14 MISC. PUBLICATION 4 4 0, U. S. DEPARTMENT OF AGRICULTURE" 



does not cause death or softening of the affected area. Green bulb 

 tissues are bitter and unpalatable, but as there is little danger of 

 decay following, the marketability of onions so affected is not greatly 

 influenced unless the lot as a whole shows this blemish quite 

 prominently. 



PINK ROOT OF ONIONS 



(Plioma terrestris Hansen) 



Pink root has been found affecting onions, shallots, garlic, leeks, 

 and chives. The disease is common to many onion-growing sections, 

 but the greatest damage occurs in the Texas, Louisiana, Iowa, and 

 California crops. Onions and other plants belonging to this group 

 are subject to attacks by the causal organism in all stages of de- 

 velopment from the seedling stage to maturity. Although primarily 

 a field trouble, this disease is also of market importance because it 

 causes a reduction in the size of the bulbs and the lesions produced 

 open the way for Fusarium species that often cause serious decay of 

 the bulbs in the field and during the marketing process. (See Fusa- 

 rium Bulb Kot, p. 11.) 



Plants affected by pink root usually show symptoms characteristic 

 of starvation or drought injury. The early symptoms noted on the 

 roots are a collapsing of the tissues, shriveling, and discoloration 

 varying from pink to dark-red and purplish hues. Many plants 

 affected with pink root show no symptoms other than the pink dis- 

 coloration of the roots. Such plants have developed new roots fast 

 enough to permit continued growth and bulb formation. Xo decay 

 of the bulb occurs when only the pink root fungus is present. 



The causal organism persists in the soil from season to season. 

 The minimum temperature for its development is 55° F. ; the opti- 

 mum is about 77° to 80° ; and the maximum is above 90°. 



No satisfactory control measures have been developed for pink 

 root. In tests for resistance to the disease it has been found that 

 most varieties of the common onion, garlic, and shallot are extremely 

 susceptible, Chives, leeks, and Welsh onions (Nebuka type) are very 

 resistant. 



(See IS, ty, 16,26, SO, 87.) 



PURPLE BLOTCH OF ONIONS 



(Hacrosporium porri Ell.) 



OCCURRENCE, SYMPTOMS, AND EFFECTS 



Purple blotch disease has been reported from time to time on onions 

 from various regions, but usually it is not of great economic importance 

 except under very favorable weather conditions. Leaf mold (Thyro- 

 spora parasitica (Thiim.) Angell) and downy mildew (Pertmospora 

 destructor (Berk.) Caspary) of onions are so commonly associated 

 with this disease that frequently the activities of the blotch organism 

 are obscured. On the stems and leaves small grayish sunken lesions 

 become visible within 4 days after inoculation ; shortly thereafter small 

 dark spots appear in the centers, later enlarging into purplish areas. 

 Infection of the bulbs occurs at harvesting time, usually al the necks 



