CONTROI, 01^ BLACKLEG DISEASE OF THE POTATO. 203 



of dry, sound potato tubers. Under ordinary conditions black- 

 leg has not been observed in Maine to spread from hill to hill 

 in the field and, as will be shown later, there is quite conclusive 

 evidence that the germs do not live over winter in the soil under 

 the climatic conditions which exist in this State. 



Observations extending over 5 years indicate that the disease 

 is much less prevalent in those seasons which have relatively 

 little rainfall between planting and blossoming time. While it 

 is quite generally distributed over the potato growing sections 

 of the State and sometimes, in very wet seasons, fields may be 

 found which show 10, 15 or even 20 per cent, of diseased plants, 

 as a general rule it occurs only as occasional, affected stalks 

 scattered over the fields. Only in exceptional cases is the crop 

 materially reduced from this cause. 



The writer has had opportunity to study numerous outbreaks 

 of rot in potato fields and in storage in Maine and other parts 

 of New England during the last 15 years and has yet to see the 

 first case of a severe epidemic of this kind that could be traced- 

 to the blackleg or stem rot disease. These epidemics are in- 

 variably preceded by outbreaks of the late blight fungus Phy- 

 tophthora infestans DeBary on the foliage, and occur with 

 equal frequency on fields which are free from blackleg as upon 

 those which are attacked with this disease. Moreover such epi- 

 demics were as common long before the blackleg disease made 

 its appearance, and now occur in those parts of New England 

 where blackleg-has never been seen, whenever the weather con- 

 ditions are favorable for outbreaks of late blight on the foliage, 

 and spraying with bordeaux mixture is not thoroughly done. 

 The nature of the decay in such cases may be soft and foul- 

 smelling and large numbers of bacteria may be found in the 

 decayed tissues, but the writer has failed in repeated trials to 

 isolate bacteria from such tubers which, upon inoculation to 

 sound tubers, would produce alone and unaided any disease 

 whatever. In all probability the soft rots observed in epidemics 

 of this kind are caused by saprophytic bacteria of the soil which 

 follow after and farther break down the tissues of the tuber 

 after they have been" killed by the late blight fungus. 



It is perfectly possible however, as has already been stated, 

 for the bacteria associated with the blackleg disease of the stem 

 to produce a rapid and complete soft rot of the tuber under 



