Nematodes in Bulbs

Wilbur D. Courtney.

A number of parasitic nematodes; eelworrns may infect ornamental bulb crops.

Root knot nematodes (Meloidogyne species), lance nematode (Hoplolaimus coronatus), and meadow nematodes (Pratylenchus species) have occasionally caused severe damage to bulb crops, especially in the warmer sections of the United States.

In cooler sections where the hardier types of bulbous irises, Easter lilies, and narcissi are grown for sale to florists for greenhouse forcing rather than for immediate flower production the bulb and stem nematodes (Ditylenchus species) and the bud and leaf nematode (Aphelenchoides olesistus) are the most serious pests. This chapter concerns the latter two nematodes as pests in the bulb industry of the Pacific Northwest.

BULB AND STEM NEMATODES usually attack the bulbs, crowns, and above-ground parts of plants. More than 350 kinds of plants have been reported as infected at some time or place. The type species, Ditylenchus dipsaci, was first discovered in 1857 causing a disease in the flower heads of fullers teasel, Dipsacus fullonum. These nematodes are less than one-sixteenth inch long.

Infected narcissus plants show characteristic discoloration and swellings in parts of leaves or stems and discoloration in bulbs.

Infected bulbous iris seldom show malformations of leaves, but spots or blotches often are present at the place the stem joins the bulb. Nematodes enter iris bulbs near the base. They produce a dark depression around the basal plate and cause the dry scales to fall away. In more advanced stages, dark streaks and blotches of lifeless tissue may be found in the fleshy scales from the base toward the bulb tip.

The nematodes on infected plant tissue appear under the microscope as tiny worms among the plant cells. Nematodes exist as males and females and reproduction is by means of eggs, from which hatch tiny larvae, which are shaped like the adults and grow rapidly. They shed their cuticles ("skins") and become second-stage larvae. The procedure is repeated to form third- and fourth-stage larvae and finally adult nematodes. The life cycle requires 25 to 30 days and is repeated as long as the plant grows.

When the narcissus plant dies, because of infection or final ripening, many nematodes leave the plant tissue and, if the soil is moist, migrate into the surrounding earth. Those that do not migrate become dried in plant debris and enter an inactive stage known as quiescence. In that condition they are able to withstand unfavorable conditions, such as extremely high temperatures, and have been revived, upon moistening, after remaining more than 5 years in dry plant tissue. The same thing happens if the soil becomes dried after the nematodes migrate from the hosts, but they remain active as long as there is enough moisture. Bulb and stem nematodes infecting narcissi have remained active in moist soil free of plant growth for 18 months. It took that much time for them to starve to death, as they can exist only on food stored in their bodies.

Control measures for nematodes in bulbous crops must be efficient enough to eradicate them entirely regulatory measures allow no tolerance of nematode-infected bulbs in salable stocks. Control measures must include sanitation, proper soil management, and the treatment of bulbs with hot-water-formalin.

The sanitary measures insure the destruction of infected plant materials and prevention of contamination by infested soil. Infected plants with adhering soil should be carefully removed from the field, placed in a deep, isolated trench, covered with quicklime, and buried with a layer of soil. Dry materials from fields and sheds may be burned. To prevent the spread of nematodes by scattering soil and debris front an infested field, adhering soil should be removed from tractors, trucks, diggers, shovels, and other tools. The implements should be washed thoroughly with water and then freely sprayed with formalin (1 part commercial formaldehyde solution U. S. P. to 9 parts of water). That solution may also be used to dip bulb trays and other equipment and to drench soil along paths to free them of the pests.

Proper management of infested soil must destroy the nematodes by an adequate starvation program or by the use of soil fumigants. The success of the starvation method is based upon the removal of all volunteer host plants from the field in question. Infested fields must be kept free of bulbs for at least 2 years following the eradication of host plants in order to starve the plant parasitic nematodes remaining in the soil. Because rapidly decaying plant material tends to lower the number of bulb and stem nematodes in the soil, the frequent use of fast-growing green-manure crops may be recommended during the starvation period. Corn, or most grains, when used as fast-growing annuals, may be used during the starvation period as cash crops.

Soil fumigants may be used to free the soil of plant parasitic nematodes after all volunteer host plants have been removed from an infested field. Fumigation probably is most economical when it is used to treat infested spots or parts of fields for eradication. The materials for fumigation may cost up to 100 dollars an acre. A grower who is thinking of fumigating soil will do well to consult local or regional horticultural officials about the source, cost, and choice of the various fumigants and applicators.

Proper hot-water-formalin treatment is the best method known at present for curing bulbs infected with nematodes. The use of heat for curing plants infected with nematodes was tried in Germany in 1909. Definite experiments with bulb crops were performed some 10 years later. As a result of those tests, Federal legislation in the 1920's, designed to protect our new bulb industry from pests and diseases carried by imports from other countries, required that nematode-infected stock be treated with hot water. The imported bulbs usually arrived in late autumn after root development was well started and were severely injured by the hot-water treatment. The injury was overcome by a year's growth, but treated bulbs could not be forced in greenhouses without being field-grown for a year. Many other methods of killing the nematodes in the bulbs were then tried, but none proved superior to hot water. It was found, however, that treatment with hot water at 110 to 111 F. for 3 to 4 hours, without the addition of formaldehyde solution U. S. P., did not kill all the nematodes.

When I was located at the Ornamentals Insect Research Laboratory at Sumner, Wash., in 1934, I added a presoak period of 2 hours in water at 75 F. in order to activate quiescent nematodes before the killing treatment. Commercial formaldehyde solution U. S. P. also was added to the treating water at the rate of 1 pint to each 25 gallons of water (0.5 percent) to kill the nematodes more rapidly and prevent the spread of rot organisms during treatment.

When the bulb industry was becoming established in the United States in the late 1920's and early 1930's, the small circular tanks used in England were found to be too small to be practical on American farms and a substitute had to be found. Therefore, equipment for treating bulbs in hot-water formalin has been largely developed by the growers themselves. Using materials a hand, they constructed tanks of wood, metal, or concrete large enough to handle many bulbs in less time. Steam from a nearby boiler, liberated near a motor-driven propeller at the bottom and end of the tank, usually heated the water. A false bottom formed a channel under the load of bulbs through which the heated water was forced to the opposite end and eventually back to the propeller. Baffle plates helped distribute the heat uniformly through the tank. A thermometer was kept immersed near the manually operated steam valve, which was opened occasionally to keep the water between 110 and 111 . Bulbs in containers were lowered to the false bottom by hand or with hoists.

After 20 years of experience, growers have made several improvements.