drops off rapidly above 27° and is much retarded below 13°. It required 33 days for 

 the plants at 8° to show above the surface of the soil. The optimum is between 15° 

 and 27°. Smut was not entirely absent at any temperature tried, but the percent- 

 age dropped off suddenly and rapidly above 27°. Below 27° there were no signifi- 

 cant differences at any temperature. There seems to be no lower Umit of infection 

 within the range of germination of the onion seeds. This suggests the possibihty 

 that smut might be avoided or diminished by delaying the planting here until the 

 soil became warm. Late planting, however, is not conducive to good crops in the 

 Connecticut Valley and, in addition, this method has been tried (50:237) here with- 

 out any beneficial effect. 



It is apparent from the results of the experiments just mentioned that the only 

 soil temperatures which could influence prevalence of smut would be from 27°C. 

 (81°F.) upward. Does the soil of the Coim'ccticut Valley onion region ever ap- 

 proach such high temperaitures during the infection period, i.e., approximately dur- 

 ing April and May? We have no local continuous seasonal records for the soil, but 

 since all infection occurs within the upper inch of soil we may approximate it from 

 the air temperature records during those months. The soil temperature during the 

 heat of the day, to be sure, may rise several degrees above that of the air, but we 

 may allow a latitude of ten degrees or more and stUl reach some conclusion. 

 Weather records at Amherst for the ten-year period 1915-24 show the mean hourly 

 temperature for the month of April to be 45.8°F. and for May, 56.5°F. Even 

 though the soil temperatiu-e in parts of the field were 20° liigher than that of the air 

 — wliich is beyond reasonable limits for any length of time — it would still not be 

 high enough to limit infection. As far as Massachusetts is concerned then, soil 

 temperature seems to be as unimportant as soil moisture. 



We may conclude, therefore, that variations in weather conditions are without 

 effect on the severity of smut infection. There is, on the contrarj^, very good evi- 

 dence that certain soil factors may have a deciding influence. A survey of the 

 Connecticut VaUey region shows that fields are very unevenly infested. Onions 

 alw^ays suffer most in, certain fields. In the same field, certain spots are very 

 heavily infested wliile other parts of the field suffer little. The location of these 

 spots remains approximately the same j'ear after j'ear although the whole field is 

 planted to onions each j^ear and the treatment of all is the same. Such a condition 

 cannot result from any lack of opportunity for the fungus to spread. It can appar- 

 ently mean only that there is some soil factor which is right for the growth of or 

 infection by the fungus in that part and not in the other parts of the field. Investi- 

 gation with the object of determining tliis factor (or factors) is now in progress at 

 this station, but it has not progressed to the point where any conclusions can be 

 drawn. 



PREVENTION OF SMUT 



When the ravages of smut first began to attract attention of New England onion 

 growers (1850-75), it was thought to be connected in some way with the character 

 of manures apphed to the soil, guano being held especially responsible. Remedies 

 were therefore sought through change of fertihzers. Thus Horace Capron (7 :224) 

 suggests: "A remedy for this disease must be sought by using less manure, or 

 manures which are less stimulating and afford less nitrogen for the fungus to feed 

 upon. AlkaUne manures are very destructive to the fungus. Wood ashes, lime, 

 gypsum and seaweed are very efficacious." 



Taylor (43:195), who first made a study of the spores and laid stress on their 

 presence in the soil as a means of carrying the disease over winter and spreading it 

 about, recommended that they be destroyed and the disease thus controlled, by 

 burning heaps of dry weeds and rubbish on the infested fields. This recommenda- 

 tion was repeated by various writers during the next fifteen years, but it does not 

 appear from the hterature that anybody tried it. Walker (47:6) has recently 

 recommended burning the onion tops after harvest and the avoidance of returning 

 onion refuse to the soil. 



Farlow (14:174) was the first to suggest rotation of crops as a means of control. 

 He was led to believe from the statements of growers that the spores do not live in 

 the soil more than four years and thought that if a smut-infested field were planted 



11 



