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Leafy Greens Diseases - Downy Mildew


Figure 8.20
Downy mildew of spinach. 

Downy Mildew
Downy mildew of lettuce.

CAUSAL AGENT:

The first documented case of spinach downy mildew in Utah occured in spring 2017 in a greenhouse.Lettuce downy mildew in Utah has only been confirmed in grocery store produce. On spinach: Downy mildew of spinach is caused by Peronospora farinosa f. sp. spinaciae. There are 16 known races of spinach downy mildew. On lettuce: Downy mildew of lettuce  is caused by Bremia lactucae. At least six races are known in the western United States.

SYMPTOMS:

Yellow to brown angular spots appear on leaves during cool wet weather, with white fuzz on the undersides. Over time, lesions turn brown and dry up.

DISEASE CYCLE:

Lettuce downy mildew can overwinter on crop residue left in the field and can be windborne. Windborne spores can be blown in from hundreds of miles away. Lettuce downy mildew develops best during cool moist weather with temperatures ranging from 41oF-75oF with an optimum temperature of 68oF. Spinach downy mildew survives in living and dead spinach plants and spores are dispersed by wind. Spinach downy mildew can survive a wide range of temperatures from near freezing to very hot temperatures. Downy mildew can also be seedborne but the importance of this type of inoculum is unknown.

MANAGEMENT:

Resistant varieties are available for both spinach and lettuce downy mildew. However, they are not resistant to all races (see table 11.4 for spinach cultivar resistance). Lettuce varieties are more selective to areas they can be grown in and therefore no variety recommendations are made.

      • Reduce leaf wetness by using drip irrigation instead of overhead irrigation to minimize dew development on leaves and reduce disease incidence.
      • Some fungicides are effective for lettuce downy mildew especially as a preventative. Apply fungicides during environmental conditions that are ideal for infection.

Spinach Varieties with Known Resistance to Specific Downy Mildew Races.

Variety High Resistance Intermediate Resistance
3665 (FI) Races 1-5, 8, 9, 11-12,14  
Anna (FI) Races 1, 3, 5,
Baker (FI) Races 1, 3, 5, 8, 9, 11, 12, 14  
C2-606 (FI) Races 1-9, 11-16  
Carmel (FI) Races 1-11, 13  
Corvair (FI) Races 1-12, 13  
Emperor (FI) Races 1-10  
F91-415 (FI) Races 1-2  
Flamingo (FI) Races 1-11  Races 12-13
Gazelle (FI) Races 1-13  
Kookaburra (FI) Races 1-13, 15  
Persius (FI) Races 1-3, 5, 8, 9, 11, 12, 14, 16  
Red Kitten (FI) Races 1-13, 15  
Reflect (FI) Races 1-11  
Seaside (FI) Races 1-12 Race 14
Space (FI) Races 1-3, 5-6, 8, 11-12  
Viceroy (FI) Races 1-2  
Woodpecker (FI) Races 1-15