GENERAL CHARACTER OF CURLY-TOP. 31 



SEVERE OUTBREAKS NOT FREQUENT IN A LOCALITY. 



A notable feature of curly-top is that, so far as has been recorded, 

 there never has been a serious outbreak two seasons in succession in 

 any locality. Therefore, the farmer may be encouraged to continue 

 the culture of sugar beets in spite of the heavy loss consequent on a 

 single attack. This opportunity is taken, however, to utter a note 

 of warning to seed producers to the effect that it will be unprofitable 

 to silo sugar beets harvested from an area badly affected with 

 curly-top, even though no beets exhibiting the characteristic symp- 

 toms are selected for siloing. Furthermore, should the beet leaf- 

 hoppers be numerous without producing more than a sprinkling of 

 plants showing visible signs of the disease, a more or less consider- 

 able percentage of loss may be expected through the development of 

 the disease in seed beets the following season. The probable amount 

 of loss will obviously depend on the number of leafhoppers present 

 during the preceding season, together with the general vigor of the 

 plants at that time and the length of time the leafhoppers had been 

 at work on the beets. 



PROGENY OF THE DISEASED SEED BEETS HEALTHY. 



Another strange feature of curly-top is that the seed from an 

 affected seed beet will produce healthy sugar beets, which in turn 

 afford vigorous and healthy seed beets unless in the mean time they 

 are attacked by more leafhoppers. Thus it appears that seed from 

 affected beets need not be discarded on account of the disease. 



GENERAL CHARACTER OF CURLY-TOP. 



It would appear from these observations and experiments that the 

 curly-top of beets should be included in the group of plant disturb- 

 ances known as " stigmonoses," as described by Dr. A. F. Woods in 

 connection with the effects of aphides, thrips, and other sucking in- 

 sects on the carnation (32, 33, 34, 35, 37), by Dr. N. A. Cobb as oc- 

 curring on apples, pears, etc. (10), and by Moulton as occurring on 

 the pear (21). Curly-top resembles these stigmonoses in that the 

 disturbances are primarily set up by the punctures of sucking in- 

 sects — the beet leafhoppers. In certain varieties of beets and in docks 

 punctured by the beet leafhopper, discoloration and spotting of the 

 leaves follow the punctures of these insects, and in young sugar beets 

 a faint mottling of the attacked leaves may be noted by transmitted 

 light (see p. 10) ; leaf distortions, though differing in character and 

 extent, are common to both. In the case of stigmonose, Woods has 

 shown that when the abnormal changes due to aphis punctures " have 



