MARKET DISEASES OF FRUITS AND VEGETABLES 11 



FUSARIUM BULB ROT OF ONIONS 



{Fusarium oxysporum Schl. f. 7 Wr., F. vasinfectum Atk. var. zonatum (Sherb.) 

 f. 1 and f. 2 (Lk. et Bail.) Wr.) 



CKXTURRENCE, SYMPTOMS, AND EFFECTS 



Fusarium bulb rot of onion is of great importance from both the 

 production and the marketing point of view. The rot causes reduced 

 yields in the field, and in transit and storage it is responsible for 

 heavy losses. It has been reported from practically all onion-growing 

 sections of the United States and is especially severe in the warmer 

 sections. On the market it has been found serious in shipments from 

 Ohio, Illinois, Wisconsin, Colorado, California, and Washington. 



The first sjmrptoms of bulb rot visible in the field are the yellowing 

 and rapid dying of the leaves from the tips down, at about the time 

 of maturity. The advancing edge of the lesion may be somewhat 

 yellow, but the dead leaves have a silvery-gray appearance. How- 

 ever, these symptoms do not invariably indicate bulb rot ; in fact, the 

 rot cannot be identified positively until the onion is pulled. Affected 

 onions pull easily because many of the roots are rotted off. The base 

 of the onion may be covered with a dense growth of white to pinkish 

 mold, and in severe cases the bulb may show a semiwatery, a mealy, or 

 a dry decay which progresses along the scales from the base upward 

 (pi. 3, D, E). Whether the rot is dry or watery usually is deter- 

 mined by the species of Fusarium causing the rot, and also by 

 moisture and temperature conditions. High moisture and high tem- 

 peratures tend to produce a dry rot. Storage onions in advanced 

 stages of decay may be converted into shriveled mummies. The 

 diseased bulbs sprout much more freely than healthy ones. 



CAUSAL FACTORS 



Although various species of Fusarium alone or in association with 

 bacteria are able to induce bulb rot, detailed study indicates that 

 Fusarium oxysporum f. 7 and F. vasinfectum var. zonatum, f. 1 and f. 

 2 are the most important. These fungi live in the soil and from this 

 they attack the growing bulb. They also overwinter in infected bulbs 

 and probably can live over in debris and containers in storage houses. 

 Continuous cropping with onions increases the amount of inoculum 

 in the soil. 



Field observations and experimental work with these species indi- 

 cate that high temperatures favor the growth of the pathogens, and 

 the inception, spread, and development of the disease. In the field 

 these organisms cause rot at temperatures between 59° and 89° F., 

 developing most rapidly between 82° and 89°. In storage there is 

 little decay at temperatures between 46° and 59°, but there is a pro- 

 nounced tendency toward premature sprouting in affected bulbs at 

 these temperatures. At 68° the rot progresses rapidly but the tissues 

 remain watery for a long time; at 86° the tissues decay and dry out 

 rapidly. The rot develops and spreads in transit and storage. Once 

 it has become established, there is a possibility of its spreading 

 through the whole lot, provided temperature and moisture conditions 

 are favorable. Wounds and insect injuries favor the development of 

 rot because they make entrance easier for the organisms. 



