96 



PROTECTION OF PLANTS, 1921-22 



brown, five to seven celled and apparently quite resistant to adverse conditions. 

 It is thus possible that spores developed later in the season may over winter. 



Control 



The ordinary methods of seed-tuber disinfection do not control this disease 

 and the only suggestion at present efficacious is that of tuber selection. 



(f) Dry Rot of Tubers 



Heavy losses in stored potatoes sometimes occar in those years when Late 

 Blight does not appear to be an important factor. Such losses are mainly due to 

 tuber rots caused bj" species of Fusarium. It is difficult accuratelj' to ascertain 

 the extent of such losses but they take a heavy annual toll from growers and 



dealers. 



Symptoms 



The rot usually starts from a 

 wound which penetrates the tis- 

 sues or which may be merely an 

 abrasion of the skin. At first the 

 affected tissues are firm and cheesy 

 and the fungus produces w^hite or 

 pink tufts of mycelium, conidio- 

 phores and conidia on the surface. 

 As the diseased part increases in 

 area and depth the tissues dry 

 out, shrive] and eventually be- 

 come quite hard. Under warm, 

 humid conditions the rot develops 

 more rapidly and is softer, at first, 

 but finally the tuber becomes 

 shrunken and hard. It Fusarium 

 species only are present there is no 

 foul odour but usualh^ bacteria 

 are to be found associated and 

 hence the rot may be softer and 

 malodorous. 



Causal organisms 



Fig. 11 — Original photograph from material col- 

 lected at Macdonald College, September 21st, 

 1921. The minute sc'erotia on the inside and 

 outside of the affected stems are shown. 



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 similar rots but they are not usually so serious^ 



