Black Mold: (fungus – Aspergillus niger) Black mold is generally a post-harvest disease, although it may be seen on mature onions in the field. The disease can be recognized by the presence of black powdery spore masses of the fungus on the outer scales. High temperatures (85oF-95oF) and moisture favor disease development. Bulbs should be protected from moisture during harvesting and shipping.
Botrytis Leaf Blight/Blast (fungi – Botrytis allii, B.squamosa, and B. cinerea): Botrytis leaf blight or blast occurs sporadically in Texas, usually early in the season. Several species of Botrytis infect onion. Seedlings may be infected (See Photo). Neck rot is caused by B. allii, leaf fleck is caused by B. cinerea, and leaf blight is caused by B. squamosa. White flecks are found along the length of the leaf (See Photo) and usually have greenish halos. With numerous flecks, the tip of the leaf may die. Non-pathogenic causes of flecks can include: cold rain, sleet, and sandblasting. Fungicides used to control purple blotch will also control Botrytis leaf blight.
Downy Mildew (fungus – Peronospora destructor): Downy mildew is an occasional disease of onion in the Lower Rio Grande Valley of Texas. Symptoms consist of white to light green spots on leaves, which later darken. A fuzzy, gray growth is seen on the leaf surface, particularly during periods of high humidity (See Photo). Lesions enlarge and leaf tissue dies. Lesions may resemble those caused by the purple blotch fungus. Fields should be monitored closely, particularly during prolonged cold, wet weather, when the disease is more likely to occur. Fungicides that are highly effective against downy mildew, such as Ridomil and Aliette, should be applied following the first report of downy mildew in the growing area.
Fusarium Basal Plate Rot (fungus – Fusarium oxysporum f. sp. cepae): The disease develops at the base of the bulb, causing it to become soft. A semi-watery decay progresses from the base of the scales upward (See Photo). The disease may not be noticed until after harvest, when the entire bulb is destroyed. The fungus is soilborne and enters the bulb through wounds, insect injuries or through root scars at the base. High soil temperatures (77oF – 82oF) favor disease development. Losses can be reduced with a 4-year rotation out of onions. Cultivars can also vary in resistance.
Leaf Variegation (Chimera): The leaves have distinct yellow or white longitudinal segments (See Photo). Affected plants occur very infrequently. This is a genetic abnormality.
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. 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 (See Photo), which can be followed by other fungi and bacteria, causing decay. Small, black-resting bodies (sclerotia) can sometimes be found on scales. 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 (See Photo). 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. Circular spots with white, powdery growth eventually occur. 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 (See Photo). The infection can encompass much of the leaf, leading to the death of tissue above the lesion (See Photo). 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 (See Photo). Not all of the roots of the plant become infected (See Photo). 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: (see Nematode)
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 (See Photo). As the rot progresses, invaded scales become soft and foul-smelling. 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. 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.
Bacterial Blight (bacterium – Xanthomonas axonopodis pv. allii): This disease is not common in Texas. The symptoms are elongated chlorotic areas on one side of the leaf. These areas become sunken, water soaked and necrotic. Eventually, the leaf dies. The pathogen is seed borne, but could spread within a field following rain or overhead irrigation. Disease development is favored by temperatures greater than 68°F. There are no control recommendations.