June, 1922 



SCIENTIFIC AGRICULTURE 



Diseases of the Potato 



By B. T. DICKSON, 

 Professor of Botany, Macdonald College 



(Group 7 Continued.) 



(f ) Dry rot of tubers. 



Heavy lo.s.st^.s in stored potatoes sometime.s 

 occur in those years when Late Blight does 

 not appear to be an important factor. Such 

 losses are mainly due to tuber rots caused 

 by species of Fusariurn. It is difficult accur- 

 ately to ascertain the extent of such losses 

 but they take a heavy annual toll from 

 growers and dealers. 



Fig. 13. — Original photograph from material 



collected at Macdonald College, September 21st. 



1921. The minute sclerotia on the inside and 



outside of the affected stems are shown. 



Symptoms 



The rot usually starts from a wound which 

 penetrates the tissues or which may be mere- 

 ly an abrasion of the skin. At first the af- 

 fected tissues are firm and cheesy and the 

 fungus produces white or pink tufts of my- 

 celium, conidiophores and conidia on the sur- 

 face. As the diseased part increases in area 



and depth the tissues dry out, shrivel and 

 eventually become quite hard. Under warm, 

 Immid conditions the rot develops more rapid- 

 ly and is softer at first but finally the tuber 

 becomes shrunken and hard. If Fusarium 

 species only are present there is no foul 

 odour but usually bacteria are to be found 

 associated and hence the rot may be softer 

 and malodorous. 



Causal organisms 



Fusarium discolor sulphureum is probably 

 the chief cause of dry rot but F. oxysporum 

 may penetrate so far into the tuber tissues 

 that it also gives rise to storage rot. Other 

 species of Fusarium occasionally cause sim- 

 ilar rots but they are not usually so serious. 



Relation to Late Blight Rot 



The rot caused by Phiftophthora infestans 

 when severe is seen at digging time and the 

 heaviest losses begin to show soon thereafter 

 in storage but Fusarium rots inflict heaviest 

 losses in the later storage period. It is ob- 

 vious, however, that small lesions caused by 

 P. infestans are eminently suitable as points 

 of entrance for Fusaria and infection by 

 Fusaria commonly occurs. The rot is then 

 not "dry" as in the case of infection by 

 Fusarium alone. 



A feature of importance is the amount of 

 rot in storage which occurs in certain years 

 even after consistent spraying with Bor- 

 deaux is practised. Murphy (Ottawa Bull. 

 44) has mentioned this and 1 have had many 

 requests for information from Quebec farm- 

 ers. During a warm wet autumn the sprayed 

 plants remain green and continue to grow 

 well on to harvest. At digging time the 

 tops are still green and under such condi- 

 tions some Late Blight is almost sure to be 

 present. Spores fall on moist tubers and 

 infection may occur which does not manifest 

 itself until later in storage. Even if no 

 Late Blight be present the tubers are im- 

 mature so far as their skins are concerned 

 and therefore are easily injured. Through 

 these injuries Fusaria enter and in the spring 

 there is the likelihood of heavy loss from dry 

 rot. It would probably be better to cut the 

 tops some seven to ten days before digging 

 to remedy this, and experiments are under 



