Agriculture: Onion and Garlic Pest Management Guidelines

Purple Blotch and Stemphylium Leaf Blight

Symptoms and Signs

Purple blotch and Stemphylium leaf blight occur as oval-shaped, tan-to-brown lesions on leaf blades. Yellow streaks, which later turn brown, extend along the blade in both directions from the lesion. As it progresses, purple blotch lesions become deep purple and may develop dark-brown concentric circles within the lesion. Purple blotch lesions may expand, girdling and killing the leaf. Likewise, in advanced stages, Stemphylium leaf blight may exhibit multiple small lesions, which coalesce and cause blighted leaves. Both diseases can also attack and girdle seed stems.

Comments on the Disease

Stemphylium leaf blight is more common than purple blotch in California, but symptoms and management are similar for both diseases. These fungi are often found infecting downy mildew lesions on onion but occur less commonly on garlic.

Stemphlyium leaf blight is a common late-season problem in processing onions grown in the Tulelake area. It commonly occurs in onions that are maturing and starting to lodge. Infection of seed stalks can reduce seed yield and quality.

Stemphylium vesicarium is a weak pathogen, so it often infects onion plants secondarily, after other diseases develop or when the plant is otherwise stressed. Stemphylium leaf blight develops in iris yellow spot lesions, as well as on dead onion leaf tips following severe heat stress. Both fungi are also commonly found infecting downy mildew lesions on onion but occur less commonly on garlic.

Moderate temperatures and increasing canopy humidity favor both diseases. Heavy dew favors their development in desert areas, but foggy and rainy weather promotes their development in other regions. The optimal temperature for disease development is in the mid-70s (°F). In most areas, these diseases are usually not a problem after the end of the rainy season. However, in the Imperial Valley and in Tulelake, they can cause damage in the absence of rain, when there are conditions of high humidity, nighttime dew, and moderate temperatures. The spores are airborne.

Management

Chemical Control

Because these diseases often occur after damage from onion DOWNY MILDEW, controlling downy mildew is a critical strategy for preventing problems with purple blotch and Stemphylium leaf blight. However, in some regions, such as in the Tulelake area, these diseases can also occur in the absence of downy mildew. In such cases, management must be focused specifically on fungicide applications for purple blotch and Stemphylium leaf blight.

The amount of time leaves are consecutively wet each night (leaf wetness) can be used to inform when to apply fungicides for control of purple blotch and Stemphylium leaf blight.

  • When leaves are wet for less than 12 consecutive hours per night, the interval between fungicide applications can be increased by several days.
  • When leaves are wet for more than 12 consecutive hours per night, fungicide application intervals for controlling these diseases can be shortened in a manner consistent with the pesticide labels.
Common name Amount per acre REI‡ PHI‡
(Example trade name) (hours) (days)
Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.
 
A. PYRIMETHANIL/FLUOPYRAM
  (Luna Tranquility) 16–27 fl oz 12 7
  MODE-OF-ACTION GROUP NAME (NUMBER1): Anilino-pyrimidine (9) and succinate-dehydrogenase inhibitor (7)
  COMMENTS: Not registered for control of Stemphylium leaf blight in California. Do not make more than two sequential applications without alternating with a fungicide with a different mode of action.
 
B. AZOXYSTROBIN/DIFENOCONAZOLE
  (Quadris Top) 12–14 oz 12 7
  MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (7) and demethylation inhibitor (3)
 
C. DIFENOCONAZOLE/CYPRODINIL
  (Inspire Super) 16–20 fl oz 12 Bulbs: 7

Greens: 14

  MODE-OF-ACTION GROUP NAME (NUMBER1): Demethylation inhibitor (3) and anilino-pyrmidine (9)
  COMMENTS: Make no more than two sequential applications without alternating with fungicides with different mode-of-action group numbers.
 
D. PYRACLOSTROBIN/FLUXAPYROXAD
  (Merivon Xemium) 5.5–11 fl oz 12 7
  MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone-outside inhibitor (11)/succinate-dehydrogenase inhibitors (7)
  COMMENTS: Make no more than two sequential applications before rotating to a fungicide with a different mode-of-action group number.
 
E. PENTHIOPYRAD
  (Fontelis) 16–24 fl oz 12 3
  MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate-dehydrogenase inhibitor (7)
  COMMENTS: Not registered for control of Stemphylium leaf blight in California. Make no more than two sequential applications before rotating to a fungicide with a different mode-of-action group number.
 
F. BOSCALID/PYRACLOSTROBIN
  (Pristine) 10.5–18.5 oz 12 7
  MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate-dehydrogenase inhibitor (7) and quinone outside inhibitor (11)
  COMMENTS: Make no more than two sequential applications before rotating to fungicides with different mode-of-action group numbers.
 
G. PYRACLOSTROBIN
  (Cabrio EG) 8–12 oz 12 7
  MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
  COMMENTS: Do not exceed 72 oz/acre per season or make more than two sequential applications before rotating to a fungicide with a different mode-of-action group number.
 
H. BOSCALID
  (Endura) 6.8 oz 12 7
  MODE-OF-ACTION GROUP NAME (NUMBER1): Succinate-dehydrogenase inhibitors (7)
  COMMENTS: Make no more than two applications before rotating to a fungicide with a different mode-of-action group number.
 
I. CHLOROTHALONIL
  (Bravo Weatherstik, Echo 720) Label rates 12 See label
  MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M5)
 
J. MANCOZEB
  (Penncozeb 75DF) 2–3 lb 24 7
  (Dithane M45) 3 lb 24 7
  MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)
  COMMENTS: For use on bulb onions, garlic, and shallots only; do not apply to exposed bulb.
Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of action. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode-of-action group numbers 1, 4, 7, 9, 11, or 17 before rotating to a fungicide with a different mode-of-action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to fungicide with a different mode-of-action group number.
Text Updated: 02/19
Treatment Table Updated: 02/19