Diseases of onion and garlic (
Pink root
Pink root is caused by the fungus Phoma terrestris. It is an important pathogen of onions, but some isolates are pathogenic on other plants also. Infected onion roots are initially light pink in color, gradually turning deeper pink and finally dark purple as the disease progresses. As new roots are produced, they are infected, turn pink, and eventually die. If infection continues, plants become stunted and may appear to be suffering from drought or a nutrient deficiency, but usually do not die.
Growth of the pathogen and disease development are optimal at temperatures of 75° to 85° F, whereas little disease occurs below 60° F. The fungal pathogen can persist in soil for some time and is spread in water and by movement of infested soil. Inoculum and disease severity increase the longer onions are grown in a field.
Long rotations of 3 to 6 years with crops not susceptible to the pathogen
will reduce but not eliminate the occurrence of disease. Some
onion cultivars possess resistance to the pathogen and should be planted
in fields with a history of pink root. Fumigation with metam sodium
or chloropicrin is effective but costly. Soil solarization can
be effective in areas such as the desert production regions of Arizona.
February 10, 2003