Onion Diseases Fungal Physiological Botrytis Blight - Botrytis squamosa Botrytis Neck Rot - Botrytis allii Purple Blotch - Alternaria porri Fusarium Basal Rot - Fusarium spp. Downy Mildew - Peronospora destructor Physiological Translucent Scale

Onion Diseases - Botrytis Blight This leaf blight, sometimes referred to by growers as “blast”, is caused by the fungus Botrytis squamosa. The white necrotic center of the lesion surrounded by a very light green halo is diagnostic of this disease.

Onion Diseases - Botrytis Blight More intense symptoms from what was seen in the previous slide. Frequently these symptoms can be confused with those caused by ozone or herbicide injury. Leaf lesions caused by ozone or herbicide injury, however, do not have the typical halo.

Onion Diseases - Botrytis Blight Here’s a field photograph of “blast” showing devastation of the crop. Most growers have to employ routine fungicide spray schedules. For control, practice a rotation with non-host crops to reduce the build up of sclerotia in the soil. Wide rows provide better air movement which promotes rapid drying of the foliage helps in control of this disease.

Onion Diseases - Botrytis Blight Key Points Pathogen: Botrytis squamosa Symptoms and signs white necrotic lesions on leaves, surrounded by a whitish halo ozone injury symptoms may mimic Botrytis symptoms but no halos are present around lesions fungus produces weather-resistant sclerotia Overwintering: onion cull piles and infected debris in soil sclerotia survive in soil for long periods of time

Onion Diseases - Botrytis Blight Control Strategies Fungicide sprays Elimination of cull piles Long rotations with non-host crops Wide row spacing Irrigation early in the day

Onion Diseases - Botrytis Neck Rot Neck rot is caused by the fungus, Botrytis allii. This disease is found primarily in storage. Typical symptoms show collapse of the neck tissue and the presence of black sclerotia, the resting stage of the Botrytis fungus.

Onion Diseases - Botrytis Neck Rot Internal symptoms consist of a watery decay that can move down from the neck and affect the entire bulb. Any conditions that provide rapid and proper curing will help prevent this problem. Cut roots under the bulb before harvest to promote in-field curing. Do not apply nitrogen later than 8-9 weeks after planting and do not space plants too closely together. Avoid injury to the neck area of the bulb at harvest. Storage conditions should be such that no moisture condensation occurs on the bulbs, and temperatures should be just above freezing for long-term storage.

Onion Diseases - Botrytis Neck Rot Key Points Pathogen: Botrytis allii Symptoms and Signs: Generally do not appear until bulbs are in storage Collapse of neck tissue Internal watery decay Sclerotia of the fungus frequently found on the neck of onion Overwintering: Sclerotia survive in soil for long periods

Onion Diseases - Botrytis Neck Rot Control Strategies Cut roots under bulbs before harvest Avoid damaging necks or bulbs at harvest cutting necks 1” above bulbs is preferable to ripping off tops Rapid and proper curing is important Proper storage conditions just above freezing prevent moisture condensation on bulbs

Onion Diseases - Purple Blotch Initial foliage symptoms are water-soaked areas that later turn brown. Fully expanded lesions often have a characteristic purple coloration. The concentric black rings in this picture are made up of the spores of the Alternaria fungus.

Onion Diseases - Purple Blotch Lesions can girdle the entire leaf leading to death of the foliage. Growers must maintain an adequate fungicide spray program. Rotation with non-host crops, maintenance of good air movement around the foliage and irrigation early in the day to ensure drying of the foliage helps to control the disease.

Onion Diseases - Purple Blotch Key Points Pathogen: Alternaria porri Symptoms and signs watersoaked areas followed by white necrotic spots on foliage girdling lesions on the stem black concentric rings on purplish lesions fungus requires rain or persistent dew for reproduction and penetration of plant tissue Overwintering: fungus can survive in crop debris

Onion Diseases - Purple Blotch Control Strategies Rotation with non-host crops Maintain good air movement around foliage Commercial growers can use currently registered fungicides

Onion Diseases – Fusarium Basal Rot Foliage symptoms of this disease appear as distortion and chlorosis of the leaves that begins at the leaf tips. The rot engulfs the bulb beginning at the basal plate. As can be seen in this slide, the roots of the onion on the left are almost totally lacking. Fusarium, a soil-borne fungus that survives in the soil for a long period of time in the absence of a host, can enter through the bulb. The fungus enters with greater ease if there is a wound. There are a limited number of varieties such as Amanda, Citadel, Copra, or Ole, that have resistance to this disease. Rotation with non-susceptible hosts aids in control.

Onion Diseases – Fusarium Basal Rot Key Points: Pathogen: Fusarium oxysporum f. sp. cepae Symptoms: Leaf tip yellowing and necrosis Affected plants may wilt Decay of bulbs progressing up from basal plate White mycelium on and surrounding basal plate Pathogen is soil borne Commonly introduced into home gardens on onion sets Infection increased by insect injury to roots, stem plate or bulb

Onion Diseases – Fusarium Basal Rot Control Strategies: Crop rotation – minimum of 4 years between onion or garlic crops Plant disease free onion sets into home garden Careful sorting of onion sets before planting helps to eliminate diseased bulbs Some disease resistant cultivars available

Onion Diseases – Translucent Scale This disorder is not caused by a pathogenic organism, but rather is a physiological condition related to excessive heat during the growing season. The tissue looks as if it has been frozen.

Onion Diseases – Translucent Scale In cross-section the scales look a bit brown, dome areas of the bulb appear water-soaked. This condition is associated with high temperatures and very high relative humidity during the growing season. Symptoms become more severe upon prolonged storage.

Onion Diseases – Translucent Scale Key Points: Cause: Frequent high temperatures (>90 F) and high relative humidity during the last 6-8 weeks of growth Symptoms: Scales are translucent and resemble freezing injury Scales become grayish and water soaked In cross section, affected scales are brownish Symptoms are most common on outer scales Affected scales shrink, giving bulb slightly irregular surface Symptoms continue to progress in storage Delays of 2-4 weeks between curing and cold storage can increase incidence

Onion Diseases – Translucent Scale Control Strategies: Little can be done to change growing environment Affected onions should be cured and temperatures in storage rapidly decreased to just above the freezing mark to minimize losses If observed in early storage, prolonged storage will result in additional losses

Onion Diseases – Downy Mildew With prolonged periods of high relative humidity and cool temperatures, downy mildew can occur in any of the onion producing areas worldwide. Leaf symptoms first appear on the older leaves; they are elongate in character and somewhat paler green than the rest of the foliage. When morning dew is present on the leaves one can sometimes see the fuzzy growth of the fungus on the leaf lesions. In time the affected leaves become chlorotic and then necrotic (dead), they bend over and collapse. Infection may progress into the bulb and in storage the outer scales may become waterlogged, turn a yellow color, and be wrinkled. Infected bulbs might also sprout in storage.

Onion Diseases – Downy Mildew This is the fungus that causes downy mildew. Seen here are the sporangiophores and the sporangia of Peronospora destructor.

Onion Diseases – Downy Mildew Key Points: Pathogen: Peronospora destructor Symptoms: Elongated leaf patches that are slightly paler green or tan When wet with dew, grayish violet and furry growth visible on lesion surface Affected leaves become pale green and soon collapse Favored by cool temperatures and wet weather Under favorable conditions, this disease rapidly destroys susceptible tissues Fungus produces oospores that can be soilborne for several years

Onion Diseases – Downy Mildew Control Strategies: Rotation of at least 3-4 years between onion crops Prompt tillage after harvest to hasten breakdown of onion tissues Protectant fungicide sprays are effective Careful irrigation to avoid prolonged leaf wetness