EXPERIMENT STATION BULLETINS. 661 



weeks before the 1919 season and almost a month before the 1920 sea- 

 son, June temperature and sunshine, as in 1919, were much above nor- 

 mal while the precipitation was below normal. Similar conditions pre- 

 vailed in July, whereas in August the temperature and precipitation 

 were above normal while the sunshine was slightly below normal. Con- 

 ditions during this season again were favorable to leafhoppers. The 

 insects emerged early and under the hot, dry weather conditions the 

 life cycles were shortened. The first nymphs to hatch out on potatoes at 

 the Upper Peninsula Experiment Station were observed on July 5 as 

 compared to July 22 when the first nymphs were observed hatching iii 

 1920. Leafhoppers were abundant in 1921 and hopperburn became se- 

 vere during the season. The timely rains which came during August 

 saved the potato crop from greater injury. 



Although the above observations cover only a period of three sea- 

 sous, they seem to indicate that whether or not hopperburn Avill be 

 serious during a given season can be predicted. Hopperburn can be ex- 

 pected to be serious cUirmg seasons tohich folloio an early or normal 

 spring during tohich the month of June is characterized by temperature 

 and sunshine "above normal" and "beloio normal" rainfall. 



RESISTANCE OF VARIETIES. 



During a survey made of potato fields during the latter part of July, 

 1919, obseiTations were made as to the resistance of varieties to hopper- 

 burn. It was repeatedly noted during the survey that early varieties 

 were more susceptible than late varieties. Observations made on the 

 variety plots at the Upper Peninsula Experiment Station, all of which 

 were planted on the same day, confirmed the findings of the survey. Of 

 the early varieties, Bliss Triumph was found to be the most susceptible 

 and was followed in order by Early Ohio and Irish Cobbler. The Idaho 

 Rural was found to be the most susceptible of the late varieties and was 

 followed in order by Green Mountain, Eusset Burbank and Rural New 

 Yorker. Although some burning has been observed on the Rural New 

 Yorker variety, it has some inherent quality whereby it is able to with- 

 stand insect and disease attacks better than other varieties. This re- 

 sistant quality of potatoes of the Rural type makes them very popular 

 with the potato growers. 



SELECTIVE TREPERENCE OP THE LEAFHOPrER 



The 1920 season which w\ts unfavorable to leafhoppers presented an 

 opportunity to make observations on the type of vines selected by the 

 over-wintering adults for egg laying. Owing to the small number of 

 adults seen in the spring a large number of hills were carefully examined 

 for nymphs during the summer months. As a result of these examina- 

 tions it was found that vines which were healthy were usually free from 

 leafhopper infestation, while nymphs were always found on vines which 

 were affected with the mosaic disease. The brighter green color of the 

 diseased hills perhaps attracts the over-wintering adults and result in 

 a greater number of eggs being laid on those vines. 



