Michael Celetti, Plant Pathologist, Horticulture Crops
Ontario Ministry of Agriculture Food and Rural Affairs
The cool wet weather experienced last fall (2014) and the recent rain in June 2015 was ideal for stem and bulb nematode (Ditylenchus dipsacci) to multiply and spread this growing season. This pest can cause significant damage to garlic crops. Obvious symptoms in garlic often appear around the time when scapes emerge in late June or early July.
Stem and bulb nematodes are often introduced into a field by planting infested garlic cloves. One stage (4th juvenile) of the nematode is particularly adapted to resist desiccation and freezing and can persist for many years under dry or cold conditions. Young juvenile nematodes within the infested cloves develop into adults during the fall and spring. When the nematodes have reached maturity, they mate and the females lay eggs. The nematodes can live from 45 to 75 days, depending upon the condition, and a single female can lay up to 500 eggs within her life span. It only takes 19 days after hatching from eggs to develop into mature adults when temperatures average around 15oC. The short period of time between egg hatch and maturity together with the frequency of reproduction during the life span of a female often results in an explosion of this pest population under cool wet conditions.
Stem and bulb nematodes feed on cells near the root plate of the garlic plant. As they feed they inject enzymes into the cells which break down cell walls resulting in a rotting around the root plated. During wet weather the nematodes leave the infest garlic and swim to neighbouring healthy garlic plants. They enter the neighbouring garlic by getting in between the scales of the garlic bulb near the soil line. Under wet conditions, the nematodes can swim a short distance up leaves of small emerging plants in the spring and then move down between the leaves in films of water left from rain or dew. Occasionally a small yellow spot or pimple is left on leaves where the nematodes were feeding as they moved down between the leaves towards the bulb. If the garlic plants become infested late in the summer or close to harvest, the nematodes may not cause noticeable damage to the mature bulbs and cloves and growers may select the infested bulb and cloves to plant in the fall which starts the cycle all over.
Often symptom development accelerates after scapes are removed. Severely infected plants appear stunted with lower leaves turning yellow and brown prematurely. Sometimes infested plants will have thicker leaves and necks, but this is not a reliable symptom in infested garlic. Infested plants can easily be pulled out from the soil with a slight tug. The basal plate (the region of the bulb where the roots attach) of severely infested bulbs may also appear to be rotten which can be easily separated from the bulbs. Bacteria, fungi and sometimes onion maggots will often invade nematode infested bulbs causing them to become mushy with soft rot and decay. Severely infested garlic bulbs tend to be soft, shriveled, discoloured and lighter in weight. Often symptoms of bulb and stem nematode in garlic look very similar to symptoms of Fusarium basal plate rot.
Rogueing out plants with obvious symptoms at this time of the season will help reduce the potential of the nematodes moving to neighbouring plants. If sympoms are observed in a garlic field, cloves from bulbs designated as next year’s seed should be tested for nematodes at a qualified pest diagnostic lab prior to planting this fall.