1. 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

2. 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.

3. 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.

4. 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.

5. 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

6. 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

7. 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.

8. 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.

9. 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

10. 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

11. 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.

12. 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.

13. 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

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

15. 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.

16. 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

17. 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

18. 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.

19. 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.

20. 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

21. 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

22. 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.

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

24. 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

25. 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