FUMIGATION OF CITRUS PLANTS. 



39 



reason for the less amount of injury in daylight fumigation with 

 liquid hydrocyanic acid. This is shown by Table III, in which are 

 presented data giving the increase in temperature at different parts 

 of a 12-foot tree covered with a canvas tent, December 17, 1919. 



Table III. — Rise in temperature at different points within a tented citrus tree in the 

 sunshine at varying periods following the covering . Thermometers placed from 6 to 10 

 inches from the canvas. 



Minutes 

 after start. 



Rise in temperature. 



Sunward 



side, 11 feet 



altitude. 



Sunward 



side, 4 feet 



altitude. 



Shade 



side, 4 feet 



altitude. 



5 

 10 

 15 

 20 



8 

 16 

 22 

 26 



3 

 9 



12 

 12 



2 

 3 

 5 

 6 



a Sun temperature at start, 75° F.; shade temperature, 69° F. Time, 10.43 a. m. 



It is shown in this table that an increase of 26° occurred toward 

 the top of the tent on the sunward side within 20 minutes after 

 covering, whereas at the same time on the same side 4 feet from the 

 ground the increase was only 12° and on the shaded side of the tree 

 at the same height only 6°. Injury in daylight work has been observed 

 to be proportionately greater at these different points in the case of 

 pot-generated gas. The lower part of the shaded side of the tree, at 

 which the temperature increase is very slight, is seldom injured even 

 when very severe burning takes place on the sunward side of the tree. 



The effect of the temperature is least felt in the case of thoroughly 

 hardened trees. In fact, the extent to which sunshine fumigation can 

 be practiced durhig the winter period is attributable largely to the 

 extent of this hardened or dormant condition of the trees. This 

 condition also offers an explanation for the safety of fumigation at 

 very low temperatures, sometimes at the freezhig point, whereas at 

 other times, especially in the early fall, while trees are active, severe 

 injury takes place at several degrees above the freezing point. In the 

 Tulare County citrus belt of California the writer has noted night 

 fumigation, with heavy dosages, carried on during the summer at 

 temperatures as high as 85° F. without apparent injury to the plants. 

 Fumigation in the coast region of southern California at such high 

 temperatures would produce severe injury. He believes that this 

 greater safety in the northern citrus region is due largely to the more 

 resistant condition of the plants brought about by the very hot, arid 

 climate during the summer. Duggar (4) states that green leaves ex- 

 posed to sunlight show a temperature from 2° or 3° to 15° higher than 

 the air, and according to MacDougal (10) the maximum temperature of 

 higher plants varies from 100° to 115° F. At or above the maximum 



