STEM AND BULB NEMATODES

Of the genus Ditylenchus, the most economically important nematode species is D. dipsaci. Other species include D. destructor, which is a localized problem on certain field and vegetable crops, but uncommon to California; and D. myceliophagus, which attacks mushrooms and may occur wherever fungal culture is carried out. Ditylenchus dipsaci attacks the bulbs and cloves of onions, garlic, and ornamentals. It also attacks field crops, including alfalfa, oats, and clover, in which young buds are attacked, preventing normal stem elongation. Thus, its parasitic habit depends partly on the type of host attacked. Ditylenchus dipsaci occurs as races that have great host specificity; for example, the race attacking alfalfa does not attack onion. More extensive field testing of this phenomenon is needed. Stem and bulb nematodes are commonly classified as migratory endoparasites, completing their life cycles within the tissues of bulbs, stem, and leaves, but rarely in roots. Repeated generations are produced within host tissues and the nematodes will enter the soil only when living conditions within the plant tissues become unfavorable.

Second stage juveniles hatch from the egg and develop through the third and fourth stages to the adult. After being fertilized by a male, a female lays up to 10 eggs a day within the host tissue and up to 500 eggs during a lifetime. The life cycle takes from 19 to 23 days on onions at 15C. The short life cycle and the large number of eggs laid by the female can result in a characteristically rapid and dramatic population increase that can be as much as a thousandfold during one season.

Because all juvenile and adult stages are vermiform, they are capable of normal nematode movement. An economically important feature of the stem and bulb nematode is its ability to survive repeated desiccation or drying. The fourth stage juvenile is especially adapted to survive desiccation. Dried fourth stage juveniles can be found as whitish clumps attached to dried stems, bean seeds, bulbs, cloves, and other infested dried plant parts. This ability facilitates survival during adverse conditions and acts as a means of dispersal for the nematode. Dry nematodes can be reactivated by moisture when infested material is replanted or when a new crop is planted in soil containing old, infested material.

Infection by Ditylenchus dipsaci causes twisting, distortion, and discoloration of stems and foliage of plants such as alfalfa and ornamentals. Ditylenchus dipsaci may produce multiple crowns in sugarbeet and increased tillering in oats. A common symptom on a variety of crop plants caused by D. dipsaci is swelling of tissues on the lower stem region aboveground, close to the root crown or bulb. Crown rot, a common feature of D. dipsaci infection on sugarbeets, is a result of secondary infection by bacterial or fungal organisms. Bulbs infected with D. dipsaci sometimes show discoloration or browning of one or more of the leave sheaths surrounding the bulb, visible in cross-section. Advanced infections of stem nematode on garlic will cause a discoloration and rotting of the base of the bulb area, and the root plate can be easily separated from the bulb.

Ditylenchus dipsaci on alfalfa, healthy on left
Ditylenchus dipsaci on garlic
Ditylenchus dipsaci on daffodil, pointing to spikkles
Ditylenchus dipsaci on daffodil, spikkles on leaves
Ditylenchus dipsaci on alfalfa
Ditylenchus dipsaci on chive
Ditylenchus sp. on iris
Ditylenchus destructor on iris
Ditylenchus destructor on potato
Ditylenchus dipsaci on daffodil
Ditylenchus dipsaci on garlic
Ditylenchus dipsaci on garlic

From: McKenry, M.V. and P. A. Roberts. 1985. Phytonematology Study Guide. University of California, Division of Agriculture and Natural Resources. Publication Number 4045.

KNOWLEDGE EXPECTATIONS