558 INSECTS AFFECTING VEGETATION 



the writer, of little consequence. Preventive measures which are 

 effective with one under Maine climatic conditions would be, in all 

 probability, equally applicable to all. Blackleg is not the same as 

 the Southern bacterial disease of the potato stem and tuber caused by 

 B. solanacearum Smith. 



Both stems and tubers are attacked. The diseased plants first 

 appear more or less unthrifty and usually undersized. The branches 

 and leaves, instead of spreading out normally, tend to grow upward, 

 forming a more or less compact top, frequently with the young leaves 

 curled and folded up along the mid-rib. Later they become lighter 

 green or even yellow and the whole plant gradually dies. If the dis- 

 ease progresses rapidly the plants may fall over suddenly and wilt 

 with very little previous signs of disease. The same general symp- 

 toms may be produced by certain other stem diseases, or even me- 

 chanical or insect injuries of the stem at or below the surface of the 

 ground. 



The appearance of the diseased stems at once differentiates 

 blackleg from other described potato diseases. Stems so attacked are 

 characterized by an inky-black discoloration extending from the base 

 of the stem, where it attaches to the seed piece, up sometimes one, 

 two or even three inches above the surface of the ground. Under 

 favorable weather conditions the disease may, in exceptional cases, 

 follow up the stem for several inches, or even out on the larger 

 branches. The seed pieces from which the diseased plants spring 

 are invariably decayed, and young tubers which have been formed be- 

 fore the destruction of the stem may sometimes be attacked by a soft 

 rot caused by the bacteria being conveyed to them along the under- 

 ground branches of the stem upon which they are produced. 



The disease is carried over from year to year by the organisms 

 living in decaying, bruised, cracked or otherwise imperfect seed 

 potatoes. They are readily killed by drying and are probably inca- 

 pable of existing in a living state on the surfaces of dry, sound potato 

 tubers. Under ordinary conditions blackleg 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. (Me. E. S. B. 194.) 



Eliminating Blackleg from Seed Potatoes. From the foregoing 

 discussion it is evident that if seed potatoes are carefully selected so 

 that only those which are absolutely sound and perfect are used for 

 seed purposes and these treated with formaldehyde that the disease 

 can be eliminated in from one to two years' time. 



For home use it is strongly recommended that only formalde- 

 hyde solution be employed. This consists of one pint of 40 per cent 

 formaldehyde in 30 gallons of water in which the potatoes should 

 be soaked 2 hours and then spread out on a clean place to dry, pref- 

 erably in the sun. Exposure to sunlight will also assist in destroying 

 the bacteria causing the disease and tend to hasten germination. 



It is only advised that the formaldehyde gas method be used 

 by the large seed dealers who must pick up a considerable amount 



