Florida Agricultural Experiment Station


 RUST
 Rust, also called black rust, yellow leaf blight, and potash hun-
ger, unlike the other Florida cotton diseases, is not caused by par-
asites, but has its origin in a disturbed nutritional condition
brought about by a number of factors. The most important of
these factors is an actual deficiency of potash in the soil. Contrib-
utory to this are poor cultural conditions due to poor drainage, de-
ficient moisture, lack of humus, improper mechanical condition,
and worn-out soils. In fact, rust is practically synonymous with
poor farming, since fields where this disease occurs, worst have
been abandoned on account of failure to use fertilizer or to keep
down grass and weeds. It is true, however, that rust also appears
in fields that apparently have received good care. In such cases
the disease is usually in spots in the field characterized by poor
drainage, or by thin or rocky soil, and it will recur from year to
year if something is not done to improve conditions in these
areas.
 Rust is a common disease in Florida and does a great deal of
damage on the lighter and poorer soils where practically total de-
foliation has been observed. The disease varies in severity in
different localities and even within the same field. This fact makes
estimates of loss for the state as a whole difficult and of little value,
because often where the disease is'worst the fields are abandoned
and are not considered in the computation of yields for the state.
If such fields were considered, the estimated yield per acre for
cotton for Florida would be even less than it is now, and the
percentage loss attributed to this disease would be greater. The
United States Department of Agriculture estimates the loss for
the entire cotton growing area of the South as 4 to 5 percent of
the crop. The loss from the disease in Florida probably exceeds
these figures somewhat, since the proportion of lighter soils used
for cotton in Florida is greater than it is in other states.

 SYMPTOMS

 Plants suffering from rust are usually small and of an unthrifty
 appearance. About the middle of the season the leaves assume a
 mottled yellow color, the areas around the veins maintaining their
 green color the longest. They later become rusty red in appear-
 ance, curl up and shed. The weakened leaves are particularly
 subject to the attacks of a number of parasites too weak to attack