CONDITIONS FAVORING AN OUTBREAK OF CURLY-TOP. 21 



bundles ; the latter soon swells out between the larger veins and pro- 

 duces the " savoyed " effect so frequently noted. To the naked eye no 

 puncture is visible, nor is any local discoloration due to the punctures 

 to be seen by reflected light, as may be seen in the case of other species 

 of hopper — for example, Eutettix strobi Fitch, which leaves a crim- 

 son-lake spot at the site of every puncture. However, on careful in- 

 spection the writer has been able to discern a faint mottling of gTeen 

 with lighter green in the parenchyma of the leaves of young beets by 

 transmitted light when the curly-top symptoms were developing. 



In some badly infested beet fields in Idaho the writer has observed 

 leaf hoppers feeding on the dock (Rumex crispus L.), and the leaves, 

 especially the inner ones, of all such docks were thickly besprinkled 

 with small, wine-colored, circular spots. These spots coalesce when 

 very numerous in a small area, and if they appear on the petioles 

 and veins the discoloration runs some distance along the fibro- vascular 

 bundles. As these leaves get older they become suffused with red 

 in the more spotted area (PI. I, fig. 2). The same phenomenon has 

 also been noted in mangel-wurzels, or stock beets, infested by the 

 leaf hopper. 



CONDITIONS FAVORING AN OUTBREAK OF CURLY-TOP. 



It has been stated by several observers that dry, hot seasons seem 

 favorable to severe outbreaks of curly-top, either because such con- 

 ditions are favorable to the activity and multiplication of the leaf- 

 hoppers or because detrimental to an early and rapid growth of the 

 beets, or a combination of both. It appears that a cold, dry, back- 

 ward spring, when the seed is slow to germinate and makes a still 

 more tardy growth after germination, is particularly favorable to 

 an outbreak of the disease. While it may prove true that hot, dry 

 weather stimulates the activity of the insects and renders the develop- 

 ment of the disease more rapid and severe, it does not yet appear to 

 be established that to keep the soil about the beets in a moist condi- 

 tion will prevent the insects from invading them, from multiplying, 

 or from producing the disease. However, some leafhoppers may be 

 drowned during an irrigation, but the reduction in numbers thus 

 brought about is not considered of much avail. 



Our experiments during 1908 and 1909 show that leafhoppers are 

 able to work and induce the disease under conditions extremely 

 varied, both as regards temperature and actual and relative humidity. 

 The crucial point seems to be the number of leafhoppers infesting the 

 beets in relation to the size and vigor of the plants. There is little 

 doubt that the weather has an important bearing on the matter, 

 mainly, it is thought, on account of its influence on the growth and 

 consequent resisting power of the beets; also quite possibly on ac- 



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