Agriculture Reference
In-Depth Information
D. dipsaci
proliferates in shoots and causes them to become twisted and distorted,
with shortened leaves and multiple side shoots, a condition termed 'bloat' in
onions (see Plate 4c). Pimple-like spots termed 'spikkles' also distort the shoots
of infected onions and garlic. Young seedlings are killed after the cotyledon
swells and collapses, and older plants may die if leaf bases are severed. Invasion
by nematodes increases secondary damage due to bacterial decay. Lightly
infested bulbs are likely to develop bacterial decay in storage, which shows as
brown, necrotic 'rings' corresponding to the infested scales. Therefore, even a
low proportion of plants attacked is serious in onion or garlic grown for storage,
since rotting makes the infected plants worthless.
The nematode invades through cracks in the outer sheaths below ground
level. There are four juvenile stages and the one adult stage. The pre-adult
stage is normally the migratory, infective stage. The life cycle lasts about 3
weeks and the pest multiplies rapidly. The life cycle may be completed within a
plant, or it may involve migration to other plants. Within the plant the
nematodes secrete enzymes that dissolve pectin, causing cells to separate,
thereby softening plant tissues and enabling the nematodes to spread. The
loosened cells are susceptible to bacterial rot. The nematodes also secrete plant
growth regulatory chemicals that inactivate auxin, and which promote
abnormal cell division and enlargement. The distorted 'bloat' symptoms of
infested plants can be simulated by damage from 'hormone type' herbicides.
The pre-adult stage is highly resistant to desiccation and tolerant of
freezing, and can survive for many years without feeding in dry soil, plant
debris or on seed-coats.
Ditylenchus dipsaci
survives for a relatively short time in
the absence of a host in warm, moist soil. Bulbs for seed production, which are
lightly infected, may survive storage and go on to produce infested flowering
plants which produce seed infected by nematodes on the seed-coat. Infected
seed can be an important primary source of spread of the pest, as can any
vegetative propagating material. In The Netherlands, onion set production
crops are inspected to ensure freedom from the pest to avoid spread in this way.
Within the field, survival and spread of the nematodes is favoured by moist
clay or silt soil, and they are shorter-lived on sandy soils. They spread only about
1 m/year by migration from an infection source in ideal moist soil conditions.
More serious, and a frequent cause of epidemic crop losses, is spread by surface
water flooding. This can follow heavy rain on poorly structured soils that form a
surface 'cap', which has a low rate of water infiltration. The result can be wide
areas of devastated alliums corresponding to the flooded zones.
Control of
D. dipsaci
requires a comprehensive series of measures to prevent
its introduction and to restrict its survival and spread. Seed crops should be
nematode free. Any infected mother bulbs that survive storage, or mother
plants showing the symptoms of infection, should be rogued from the seed
crop. Nematodes can be destroyed by fumigating infected seed with methyl
bromide. Onion sets, shallots and garlic cloves for planting should be grown on
nematode-free land and inspected. Eradication from infected sets, shallots and
