understood, however, that the mycehura from a single point of infection grows 

 throughout the whole length of the cotyledon. On the contrary, a single mycelium 

 probably never extends more than a quarter of an inch from the point of infection 

 and the ripe sorus is developed at that point. If there are a number of sori in the 

 cotyledon — which is usually the case — they are the result of the same number of 

 separate infections. Very long sori in the cotyledon are the result of anastomosing 

 shorter sori. 



Just at the junction of the young cotyledon and the first root there is a slight 

 swelling known as the root joint. Immediately^ above this is the growing point, — a 

 region of less than a half inch, — from which place all new leaves start. This point 

 remains stationary throughout the further development of the onion bulb. If an 

 infection occurs near the growing point the mycelium reaches tliis rapidly dividing 

 tissue and as each leaf grows up it carries with it some of the infecting myceUum 

 which spreads and sporulates in the leaves. When an onion once becomes infected 

 in the growing point, it never recovers. If, however, the cotyledon is infected only 

 in the upper part the mycelium does not reach the growing tissues and the disease 

 is sloughed off with the cotyledon and the plant develops normally. 



RELATION TO ENVIRONMENTAL FACTORS 



Most plant diseases vary greatly in prevalence and severity according to the 

 season. Certain kinds of weather favor them, others retard them. They ruin the 

 crop one year and are entirely absent during other years. Onion smut, however, 

 differs from most common diseases in this respect: It is about the same every year. 

 Some growers thinl<; they find it more severe during a dry year; just as many others 

 think a wet year favors it. The writers ha^■e had occasion to watch the course of the 

 disease for many years but have never noticed any marked difference in its severity 

 from year to year on the same field. Walker and Jones (50:240) conducted con- 

 trolled experiments in the greenhouse to determine the effect of variation in the 

 moisture of the soil on the percentage of smut infection and concluded "that soil 

 moisture does not function as a factor limiting infection with onion smut witliin the 

 limits at either extreme where good germination and growth of the host occur." 



A similar experiment was conducted by the writers at the Massachusetts Experi- 

 ment Station in which the seedlings were grown in pots of naturally infested soil 

 kept at moisture contents at intervals of 2 per cent, from 12 to 40 per cent, of the 

 dry weight of the soil. Below 12 per cent, this soil was too dry for good germina- 

 tion, while at the upper limit it began to dechne rapidly from 34 per cent. Within 

 this range the smut loss was almost total. Above 34 per cent, there was a decline 

 in the percentage of infection, but the soil was very muddy and it is not likely that 

 any grower would plant onion seed in soil so wet as this. It is conceivable, however, 

 that rains immediately following the planting might keep the ground so wet that 

 infection would be reduced, but probably the loss of germination on account of wet 

 soil would more than counterbalance the gain from decreased infection. The prac- 

 tically identical results of these two experiments conducted independently in Wis- 

 consin and Massachusetts indicate that the moisture condition of the soil in the 

 spring is without influence on prevalence of the disease during that year. 



Neither is there any indication that a warm spring in the Connecticut Valley is 

 any more or less favorable than a cold one. All the onions here are planted in the 

 early spring before the soil becomes very warm. Wallcer and Jones (50:247) con- 

 ducted experiments on the effect of soil temperature on infection and concluded 

 "that a high percentage of infection maybe expected up to 25° C. ( = 77°F.) above 

 which there is a rather abrupt reduction, leading to complete inhibition at 29° C. 

 ( = 84° F.). There appears to be no lower limit of temperature for infection within 

 the range where onion seeds will germinate and normal growth occur." They ex- 

 plain the absence of smut from the southern onion regions, e.g., Texas and Louisiana, 

 as due to the fact that in those regions the seed is planted in late summer when the 

 soil temperatiu-e is too high for infection to occur. 



The temperature experiments of Walker and Jones were also duplicated at the 

 Massachusetts Experiment Station and the results were practically the same. 

 Onion seed germinated at all temperatures between 8° and 35°C., but the percentage 



10 



