG. 47--PORTION OF RUSTED ASPARAGUS STEMS]

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.

METHODS OF TREATING THE RUST

All the cultivated varieties of asparagus are readily affected by the
rust, although it has been found that some varieties, notably Palmetto,
are less susceptible to its attacks than others. The most effectual
means of controlling the disease are spraying, burning of the brush,
cultivation, and irrigation.

_Spraying._--Dr. Halsted, in his first experiments, used soda-bordeaux,
hydrate-bordeaux, and potash-bordeaux. The sp