Agriculture Reference
In-Depth Information
cloves is possible by immersion in hot water at 45°C for 2-3 h. Also effective is
dipping in various nematicidal chemicals, either alone or in addition to hot-
water treatment. Transplants should be raised using nematode-free seed and
growing media.
Chemical sterilants and solarization can destroy the pest in the soil, but
these treatments are expensive. After such treatments it is important to reduce
the rate of spread from surviving foci of infection. The risk of surface water
movement can be minimized by good drainage and soil husbandry that
conserves soil structure and avoids 'capping'.
In temperate climates the nematode survives for too long on clay loams and
fine silts, and has too many alternative hosts, for rotation to be a practicable
control once such soils are infested. However, on lighter soils, rotations involving
wheat, barley and brassicas can lower the risk to following alliums. The short
persistence of the nematode in warm, moist soil makes control possible by 2-3-
year rotations and soil solarization in warm climates (Duncan and Moens,
2006). Field hygiene to avoid the spread of infected waste from any of the host
plants should always be followed.
Theoretical models have been developed describing how the influence on
yield of nematode pests depends on their initial population density in the soil
(Schomaker and Been, 2006). The effects of nematicides and host resistance on
the pest population in the soil and crop losses have been modelled. The model
that applies to stem and bulb eelworm, where an infected plant is totally
worthless, differs from the model applicable to root parasites, where loss in
yield varies continuously with the intensity of root damage.
Symptoms of yellow mottling (mosaic) or striping on leaves, distortion of
shoots and reductions in yield caused by viral infections can occur in allium
vegetables. The viruses are sub-cellular particles consisting of a protein coat
surrounding a central strand of RNA, which carries the genetic information
(Agrios, 2005). The majority of viruses infecting allium vegetables are rod-
shaped and flexuous (bendy), the longest being about 800 nm long
12 nm
wide. After infection, host cell replicative processes are diverted towards virus
proliferation. Sometimes this disruption results in obvious symptoms in the
host plant, but 'latent' viruses proliferate within the host without symptoms.
The viruses are transmitted from plant to plant by aphids, mites or thrips,
depending on species, and therefore viral disease epidemiology is linked with
the biology of these arthropod pests (see 'Arthropod Pests', above). Thus, virus
transmission depends on the occurrence, numbers, infectivity and migration
pattern of these vectors. The main viruses causing disease in allium crops have
a narrow host range, primarily restricted to alliums. Fortunately they are not
transmitted in seeds. These two facts make crop rotation plus crop hygiene an
