)46 DISEASES OF VARIOUS CROPS 



form a diastase which readily hydrohzes starch when it occurs in a gelati- 

 nous condition. 



1130- Fusar/um rad/c/cola, the Cause of Rot in Potato Tubers in the United 



States. — I'RATT, O. A., in The Journal of Agricultural Research, Vol. VI, No. 9, 

 pp. 297-309, ri. XXXIV- XXXVII, Washington, D. C, 1916. 



Fnsarium radicicola Wollenw. gives rise to two distinct types of rot in 

 potatoes : a dry rot and a soft or gelatinous rot. 



Dry rot (" black rot ") is characterised by blackening of the attacked 

 tissues, which in time always acquire a dark sepia-brown colour. The fun- 

 gus invades the host by three rhaimels : i) the point of the branch where the 

 swelling of the tuber begins (" stem-end ") ; 2) lenticels ; 3) ej^es. In the 

 first case the parasite develops' and extends throught the vascular system 

 which turns black and dies, in the second case it spreads more or less com- 

 pletely round the tissues ; finally, in the third case, it passes uj) the secondary 

 vascular branches but scarcely ever reaches the central axis. 



Externally, infected tubers have a sunken brownish-black region. This 

 type of rot is, especially noticeable in potatoes with round tubers such as 

 " Idaho Rural " and " Pearl ". In every case where infected material 

 was isolated, F. radicicola and sometimes F. oxysporum as well were found. 

 The latter should be considered as an occasional parasite, which gets into 

 the tubers by the necrosed vascular bundles. When the spores of F. ra- 

 dicicola were injected into the tubers and stolons of the potato the results 

 obtained were definitely positive ; after longer or shorter periods of time, 

 which vary according to thermal conditions, the characteristic symptoms 

 of " black rot " appeared. 



In cases of soft rot (" jelly-end "), the fungus enters the tuber at the 

 point of its formation and the infection spreads inwards ; although it spreads 

 more quickly along the vascular bundles, it effects all the tissues to some 

 extent. It is not long before the diseased parts become brown. This type 

 of rot is found principally in potatoes with oval tubers, the Bnrbank group, 

 for example " Netted Gem ". There is no doubt, judging from the positive 

 results of inoculations, that F. radicicola is able to produce jelly-end fot. 

 But in isolating the pathogenic germs from naturally infected material it 

 could be proved in almost all cases that, in addition to F. radicicola, there 

 were also present other species of Fusarium such as F. trichoihecioides and 

 F. oxysporum. The last, as shown by Carpenter's researchs, can give rise 

 to a soft form of rot in tubers, and considering its almost constant asso- 

 ciation with F. radiciola in jelly-end rot, the author is inclimed to believe 

 that it is one of the factors in causing this disease. When tul.iers infected 

 with " black rot " are sown the crop obtained is to a large extent contami- 

 nated, while sowing tubers infected with " jelly-end " rot does not seem to 

 entail harmful results. 



F. radicicola is widely distributed in Europe and in America. In the 

 United States it has ben reported from the following states : Idaho, Oregon, 

 California, Nevada, Mississippi, New York, Virginia, and the district of 

 Columbia. It is probably well distributed throughout the desert soils, 

 where the damage caused to the potato crop by this parasite can attain to 



