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Published on January 12, 2008

Author: Carla

Source: authorstream.com

Onion Diseases:  Onion Diseases Translucent Scale Botrytis Blight - Botrytis squamosa Botrytis Neck Rot - Botrytis allii Purple Blotch - Alternaria porri Fusarium Basal Rot - Fusarium spp. Downy Mildew - Peronospora destructor Onion Diseases - Botrytis Blight:  Onion Diseases - Botrytis Blight Onion Diseases - Botrytis Blight:  Onion Diseases - Botrytis Blight Onion Diseases - Botrytis Blight:  Onion Diseases - Botrytis Blight Onion Diseases - Botrytis Blight Key Points:  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 Blight Control Strategies Onion Diseases - Botrytis Neck Rot:  Onion Diseases - Botrytis Neck Rot Onion Diseases - Botrytis Neck Rot:  Onion Diseases - Botrytis Neck Rot 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 Key Points 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 - Botrytis Neck Rot Control Strategies Onion Diseases - Purple Blotch:  Onion Diseases - Purple Blotch Onion Diseases - Purple Blotch:  Onion Diseases - Purple Blotch Onion Diseases - Purple Blotch Key Points:  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 - Purple Blotch Control Strategies Onion Diseases – Fusarium Basal Rot :  Onion Diseases – Fusarium Basal Rot Onion Diseases – Fusarium Basal Rot Key Points::  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::  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 :  Onion Diseases – Translucent Scale Onion Diseases – Translucent Scale :  Onion Diseases – Translucent Scale Onion Diseases – Translucent Scale Key Points::  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::  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 :  Onion Diseases – Downy Mildew Onion Diseases – Downy Mildew :  Onion Diseases – Downy Mildew Onion Diseases – Downy Mildew Key Points::  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::  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

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