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.