20 
BULLETIN 8*79, U. S. DEPARTMENT OF AGRICULTURE. 
^irus of the mosaic of cucurbits have been less extensive, they indi- 
cate that it. is similar in many respects to that of tobacco and other 
plants. The power of infection may be destroyed in each case and 
is definitely connected with temperature, moisture, age, and other 
factors. 
EFFECT OF HEAT ON THE POWER OF INFECTION. 
The expressed juices of mosaic cucumber plants lose their power 
of infection when heated above 70° C. This temperature has proved 
the limit in all experiments, as is shown in Table III. In these tests 
the plant juices were expressed and filtered through filter paper 
before use. Small test tubes were used and 5 c. c. portions of the 
juice taken to insure rapid heating to the desired temperature. A 
constant- temperature water bath was used and the tubes heated 10 
minutes and then immediately cooled to room temperature. Inocu- 
lations were made from the different tubes to healthy cucumber 
plants, inoculating into the stem, at the tip and base, and also into 
two of the young leaves. These plants were protected from possible 
outside infection by covering them with insect-proof cages. 
Table III. — Effect of heat on power of infection of expressed juices of mosaic cucumber 
plants. 
Temperature (°C). 
Number 
of plants 
inocu- 
lated. 
Results. 
Date. ' 
Number 
of mosaic 
plants. 
Date last 
observed. 
Field tests: 
Sept. 3,1916 
45 
50 
55 
60 
65 
70 
75 
80 
Unheated. 
60 
65 
70 
75 
80 
85 
90 
95 
Unheated. 
55 
60 
65 
70 
75 
80 
85 
90 
Unheated. 
3 
3 
3 
3 
3 
3 
3 
3 
3 
4 
4 
4 
4 
4 
4 
4 
4 
4 
6 
6 
6 
6 
6 
6 
6 
6 
6 
2 
2 
3 

2 
2 


3 
3 
2 
4 





4 
4 
5 
2 
3 




4 
Sept. 18,1916 
Do. 
Do 
Do 
Do. 
Do 
Do. 
Do 
Do. 
Do 
Do. 
Do... 
Do. 
Do 
Do. 
Do 
Do. 
Aug. 9,1917. 
Aug. 21,1917 
Do. 
Do 
Do 
Do. 
Do 
Do. 
Do 
Do. 
Do 
Do. 
Do 
Do. 
Do 
Do. 
Do 
Do. 
Greenhouse tests: 
Oct. 30, 1917 
Nov. 10,1917 
Do 
Do. 
Do 
Do. 
Do 
Do. 
Do 
Do. 
Do 
Do. 
Do 
Do. 
Do 
Do. 
Do 
Do. 
EFFECT OF AGE ON THE POWER OF INFECTION. 
The expressed juices of mosaic cucumber and other cucurbitaceous 
plants retain the power of infection for only a short period after their 
extraction. Repeated tests have shown that the juices of mosaic 
plants of cucumber, squash, pumpkin, muskmeion, and Micrampelis 
lobala are never infectious for more than three to five days and in most 
cases lose their virulence within 24 to 48 hours. This is in sharp 
contrast with results secured by Allard (4) with the juices of mosaic 
