and bulb nematodes (Ditylenchus dipsaci)
microscopic (0.9 - 1.8 mm long) roundworms
that live in soil and plant tissues. They
feed on stems, leaves and bulbs by puncturing
and sucking cell contents with a needle-like
mouthpart called a stylet. There are thought
to be about twenty morphologically similar
forms of D. dipsaci, each with a
specific host group. Throughout Europe this
organism attacks more than 1,200 species
of wild and cultivated plants.
Except in cold weather, reproduction occurs
throughout the year with a typical life cycle
taking between 19 and 23 days at 15°C.
When sexually mature, the nematode lives for
between 45 and 75 days, each female being
capable of producing up to 500 eggs. It is
the fourth stage juvenile that is the nematode
survival stage and it can go into a state
of diapause ('slowed life') on or below the
surface of plant stems, petioles and bulbs
for up to five years.
The stem and bulb nematode is a migratory
parasite and at the beginning of a new season
the fourth stage juvenile will enter young
tissues especially seedlings just below the
soil surface. Migration on plant parts above
ground level requires water and juveniles
and adults will take advantage of rain and
irrigation water to travel above the soil.
infested with stem and bulb nematode has
distorted and swollen tissues with a spongy
appearance. The plants become stunted with
shortened and thickened leaves often with
brown or yellowish spots. The bulb tissue
begins softening at the neck and gradually
proceeds downwards; scales appear grey and
the bulbs desiccate and split at the base
under dry conditions. Under wet conditions
secondary invaders such as bacteria, fungi
and onion maggots may induce soft rot and
decay of the bulbs.
Management & Control: It
is important to know the cropping history
to be planted; if the field is known to
be free from stem and bulb nematode then
should be done with clean, uninfected cloves.
Where infestation of planting stock is
evident or suspected then treating bulbs
water (46°C for 2 hours) has been shown
to be effective in eradicating nematodes.
Rotation with non-host crops is the most
effective way of reducing nematode populations.
Two years of rotation with crops such as carrots,
lettuce and potatoes should reduce nematodes
to below levels that would cause economic
losses. Also, avoid infesting new land by
cleaning machinery and equipment thoroughly
and by preventing the movement of infested
Stem and bulb nematode can also be controlled
by the use of Fenamiphos (Nemacur 15G) at
a rate of 7 - 14 Kg/acre applied with the
garlic cloves at planting.
An organic alternative for areas heavily
infested with nematodes is to grow dwarf French
marigolds on the infested land. It has been
shown that solid planting of marigolds suppresses
nematode populations and greatly reduces the
numbers found in the roots of susceptible
hosts. Compounds toxic to nematodes have been
identified in the growing roots of French
marigolds and these have been shown to be
released by the plants throughout the growing
season and for the benefits to be evident
after a period of three to four months. For
maximum benefit marigolds should be planted
30 cm apart in May and grown through until
autumn when the tops should be removed and
the roots ploughed into the soil.