Ditylenchus dipsaci (stem and bulb nematode)

In general, this nematode causes swellings and distortion of aerial plant parts and necrosis or rotting of stem bases, bulbs, tubers and rhizomes (see section on Biology).Allium spp. (onions, garlic, leeks, etc.)Penetration of onion leaves by D. dipsaci causes leaf deformation and leaf swellings or blister-like areas on the surface. The leaves grow in a disorderly fashion, often hang as if wilted and become chlorotic. Young plants can be killed by high infestations. The inner scales of the bulb are usually more severely attacked than the outer scales. As the season advances the bulbs become soft and when cut open show browning of the scales in concentric circles. Conversely, D. dipsaci on garlic does not induce deformation or swellings, but causes leaf yellowing and death (Netscher and Sikora, 1990).LucerneThe crop declines in patches in the field and damage is more serious in humid climates. The whole plant becomes desiccated and presents symptoms of stunting and swelling at the base of the stem with conspicuous shortened internodes. With heavy infestation, plants can be killed.TobaccoInvasion by the nematode of the lower part of the stem causes stunting and deformation of the plant followed by 'stem break'.Faba beans, Vetch, Chickpea, Pea and Lentil D. dipsaci causes swelling and deformation of stem tissue or lesions which turn reddish-brown then black, depending on cultivar and environmental factors. Newly formed pods take on a dark-brown appearance. The lesions envelop the stem and increase in length, often advancing to the edge of an internode. Leaf and petiole necrosis is also common under heavy infestations, but can be confused with symptoms induced by fungal leaf pathogens. Infected seeds are darker, distorted, smaller in size and may have speckle-like spots on the surface. Heavy infestations often kill the main shoots, stimulating secondary tiller formation. The more severe symptoms are usually induced by the 'giant race' on faba beans (Sikora and Greco, 1990). On faba bean (V. faba), D. dipsaci induces necrosis or swelling of the tissue. Infested stems of lentil and vetch (Vicia spp.) are swollen and show shortened internodes. D. dispaci induces local necrosis on pea and a total necrosis of the stem on vetch (Caubel et al., 1998).Cereals (oats and rye)D. dispaci causes the production of extra tillers at the base and the plants become swollen to give a typical 'tulip-root' appearance.NarcissusNarcissus leaves are distorted and often have characteristic pale swellings called 'spikkels'; infested bulbs usually have brown rings when sliced.

Cultural Control

Control by crop rotation is limited by the polyphagous habit of some races of D. dipsaci and by persistence of the nematode in clay soils but a 3-4 year rotation to lettuce eliminated the nematode in New York State (Lorbeer et al., 1997). Chemical treatments of the soil are not an economic proposition for large areas. However, it may sometimes be worth treating small patches, after lifting and destroying the affected plants (bulbs) together with a margin of surrounding healthy ones, to eradicate a slight infestation before it spreads. Systemic nematicides may be effective to some extent in controlling D. dipsaci in some ornamental crops. The use of tolerant or resistant cultivars can also reduce the damage. Nematophagous fungi such as Verticillium balanoides may be suitable for development as biological control agents for D. dipsaci (Hay and Bateson, 1997).

Soil solarization has been tried in Israel (Siti et al., 1981) and Italy (Greco et al., 1985; Greco and Brandonisio, 1990). Excellent results were seen in the Israel trials.

Sanitation

Certified nematode-free seeds and planting material are most essential to prevent crop damage by D. dipsaci. Hot-water treatments with different temperature-time combinations, depending on type and state of seed material, are operational and efficient to control D. dipsaci (Gratwick and Southey, 1972). Hot-water treatment of narcissus bulbs infected with stem nematodes consist either of storage for 1-2 weeks at 25-30°C, followed by soaking in water for 24 h and hot-water treatment at 45°C for 4 h, or storage for 1-2 weeks at 25-30°C, followed by hot-water treatment at 47°C for 4 h. Routine hot-water treatment for the control of other pests and diseases should be carried out at 43.5°C for 3 h to control any slight, possibly unnoticed nematode infection (Windrich, 1973).

 

D. dipsaci is one of the most devastating plant-parasitic nematodes, especially in temperate regions. Without control, it can cause complete failure of host crops such as onions, garlic, cereals, legumes, strawberries and ornamental plants, especially flower bulbs.D. dispaci is sometimes associated with other pathogens. For example, it transmits Corynebacterium insidiosum to lucerne plants (Hawn, 1963). Experimental infection of onions with D. dipsaci and Peronospora schleidenii together was 36.5% greater than with nematodes alone (Yakimenko and Efremenko, 1973).Sturhan and Brzeski (1991) indicate that in heavy infestations, crop losses of 60-80% are not unusual. In Italy up to 60% of onion seedlings died before reaching the transplantation stage. On garlic, losses of 50% were recorded from Italy and more than 90% from France and Poland.