RANKIN: DUTCH ELM DISEASE CONTROL 
61 
County in an area of 140 square miles and a population of about 240,000 
elms over 5 inches, the incidence has been reduced from 1032 diseased 
elms the first year to 280 in 1938. In this region the disease had gained 
considerable headway by 1934, when many areas showed 10 to 30 or 
more diseased elms to the square mile. Estimates at that time indicated 
that the rate of increase was about 5 times annually. The 36 per cent 
reduction in this region in 1937 below the 1936 figure, and the 40 per cent 
reduction in 1938 below 1937, indicates that additional progress can be 
made here and that the eradication method can be depended on to hold 
the incidence to its present low figure of about one-tenth of one per cent 
of the elm population in this region of high elm values. 
The effectiveness of the eradication measures in preventing local 
spread is best shown by the results obtained in northern Westchester 
County, in an area of 234 square miles where the number of diseased 
elms found has increased annually as follows: 9 diseased elms in 1934, 
20 in 1935, 67 in 1936, 138 in 1937 and 136 in 1938. Delay in scouting 
this area in most summers until August may account for these increases. 
The record is definite, however, that the prompt eradication of the dis¬ 
eased elms found each year and local sanitation has been effective in 
87 per cent of the cases in preventing any further occurrence of the dis¬ 
ease in the vicinity. Of the total of 150 locations where single diseased 
elms or groups of diseased elms were found only 22 have ever showed 
recurrence of the disease within a 500 foot radius. Of the 22 cases that 
did show apparent recurrences only one showed infection persisting into 
the third year. These records for a large rural area of light infection as 
well as those for the southern portion of Westchester County in a resi¬ 
dential area show the striking effectiveness of finding the diseased elm, 
destroying it and sanitating for beetle breeding material. The large 
majority of the diseased elms found each year both in the heavy and 
lightly infected zones occur in new locations that have no connection 
with past known cases or any failure to prevent local spread from 
known locations. 
These facts indicate that the future success of the eradication program, 
if it is to accomplish more than suppression and local control, depends 
on the development of better detection methods to find sources of spread 
that are now being over-looked. Whether a more efficient personnel 
and timely coverage of the territory would detect these sources as trees 
showing symptoms or whether there may be a persisting residue of hid¬ 
den sources of spread, should be determined as soon as possible because 
