MARKET DISEASES OF VEGETABLES. 47 



POTATO: FUSARIUM TUBER ROT. 



Cause: Fungi (Fusarium species) . 



Fusarium tuber rot is generally marked by sunken, shriv- 

 eled, wrinkled, or broken areas* on the tuber surface. These 

 areas may be brown to black in color and on them may 

 appear masses of whitish or brightly colored mold. 



The diseased tissue underlying such discolored and 

 shrunken areas may be dry and brittle, and may contain 

 cavities lined with a white or bright colored growth of the 

 fungus responsible for the rot; or this tissue may be watery 

 but intact, depending upon the species of fungus responsible 

 for the rot and the conditions under which the affected tuber 

 was kept. Usually, if affected tubers are kept in a dry cool 

 place, the dry, brittle, and hollow type of powdery dry rot 

 results. If they are kept in a warm moist place the soft 

 intact type of rot is usually the result. Affected tissue may 

 be gray or brown to black in color. 



The rot may proceed from the stem or seed end, from the 

 eyes or from broken places in the skin, such as cuts and 

 bruises. At times the infection is confined to a mere dis- 

 coloration of the vascular ring proceeding from the stem 

 end of the tuber. This is not visible until the tuber is cut. 

 This discoloration cannot always be positively differenti- 

 ated, without miscroscopic examination and cultural test, 

 from discolprations due to freezing injury or to varietal 

 characteristics. Fusarium discoloration penetrates much 

 farther into the tuber, however, than other vascular dis- 

 coloration. In southern potatoes, this discoloration in the 

 ring can be told from a similar discoloration due to brown 

 rot by the absence of the slimy bacterial masses or droplets 

 which ooze out when tubers affected with brown rot are cut. 



This stem-end infection and some lenticel and eye infec- 

 tions occur in the field. Stem-end invasion is very common 

 in potatoes produced by plants affected with Fusarium wilt. 

 Jelly-end is another type of stem-end infection which occurs 

 in the field. However, most of the tubers rotted by Fusa- 

 rium species are infected through breaks in the skin caused 

 during and after the harvest. 



Tubers with areas injured or killed by freezing are very 

 subject to Fusarium rots. The rot usually starts in at some 

 point which did not heal over completely with cork or was 

 not entirely sealed with starch following the drying out of 

 the frozen tissue. At times it is very difficult to distinguish 

 between potatoes with frozen areas which were subse- 

 quently infected with Fusarium tuber rot, and potatoes 

 which were originally affected with the rot and then kept 

 under moist conditions. The end result in both cases is 

 usually a wet brown rot. This wet type of rot is especially 

 marked in the Burbank and Netted Gem potatoes from 

 Idaho. It may be practically impossible at times to deter- 

 mine the cause of the decay by examination of a single tuber 



