NEWA Model References

The Onion Pest Descriptions Page

The Botrytis Leaf Blight pathogen overwinters as sclerotia (compact mass of fungi capable of surviving unfavorable environmental conditions). These are produced in infected onion bulbs left in cull piles, in mother bulbs stored for seed production, and in bulbs left in the field. The latter result in volunteer onion plants the following spring. Sclerotia also overwinter directly in the soil and on leaves that persist as debris in commercial onion fields. The sclerotia are formed on infected leaves and the necks and upper portions of bulbs before or after harvest. Infected leaves may be raked or washed together and persist as leaf tissue debris in which many sclerotia can be found. Sclerotia in the soil result from the disintegration and decay of infected leaves on which sclerotia were formed.


Sclerotia on onion bulbs in cull piles, on mother bulbs in seed fields, and on volunteer onion plants in commercial onion fields produce conidia (spores) that infect leaves on sprouted bulbs and onion plants in commercial fields. Sclerotia on the surface of the soil in commercial onion fields also produce conidia that can infect the leaves of nearby onion plants. Sclerotia on leaf debris produce conidia and also ascospores (sexual spores) that infect leaves of onion plants. Because ascospores are the result of sexual reproduction, they may serve as the source of new strains of the pathogen that are tolerant to fungicides used to control BLB. The ability of sclerotia to germinate and produce conidia repeatedly (up to four times) results in the production of conidia over an extended period of time. Sclerotia on the bulbs of volunteer onions produce conidia that infect either leaves of the same plant or those of onion plants in commercial fields. In the absence of seed fields and cull piles, it is suspected that sclerotia in the soil and sclerotia on volunteer plants provide the primary source of inoculum for outbreaks of BLB in commercial onion fields.


The dense, tangled growth of leaves that develop from bulbs in cull piles provides conditions (little air movement and high relative humidity) that are favorable for subsequent production of spores on dead leaf tissue. This results in secondary cycles of infection in the cull piles. Similar secondary cycles occur slightly later in seed fields. Conidia are blown from the seed fields and cull piles to commercial bulb production fields and the disease cycles continue. Leaves of onion plants in commercial fields can be infected by conidia that develop from sclerotia in the soil. These infected leaves also serve as secondary sources of inoculum once conidia are produced on the dead leaf tissue. Leaves of volunteer onion plants infected by conidia produced by sclerotia on the same plant also serve as a secondary source of inoculum. Leaves of onion plants grown for commercial production are repeatedly infected, and these serve as the source of inoculum for infecting new leaves throughout the growing season.

Downy mildew is characterized by pale-green to yellowish areas of irregular size and shape (oval to cylindrical) on infected leaves or seed stalks. These areas may consist of alternating yellow and green layers of tissue. The causal organism of DM produces fruiting bodies and spores called sporangia on the surface of the leaves and seed stalks. The masses of spores are at first transparent to greyish, and then rapidly become violet in color. Leaves become girdled in the region where mildew develops and the leaves collapse. This results in dead leaf tips that are rapidly colonized by purple blotch, which is dark in color and obscures DM. DM seldom kills onion plants, but bulb growth may be reduced. Bulb tissue, especially the neck, may become spongy and the bulb may lack keeping quality.

The onion maggot adult is a 0.25 inch (6mm) long, bristly, greyish-brown fly similar in appearance to, but slightly smaller than the housefly. Compared to the housefly, onion flies have longer legs, are more slender, and overlap their wings when at rest. The genus Hylemya contains other similarly appearing root or seed maggot flies that can be confused with the onion fly. One is the seed corn maggot fly Hylemya platura, which appears on the muck soils prior to the emergence of onion flies. The greyish-brown seed corn fly appears almost identical to the onion fly except that it is approximately 1/2 the size of the latter. The seed corn maggot is not known to be a serious pest of onions, but it is often found feeding on previously damaged or decaying onion bulbs. Another similar root maggot fly is the cabbage maggot, Hylemya brassicae. The adults are usually not found near onion fields unless cruciferous hosts are growing nearby. Cabbage maggot flies have smokey-grey wings, black legs, and dark grey bodies with three black stripes on their back. They are slightly smaller than onion maggot flies. The spring brood of onion flies emerge from pupae (resting stage prior to adult), which overwinter in the soil in mid-May and peak about two weeks later in the northeast. Females begin laying eggs approximately 7-10 days after emergence. Onion flies can survive for 2-4 weeks and may lay several hundred eggs. The number of eggs laid is believed to be due to the amount of food (such as wild flowers and other as yet unidentified sources near the crop) available to the adults. The second or summer brood begins emerging in early July with peaks in mid to late July. Emergence of the final or fall flight begins in late August, peaks in early to mid-September, and may continue into October. EGGS The onion fly deposits white elongated eggs (Fig. 2) about 1/25 inch (1.25mm) in length on the soil near the stem and occasionally on the young leaves and neck of the onion plant. Eggs hatch into maggots two to three days after being laid. LARVAE The legless maggots are tapered, creamy-white in color, and reach a length of about 1/3 inch (8mm) when fully mature (Fig. 3). Maggots develop through three larval stages in 2 to 4 weeks depending on the temperature. Most newly hatched larvae crawl below the soil surface and feed upon the roots or burrow the basal plate of developing bulbs. Some maggot larvae may enter into the sides of onion bulbs rather than through the basal plate, after windrowing or undercutting has occurred. Any injury site on the onion bulb facilitates the maggot's entry. PUPAE Once mature, the maggot leaves the onion plant and enters the soil to pupate at a depth of one to four inches (5-10 cm). The pupa is chestnut brown and 1/3 inch (7mm) long. First and second generation pupae remain in the soil for two to four weeks before adult emergence. Larvae of the third (fall) generation develop into pupae and pass the winter in that stage. Flies emerging the following spring constitute the spring flight.

Onion thrips

vary in color from white to yellow to brown. The adults are very small, 1/16 inch (2 mm) long, slender, and pointed at both ends. The males are wingless, extremely rare, and are not needed for reproduction. The adult females have four slender wings which, when folded, extend slightly past the tip of their abdomen. Wings are fringed with long hairs. In the field, these characteristics can be identified with a 10X hand lens. Without magnification, adult thrips may be identified by their small size and rapid movement on the leaf surface. The immature stages are similar in form to the adults, but lighter in color, wingless, smaller in size, and slower moving. Both adults and larvae spend the winter in clover, alfalfa, and wheat rather than in onion or cabbage residues left in the field. In late spring and early summer, populations move from their overwintering sites to vegetable crops. The female thrips lays white, bean-shaped eggs in plant leaves. Eggs hatch in 5-10 days. There are two larval stages followed by a prepupal and pupal stage (resting stage prior to adult). These last two stages are reportedly spent in the soil at the base of the plant in onions. However, with cabbage they develop within the head. Developmental time from egg to adult may range from 10 to 30 days, depending upon temperature. After the pupal stage, the adult female lays eggs for another generation. . . .