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Plant NameTypeDescription 
Basal rotOnions, GarlicDiseaseBasal rots of onion, garlic, and related species are caused by fungi. Affected plants are yellowed, wilted, and grow poorly. Reddish discoloration may be present on affected garlic, while onions may show a red-brown rot beginning where roots are attached to the bulb at the basal plate. Initial infection typically occurs on the roots or the basal plate. The rot progresses upward through the bulb and can also occur in stored infected bulbs when sufficient moisture is present. Plants attacked by onion maggots or other insects are more likely to be infected.
Downy mildewOnions, GarlicDiseaseDowny mildew is a fungal disease affecting onions and related species, including weedy species. Leaves initially develop pale, elongate patches. The spots may be water-soaked at first, then later appear somewhat purplish. The spots become covered with a downy, grayish or white fungal growth. Affected leaves often die back. Bulbs of affected plants are smaller than normal and of poor quality. Infected bulbs may become shriveled and discolored in storage, or may sprout prematurely. Cool, damp weather favors spread of the disease.
Gray moldOnions, GarlicDiseaseGray mold is a fungal disease primarily found on garlic. It attacks the tissues of the neck and bulb and occasionally causes a leaf blight. Symptoms of infection typically appear on the neck around the time of harvest, when tissues take on a water-soaked appearance with yellow discoloration. A gray, fuzzy mold develops between the scales, and small black fungal structures may be present around the neck. Gray mold eventually results in a rot of the bulbs. Infection typically occurs at the neck or through wounds in the bulb. Plantings that are in heavy, damp soil or that are overwatered have the greatest risk of infection. The disease often develops in stored bulbs and is favored by high humidity. The fungus causing gray mold can overwinter in the soil.
Neck rotOnions, GarlicDiseaseNeck rot is a fungal disease which can affect onions and related species. Infection typically occurs in the field, but symptoms often do not appear until bulbs are in storage. The fungus infects the neck tissue and may also attack the bulb via wounds. Affected tissues appear water-soaked and yellow. The disease progresses from the neck down the bulb, with a gray mold forming on the infected portions in humid conditions. Hard, black fungal structures may be present, especially around the neck. Infected bulbs decay into a soft mass. White onions are particularly susceptible to damage, although yellow and red varieties may also be affected. The fungus can overwinter in infected plant debris.
White rotOnions, GarlicDiseaseWhite rot is a fungal disease which affects onions and related species. Cool, wet conditions may be followed by wilting and yellowing of leaves beginning at the base of the plant. Affected plants eventually die. Roots are destroyed and a white, fluffy mass of fungus may cover the bulbs. Small, black fungal structures may be present in the white fungal growth. These structures (sclerotia) are smaller and rounder than those of Botrytis neck rot. They are often found near the base of the bulb. Infected bulbs will continue to decay in storage, turning soft and watery. The fungus can persist in the soil for long periods. This disease is a serious problem of onions and related crops.
Onion maggotOnions, GarlicInsectThe onion maggot is the larvae of a slender gray fly about 1/4" in length. Female flies lay eggs near the base of host plants including onion, garlic, leek, and shallot. The white larvae tunnel through the underground portions of the stem and into the bulb, where they feed. Soft rots often infect these injuries, causing further damage. Affected plants appear yellow and flabby. Infested seedlings typically die, while larger onion bulbs may be hollowed out by one or more larvae feeding inside. Mildly damaged bulbs usually rot in storage. Mature larvae are dirty white and about 1/3" long. They pupate in the soil, with up to three generations occurring in a season. The maggots overwinter in the soil or in infested onions, emerging in the spring.
Onion thripsOnions, GarlicInsectOnion thrips are tiny, slender insects about 1/16" long. They are yellowish to dark in color, depending on age. They feed on tender inner leaves, causing longitudinal silvery to white mottling or streaking along the leaves. Often, tiny black specks of frass (excrement) may be present. The mottled areas later turn yellow or brown. Damaged leaves may wither and droop. Severe damage may kill seedlings and reduce yield. Thrips are also found on debris of the host plants and on weeds.