Mushy Rot
(fungus - Rhizopus spp.): Bulbs have soft areas around the neck. In the neck area, there is a white fuzzy growth with black speckling [Picture]. This is a post-harvest problem that occurs when onions that are not properly cured or stored are transported at high temperatures.

Neck Rot
(fungus - Botrytis allii, Botrytis sp.): This disease is frequently not noticed in the field because damage usually occurs during transit and storage. Diseased tissue at the base of the crown becomes sunken and watersoaked in appearance. A gray fungal growth later forms on the surface [Picture], which can be followed by other fungi and bacteria, causing decay. Small, black-resting bodies (sclerotia) can sometimes be found on scales [Picture]. Careful handling of the crop at harvest and prompt drying of onions with heat and air ventilation are the best means of controlling this disease.

Pink Root
(fungus - Phoma terrestris): Pink root is a soilborne disease that affects roots. Diseased roots turn pink, shrivel and die [Picture] As the plant sends out new roots, they also become infected and die. Affected plants do not usually die, although they may develop tip blight. Severe infection will reduce bulb size. The fungus can be introduced to a field by using transplants grown in infested soil. Once a field becomes infested, the fungus remains in the soil for many years. Soil fumigation has been shown to be an effective, but expensive, control measure. Resistant onion cultivars are one management approach, as is a long rotation out of onions.

Powdery mildew
(fungus - Leveillula taurica): Powdery mildew occurs rarely in the Lower Rio Grande Valley of Texas. The earliest symptom is a pale discoloration of the leaf [Picture]. Circular spots with white, powdery growth eventually occur [Picture]. There are no control recommendations, since the disease is not a serious problem.

Purple Blotch
(fungus - Alternaria porri): The fungus usually infects dead or dying leaf tissue. The first symptoms are small, white, sunken lesions. These lesions develop purple centers and enlarge [Picture]. The infection can encompass much of the leaf, leading to the death of tissue above the lesion [Picture]. The disease can be controlled with fungicides. In the Rio Grande Valley of Texas, the need to apply fungicides can be determined by monitoring leaf wetness during the growing season. Leaf wetness occurs as the result of dew, fog or rain. The action threshold is 12 hours of continuous leaf wetness.

Pythium Root Rot
(fungus - Pythium sp.): This disease is most serious with young plants growing under conditions of high soil moisture and cool temperatures. Infected roots become water-soaked and flimsy [Picture]. Not all of the roots of the plant become infected [Picture]. The plants will not usually die, but severe infection can result in small bulbs. The loss of a substantial amount of roots will lead to tip dieback. Planting on raised beds will minimize the impact of this disease.

Root Knot Nematode
(nematode - Meloidogyne spp.): Portions of the roots have swollen areas [Picture].

See section on Root Knot Nematodes

Soft Rot
(bacteria - Erwinia carotovora subsp. carotovora and other species): Soft rot is one of the more prevalent causes of loss in storage onions. The soft rot bacterium can enter the neck tissues as plants approach maturity. In the field, plants wilt and die [Picture]. As the rot progresses, invaded scales become soft and foul-smelling [Picture]. Onions with mechanical injuries, sunscald, or bruises are particularly susceptible to bacterial soft rot, especially if they have been held under warm, humid conditions

Stemphyllium Blight
(fungus - Stemphyllium vesicarium): Lesions are initially light yellow to brown and water-soaked. They elongate, often reaching the leaf tips, and become dark brown to black. The disease can become serious following periods of more than 24 hours of rainy weather. Fungicides used to control purple blotch will also control this disease.

Tip Blight
(several causes): Infection by several species of fungi infecting leaves or roots can result in tip dieback. There can also be many non-pathogenic causes [Picture]. These include: overcrowding, insect injury (particularly thrips and leaf miners), drought or salt stress, wind dessication, and occasionally, damage by ozone gas produced by lightning during severe thunderstorms.