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Onion Pink Root - Leaves die back from tips, roots turn a pale pink , sometimes light red and purple. Stunted Growth.
Pink root is a fungal disease [Phoma terrestris] that attacks alliums such as onions, garlic and leeks. Grass and grain plants are at times carriers, but are not as severely effected. The fungus can lay dormant in the soil for years with no host crop, but rapidly emerges from its dormancy once it finds a suitable host. Weakened and stressed plants are naturally more susceptible.
Pink root symptoms are frequently misdiagnosed as drought stress or a nutrient deficiency. Pink Root rarely results in the death of the plant and the disease is limited to roots and bulb, which basically are the Onion crop. Severe cases will reduce the bulb size, all cases will result in diminished quality.
Crop rotation with unrealted non host crops is an important step in curtailing its spread. Grasses, grains, corn are host crops - avoid them in the vicinity of where you plan to grow , or are growing Onions or other alliums.
Fungicides are somewhat effective, and not always worth the expense for smaller plantings given the success rate they have displayed. Fungicides that contain chloropicrin are effective against the widest range of pink Root strains. Unfortunately it is a restricted use chemical, unavailable to Home Gardeners. Products containing boscalid are less effective, but readily available.
Proper cultural practices are a defense against onion pink root.Remove and destroy all plant refuse in the fall and use deep cultivation to bury any remaining refuse.
Do not place diseased plants in the compost heap, as this will only serve to carry bacterial or fungal infestation into the next growing season.
Being that that above ground symptoms mimic those of drought stress and nutrient deficiencies the only way to confirm pink root onion disease with absolute certainty is to uproot suspected onion plants and inspect the roots.
You can slow the advance of pink root but creating a habitat that is not conducive to the diseases spread. Cut back on watering, pink root, like all fungus thrives in moist wet conditions. Allow your onions to go dry around the base before watering.
Keep the plants as healthy as possible, weaker plants are more susceptible. Increase fertilizer applications - do not over do it, just bump it up a tad so as to avoid fertilizer burn.