:Marth. 1922. 



scir-:xTiFic agriculture 



235 



Life History of the Organism. 



Selerotia remain donnant in the soil 

 until early summer when they may germ- 

 inate giving rise to apotheeia bearing as- 

 cospores. The aseopores are discharged 

 and infect the older leaves of the stem in 

 the lowest axils. 



The selerotia may also live over winter 

 in the soil and germinate by mycelium 

 which can infect the plants. 



In England and Ireland the organism is 

 tnown as Selerotinia sclerotiorum but for 

 the selerotinia found by Lachaine in New 

 Brunswick no definite name can yet be 

 given although it appears to be 8. liber - 

 tiana. 



Control. 

 The only satisfactory means of control 

 ^re crop rotation so that susceptible plants 

 are not available to the fungus and careful 

 removal and destruction of diseased plant 

 parts. 



In England, Cotton finds that late plant- 

 ing has proved successful since fewer old 

 leaves are available at the time of spore 

 discharge. 



Group 6. 

 Disease Caused by a Basidiomycete. 



Dry Stem-Rot and Black Scurf 

 This disease is known under a variety of 

 common names of which the chief are : — 

 dry stem-rot, black scurf, black scab, rus- 

 set scab. Rhizoctonia disease, little potato, 

 aerial potato, rosette, black speck sdab 

 and collar fungus. 



The first description of Rhizoctonia was 

 given by Duhamel in 1728 as causing a 

 disease of saffron in France. De Candolle 

 in 1815 gave the fungus the name Rhizoc- 

 tonia when he discovered a similar disease 

 on lucerne. In 18.51 the Tulasne brothers 

 classified all the then known rhizoctonias 

 as R. violacea but Kuhn in 1858 described 

 a species on potato which he named R. so- 

 lani. Webber (1890) first reported the 

 fungus in America and in 1901 Duggar 

 and Stewart (Bull. 186. N.Y.) gave a list 

 of hos'^s attacked by Rhizoctonia. It is 

 now known to occur generally in the 

 Ignited States and Canada and is reported 

 from the West Indies, India. Australia and 

 S"outh America. The perfect stage Avas 

 found by Rolfs in 1903 on potato stems and 

 described as Corticium vagum B. & C. i-ar 

 solani by Burt. 



Other Hosts. 

 The complete list of other hosts would be 



too long to include here but mention may 

 be made of tomato, bean, lettuce, carrot, 

 cabbage, pea, pumpkin, beet, carnation and 

 pansy. 



Symptoms. 

 Tliese vary considerably with climatic 

 conditions, age of the plant at infection, 

 and the soil type. The sclerotial stage is 

 \ev\ common on tubers where many black 

 selerotia of varying shape and size (but 

 usually small) are found on the surface. 

 They "do not cause any apparent injury 

 since they are superficial. Frequently, 

 jiowever, the skin is more or less cracked 

 and russetted and a still more advanced 

 stage may give rise to scabbing. In this 

 condition drying out in storage or the en- 

 <-rv of secondarv organisms is facilitated. 



Fig. 12. — Dry stem-rot of potato. Note that 

 lesions occur about at ground level and that 

 the plant may branch out again from below. 

 (After Bull. 85. Mich.) 





