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Fusarium or Bulb & Stem Nematode?

This season a number of garlic growers have complained about yellowing and dieback of leaves. Tip yellowing and dieback in alliums, including onions, garlic, chives and shallots can be caused by abiotic factors such as plant overcrowding, drought, salt stress, wind desiccation and occasionally ozone damage. As well, these symptoms can be caused by biotic factors like Fusarium basal plate rot (Fusarium oxysporum cepae) and the notorious bulb & stem nematode (Ditylenchus dipsaci).

Both Fusarium and bulb & stem nematode are soil organisms capable of surviving for long periods of time. Both can enter healthy bulbs directly, or through new or old wounds such as those caused by insect injury, pink root infection, or cultivation injury. Spread occurs season to season through infected soil or via contaminated sets or cloves. Within infected fields, affected plants can be either localized or scattered throughout the field. Although it is not believed that bulb-to-bulb movement in storage is significant, this may be a concern for garlic growers who store bulbs for garlic seed stock.

Roots and basal plates can be infected at any age. In the case of Fusarium, symptoms include a gradual yellowing and dieback of leaves. When infected plants are pulled the roots are pinkish-brown and if cut vertically, a discolouration of the plate is evident. Optimum disease development occurs in wet soils when soil temperatures reach 25 to 28°C. Bulb & stem nematode on the other hand usually result in a sudden yellowing and dieback of leaves. When pulled, portions of the roots are completely missing and there is often a separation of the basal plate from the cloves.

If you suspect you have either of these organisms, have some samples tested. It's the only way to be sure and to implement mitigation measures for next year's crops.

For full details and pictures see OMAFRA Vegetable Update at

http://apps.omafra.gov.on.ca/scripts/english/crops/agriphone/article.asp?ID=1717